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Globally, normal-fault displacement bends and warps rift flanks upwards, as adjoining basins drop downwards. Perhaps the most evident manifestations are the flanks of the East African Rift, which cuts across the otherwise minimally deformed continent. Flank uplift was explained by Vening Meinesz (1950, Institut Royal Colonial Belge, Bulletin des Seances, v. 21, p. 539-552), who recognized that isostasy should cause uplift of a normal-faulted footwall and subsidence of its hanging wall. Uplift occurs because slip on a dipping normal fault creates a broader root of less-dense material beneath the footwall, and a narrowed one beneath the hanging wall. In this paper, we investigate the potential influence of this process on the latest stages of Sierra Nevada uplift. Through theoretical calculations and 3D finite element modelling, we find that cumulative slip of about 4 km on range-front faults would have produced about 1.3 km peak isostatic uplift at the ridge crest. Numerical models suggest that the zone of uplift is narrow, with the width controlled by bending resistance of the seismogenic crust. We conclude that footwall unloading cannot account for the entire elevation of the Sierran crest above sea level, but if range-front faulting initiated in an already elevated plateau like the adjacent Basin and Range Province, then a hybrid model of pre-existing regional uplift and localized footwall unloading can account for the older and newer uplift phases suggested by the geologic record. [ABSTRACT FROM AUTHOR]

Obliquity of the strain velocity field to the deformation zone boundary requires strain to be triaxial. At the plate boundary scale, fault geometries predicted by transtensional theory better explain observed fault patterns in the northern Eastern California shear zone-Walker Lane area than do 2D plane strain predictions. Structural provinces in the Eastern California shear zone-Walker Lane are defined by variations in the orientation of the eastern margin of the Sierra Nevada microplate, and include the Honey Lake-Pyramid Lake, Lake Tahoe, Mono Lake-Long Valley, Owens Valley, and Coso subprovinces. The local geometry of the transtensional zone boundary and the microplate transport direction determines orientations of the instantaneous strain axes for each province. Fault orientations predicted with respect to these axes are consistent with those observed in each structural province. The variation of zone geometry among predominantly coaxially dominated strain provinces over a consistent period of deformation does not result in dramatic changes in shape or orientation among finite strain ellipsoids. Although Quaternary faulting strongly reflects transtensional kinematics, the magnitude of estimated finite strain for each province suggests that present-day transtensional deformation has not accommodated large amounts of extension, vertical thinning, or strike-slip offset. K-values for all structural provinces plot in the k>1 constrictional field on the logarithmic Flinn (Ramsay) diagram, indicating non-plane, transtensional strain. Collectively, these theoretical applications and structural-tectonic observations have implications for evaluating kinematics and faulting during brittle non-plane strain deformation, and for the overall evolution of the transtensional Sierra Nevada-North America plate boundary. [ABSTRACT FROM AUTHOR]

The Miocene Stanislaus Group (Stanislaus, Calaveras, Tuolomne, Alpine and Mono counties, CA), composed of intercalated latite and quartz-latite (trachyandesite and trachyte/trachydacite) lavas and ignimbrites, provides an important marker for reconstructing the elevation history and tectonic development of the Sierra Nevada and Walker Lane. We present new 40Ar/39Ar geochronology and magnetostratigraphy indicating that the Stanislaus Group was emplaced in two pulses: (1) major outpouring of latite lavas at ca. 10.4 Ma and (2) ignimbrite eruptions alternating with lesser lava flow eruptions during ca. 9.4-9.8 Ma. These two events filled the ancestral Stanislaus River drainage in the region of the present Sierra Nevada crest, whereas the ignimbrite eruptions formed the Little Walker Caldera. Our new and previous results date these topographic changes and yield improvements to the Stanislaus Group stratigraphy. [ABSTRACT FROM AUTHOR]

The modern Sierra Nevada and Great Basin were likely the site of a high-elevation orogenic plateau well into Cenozoic time, supported by crust thickened during Mesozoic shortening. Although crustal thickening at this scale can lead to extension, the relationship between Mesozoic shortening and subsequent formation of the Basin and Range is difficult to unravel because it is unclear which of the many documented or interpreted extensional episodes was the most significant for net widening and crustal thinning. To address this problem, we integrate geologic and geochronologic data that bear on the timing and magnitude of Cenozoic extension along an ∼200 km east-west transect south of Winnemucca, Battle Mountain, and Elko, Nevada. Pre-Cenozoic rocks in this region record east-west Palaeozoic and Mesozoic compression that continued into the Cretaceous. Little to no tectonism and no deposition followed until intense magmatism began in the Eocene. Eocene and Oligocene ash-flow tuffs flowed as much as 200 km down palaeovalleys cut as deeply as 1.5 km into underlying Palaeozoic and Mesozoic rocks in a low-relief landscape. Eocene sedimentation was otherwise limited to shallow lacustrine basins in the Elko area; extensive, thick clastic deposits are absent. Minor surface extension related to magmatism locally accompanied intense Eocene magmatism, but external drainage and little or no surface deformation apparently persisted regionally until about 16-17 Ma. Major upper crustal extension began across the region ca. 16-17 Ma, as determined by cross-cutting relationships, low-temperature thermochronology, and widespread deposition of clastic basin fill. Middle Miocene extension was partitioned into high-strain (50-100%) domains separated by largely unextended crustal blocks, and ended by 10-12 Ma. Bimodal volcanic rocks that erupted during middle Miocene extension are present across most of the study area, but are volumetrically minor outside the northern Nevada rift. The modern physiographic basins and ranges formed during a distinctly different episode of extension that began after about 10 Ma and has continued to the present. Late Miocene and younger faulting is characterized by widely spaced, high-angle normal faults that cut both older extended and unextended domains. Major widening of the Basin and Range at this latitude thus took place during a relatively brief interval in the middle Miocene, and the lack of major shortening west of the Sierra Nevada at this time suggests that the change in the plate margin from microplate subduction to lengthy transtensional strike-slip played an important role in allowing extension to occur when it did, as rapidly as it did. The onset of extension ca. 16-17 Ma was coeval with both Columbia River flood-basalt volcanism and the hypothesized final delamination of the shallow Farallon slab that lay beneath the western USA in the early Tertiary. However, it is unclear if these events were necessary prerequisites for extension, simply coincidental, or themselves consequences of rapid extension and/or reorganization of the plate boundary. [ABSTRACT FROM AUTHOR]

The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young ( < 6 Ma); however, recent work has provided evidence that the range is very old (>80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ∼2-5 Myr of subduction-induced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11-10 Ma, records the birth of the 'future plate boundary' by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8-7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3-3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial- and mass-wasting deposits, and greater degree of encroachment by Walker Lane-related faults beginning at 10-11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events. [ABSTRACT FROM AUTHOR]

The Sierra Nevada and Great Valley are coupled, and behave as a semi-rigid microplate. The microplate formed as it was calved off the western edge of the Nevadaplano in the late Miocene, at which time westward regional tilting began. Tilting is controlled by west-side-up normal faulting primarily along the eastern Sierra escarpment system. Uplift and exhumation along the eastern Sierra are balanced by subsidence and sedimentation along the western Great Valley. The west tilt of the microplate is expressed by the west slope of a regional relict landscape surface that developed across much of the Sierra Nevada basement, and by the westward continuation of the surface as the basal Eocene nonconformity of the west-dipping Great Valley Tertiary section. The rigid behaviour of the microplate breaks down along its southern ∼100-150 km segment as expressed by seismicity, pervasive faulting and the development of a deep marine basin, the San Joaquin Basin (SJB), whose facies and palaeogeographic patterns diverge from regional patterns of the rest of the Great Valley. The disrupted state of the southern segment of the microplate resulted from its Late Cretaceous position above a regional lateral ramp in the underlying Franciscan-related subduction megathrust. The Kern Canyon fault system began its polyphase history as a complex oblique dextral shear zone above the megathrust lateral ramp. It was remobilized in the Neogene as an oblique transfer structure partitioning differential extension between the southern Sierra Nevada and the SJB. In Quaternary time, the Kern Canyon zone was again remobilized as a west-side-up normal fault system whose geomorphic and structural expressions are best developed south of ∼36.4° N. This normal fault system controls the west tilt of the relict landscape surface in the southern Sierra region, as well as the west dip pattern in the strata of the adjacent SJB. To the east of the Kern Canyon normal fault system, the relict landscape surface slopes continuously southwards from the high eastern Sierra into a low-lying, multiply extended terrane. Thus, from ∼36.4° N southwards, the west tilt along the western Sierra and the west dip of the adjacent Great Valley strata are controlled by the Kern Canyon system. Fresh normal scarps along the eastern Sierra escarpment system become more subdued and ultimately die out southwards from ∼36.4° N. Thus, currently, the controlling structure for the west tilt of the microplate steps westwards in the south from the eastern escarpment system over to the Kern Canyon system. [ABSTRACT FROM AUTHOR]

Geometrical properties of an earthquake population can be described by summation of seismic potency tensors that provide a strain-based description of earthquake focal mechanisms. We apply this method to ∼170 000 potency tensors for southern California earthquakes recorded between January 1984 and June 2003. We compare summed tensors for populations defined by faulting region and earthquake magnitude to investigate the relation between earthquake characteristics, tectonic domains and fault-related length scales. We investigate spatial scales ranging from ∼1–700 km and use the results to identify systematic differences between seismic behaviour for different faults and different regions. Our results show features that are indicative of both scale-invariant and scale-dependent processes. On the largest scale the overall potency tensor summation for southern California earthquakes over ∼20 yr corresponds closely to a double-couple (DC) mechanism with slip direction parallel to relative plate motion. The summed tensors and derived quantities for the different regions show clear persistent variations that are related to the dominant tectonic regime of each region. Significant differences between the non-DC components of the summed tensors, which we relate to fault heterogeneity, indicate systematic differences in deformation associated with earthquake populations from different fault zones or different magnitude ranges. We find an increase of heterogeneity for populations of smaller earthquakes and for regions where faulting deviates strongly from the overall sense of deformation, even when corrected for quality. The results imply an overall organization of earthquake characteristics into domains that are controlled to first order by geometrical properties of the largest faults and the plate motion. Smaller scale characteristics are related to local variations in the orientation, complexity and size of faults. [ABSTRACT FROM AUTHOR]

We present the GIS extension CalHypso to automatically extract multiple hypsometric curves from digital elevation models (DEM). This extension is programmed in Visual Basic 6 and uses the ArcObjects architecture of ArcGIS. It employs a new and easy methodology to extract the hypsometric curve by using the integer data-model properties of ArcGIS and summations algorithms. This provides an optimal integration within the program environment, allowing the representation and comparison of curves directly within the GIS main program. The CalHypso extension can also calculate the main statistics related to the hypsometric curve by applying polynomial fits. We have tested this tool in several basins of the eastern border of the Sierra-Nevada dome in the Betic Cordillera (SE Spain), showing important differences in the north- and south-slope hypsometric curves. These variations are probably related to the tectonically controlled drainage evolution of the southern border of Sierra Nevada. An eastward decrease in tectonic-driven erosion is also suggested by the hypsometric curves analyzed. [Copyright &y& Elsevier]

We determine the paleoelevation of the northern Sierra Nevada (California) in the Oligocene based on hydrogen stable isotope compositions of meteoric water preserved within volcanic glass from ignimbrites sampled across the range. A 48‰ decrease in the isotopic composition of hydrated glass from ignimbrites located near paleosea level to ignimbrites 100 km to the east reflects the effect of ancient high topography on precipitation. These data show that 31-28 Ma ago, the northern Sierra Nevada had a steep western gradient and elevations similar to the present. This study, placed in the context of other paleoaltimetry studies, suggests that the range was a high topographic feature throughout the Cenozoic and that the majority of uplift occurred in the Late Cretaceous to early Cenozoic, much earlier than some studies have proposed. [ABSTRACT FROM AUTHOR]

Abstract: Assembly of the supercontinent Columbia at about 1.85–1.90 Ga coincided with several events that affected the entire earth. The oldest worldwide network of orogenic belts formed at the same time. Although some granite–granodiorite (GG) suites had formed earlier, the GG suites became common in the 1.8–1.9 Ga orogenic belts. These suites succeeded the older tonalite–trondhjemite–granodiorite (TTG) suites, which were not produced after 1.8 Ga. Changes on the earth''s surface at 1.8–1.9 Ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes. All of these surface changes occurred as Columbia accreted, and the assembly of Columbia may have contributed to the drastic changes in the earth''s surface environment as well as to the evolution of primitive life forms. [Copyright &y& Elsevier]

Structural and stratigraphic analyses along the western margin of the Salton Trough show that the San Andreas fault system was reorganized in early Pleistocene time from a system dominated by two fault zones (the San Andreas fault and the West Salton detachment fault) to a network of dextral faults that include the San Andreas and at least four dextral faults to the southwest. The San Felipe fault zone, one of these dextral faults, has ~5.8 ± 2.8 km of right separation and consists of three principal faults in the Peninsular Ranges. These are the San Felipe fault in the WNW, Sunset fault in the middle, and Fish Creek Mountains fault in the ESE. They form a left-stepping array and bound domains in which the Sunset Conglomerate, the older West Salton detachment fault, and Cretaceous mylonitic rocks below the detachment are folded about WNW-trending folds. A complex flower structure within the left-stepovers probably produced this fault-parallel folding. Because all the rocks within stepovers of the San Felipe fault zone, from Cretaceous to Pleistocene, are deformed about WNW-trending folds and record broadly similar shortening strains, we infer a Quaternary age of deformation. Parts of the San Felipe fault zone cut latest Pleistocene to Holocene surficial deposits, and the fault zone is likely active. Evidence for early Pleistocene initiation of the San Felipe fault zone is preserved in conglomerate NE of the Sunset fault. Poorly sorted angular boulder conglomerate and pebbly sandstone of the Sunset Conglomerate are ~600 m thick and lie in angular unconformity on the Pliocene Palm Spring Group. The conglomerate coarsens upward and toward the fault, and is dominated by plutonic clasts derived from SW of it. Conglomerate beds contain up to 10% sandstone clasts recycled from older basin fill and accumulated in proximal to medial alluvial fans that were shed to the NE from uplifted rocks along the then-active Sunset fault. Based on lithologic, stratigraphic, structural, and compositional similarities, we correlate the Sunset Conglomerate to the Pleistocene Ocotillo Formation. Clasts of recycled sandstone record erosion of detachment-related basin fill that predates the San Felipe fault and once covered the Vallecito and Fish Creek mountains. These crystalline-cored mountain ranges first emerged from beneath basin fill during early slip above the nascent San Felipe fault ca. 1.1-1.3 Ma. Later, the San Felipe fault zone cut upward, folded, cut across, and deactivated the West Salton detachment fault within a ~9-km-wide contractional bend and pair of left-steps. Areas that accumulated sediment within this step-over zone between ca. 1.1 and ca. 0.6 Ma are currently being inverted and folded. Initiation of the San Felipe fault in early Pleistocene time was a significant event in the reorganization of the southern San Andreas fault system. The Quaternary dextral faults broadened the plate boundary zone southwestward from roughly 25 km (during coeval slip on the San Andreas fault and West Salton detachment fault) to 50-70 km, and mark a change in the dominant structural style from transtension to distributed dextral faulting south of the Big Bend. [ABSTRACT FROM AUTHOR]

The northern Sierra Nevada and adjacent Basin and Range, western United States, are marked by a widespread regional unconformity at the base of the Eocene-Miocene volcanic and sedimentary section that overlies the Mesozoic batholith and its wall rocks. To help address controversial questions about the origin, uplift, and erosion of the batholith, we compiled a subcrop geologic map of the unconformity prior to Tertiary extension. This simple but underutilized technique reveals the distribution of rock units that could have contributed detritus to the Eocene-Miocene river channels crossing the Sierra Nevada and demonstrates that the Mesozoic Sierra Nevada batholith was continuous to the northeast across the northwestern Basin and Range. More speculatively, the subcrop map implies that Late Cretaceous-Eocene erosional stripping may have been greatest above the axis of the batholith and decreased to the east; thus, drainage in this area may have been eastward and then switched westward in Eocene-Miocene time. [ABSTRACT FROM AUTHOR]

The precise tectonic role of the left-lateral Garlock fault in southern California has been controversial. Three proposed tectonic models yield significantly different predictions for the slip rate, history, orientation, and total bedrock offset as a function of distance along strike. In an effort to test these models, we present the first slip-rate estimate for the western Garlock fault that is constrained by radiocarbon dating. A channel (referred to here as Clark Wash) incised into a Latest Pleistocene alluvial fan has been left-laterally offset at least 66 ± 6 m and no more than 100 m across the western Garlock fault, indicating a left-lateral slip rate of 7.6 mm/ yr (95% confidence interval of 5.3-10.7 mm/ yr) using dendrochronologically calibrated radiocarbon dates. The timing of aggradational events on the Clark Wash fan corresponds closely to what has been documented elsewhere in the Mojave Desert, suggesting that much of this activity has been climatically controlled. The range-front fault, located a few hundred meters northwest of the Garlock fault, has probably acted primarily as a normal fault, with a Holocene rate of dip-slip of 0.4-0.7 mm/yr. The record of prehistoric earthquakes on the Garlock fault at this site, though quite possibly incomplete, suggests a longer interseismic interval (1200-2700 yr) for the western Garlock fault than for the central Garlock fault. The relatively high slip rate determined here indicates that the western and central segments of the Garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data. The high rate of motion on the western Garlock fault is most consistent with a model in which the western Garlock fault acts as a conjugate shear to the San Andreas fault. Other mechanisms, involving extension north of the Garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault, but they cannot explain a high rate of slip on the western Garlock fault. [ABSTRACT FROM AUTHOR]

We propose a new approach to quantifying fault system geometry, using an objective fit of the fault geometry to a test function, specifically here a fault branch. Fitting a Y-shaped object using a cost function to dextral faults in California, we find a number of significant results arising from use of a a systematic, objective, quantitative approach. (1) The largest angle of the branch structure is generally very close to 180°, implying that the branch is a splay fault off the primary through-going fault. (2) The distribution of the smallest angle, the splay angle, has a peak near ±17°, symmetric about the primary fault. (3) These features appear independent of scale. These results are not yet explained by any theory, and they pose new questions and constraints for the physics of fault system formation and behavior. [ABSTRACT FROM AUTHOR]

We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting. [ABSTRACT FROM AUTHOR]

Continental sedimentation reflects a complex interplay of tectonics and climate. A 2000-km transect from coastal California to the western Great Plains documents a major increase in sedimentation (ca. 16–6 Ma) coeval with deposition of the hemipelagic Monterey Formation along the California coast. Basin and Range-style regional extension following elongation of the Pacific–North American transform boundary at ca. 17.5 Ma provided fault-bounded basins for accommodation space, but sedimentation also occurred on unextended erosional surfaces of the Great Plains and Colorado Plateau. Two global climate transitions bracket this sedimentary interval. The middle Miocene transition (ca. 17–12 Ma) records the global change from equatorial to meridional circulation caused by: (1) closing of the eastern Tethys Seaway (ca. 18 Ma); (2) opening of the Arctic–North Atlantic connection (ca. 17.5 Ma); (3) growth of the East Antarctic Ice Sheet (ca. 14 Ma); and (4) closing of the Indonesian Seaway (ca. 12 Ma). Upwelling of cold waters along the California coast, abetted by domination of La Niña phases of El Niño–Southern Oscillation (ENSO), progressively aridified the Southwest as reflected in sedimentary and biologic records. The second climate transition occurred as opening of the Gulf of California (ca. 6 Ma) intensified the North American monsoon, resulting in integration of drainages, incision of uplifts, and exhumation of basin fills. The Miocene ended with the driest climate of the Tertiary (both regional and global) accompanied by conversion of savanna to steppe or scrub desert, spread of C4 grasses, and the greatest mammal extinction of the Neogene. [ABSTRACT FROM AUTHOR]

We apply a kinematic finite-fault inversion scheme to Pnl displacement waveforms recorded at 14 regional stations (Δ < 2°) to recover the distribution of coseismic slip for the 2004 Parkfield earthquake using both synthetic Green's functions (SGFs) calculated for one-dimensional (1D) crustal-velocity models and empirical Green's functions (EGFs) based on the recordings of a single Mw 5.0 aftershock. Slip is modeled on a rectangular fault subdivided into 2 x 2 km subfaults assuming a constant rupture velocity and a 0.5 sec rise time. A passband filter of 0.1-0.5 Hz is applied to both data and subfault responses prior to waveform inversion. The SGF inversions are performed such that the final seismic moment is consistent with the known magnitude (Mw 6.0) of the earthquake. For these runs, it is difficult to reproduce the entire Pnl waveform due to inaccuracies in the assumed crustal structure. Also, the misfit between observed and predicted vertical waveforms is similar in character for different rupture velocities, indicating that neither the rupture velocity nor the exact position of slip sources along the fault can be uniquely identified. The pattern of coseismic slip, however, compares well with independent source models derived using other data types, indicating that the SGF inversion procedure provides a general first-order estimate of the 2004 Parkfield rupture using the vertical Pnl records. The best-constrained slip model is obtained using the single-aftershock EGF approach. In this case, the waveforms are very well reproduced for both vertical and horizontal components, suggesting that the method provides a powerful tool for estimating the distribution of coseismic slip using the regional Pnl waveforms. The inferred slip model shows a localized patch of high slip (55 cm peak) near the hypocenter and a larger slip area (∼50 cm peak) extending between 6 and 20 km to the northwest. [ABSTRACT FROM AUTHOR]

Non-volcanic tremor (NVT) is an extended-duration and non-impulsive seismic signal observed away from volcanic regions. Although NVT has been found along the circum-Pacific subduction zones and the transform plate boundary in California, it is not clear whether NVT occurs in other tectonic environments. NVT is often associated with slow-slip events, and can be triggered instantaneously during the surface waves of teleseismic events. However, the underlying mechanisms of tremor generation remain mysterious. Here we show clear evidence of NVT beneath the Central Range (CR) in Taiwan triggered by the 2001 Mw 7.8 Kunlun earthquake in northern Tibet. Tremor occurs when the Love wave displacement is to the southwest (the lateral motion direction for the CR), suggesting a simple frictional response to the driving force. We hypothesize that tremor occurred on the weak basal detachment fault beneath the CR. Our observations indicate that tremor may exist at a wide range of active tectonic environments. [ABSTRACT FROM AUTHOR]

This paper documents a newly discovered pattern of biological disjunction between NW and SE New Caledonia. The disjunction occurs in 87 (mapped) taxa, including plants, moths and lizards, and correlates spatially with the West Caledonian fault. This fault is controversial; some geologists interpret it as a major structure, others deny that it exists. It may have undergone 150–200 km of lateral movement and it is suggested that this has caused the biological disjunction by pulling populations apart. The disjunction matches similar dextral disjunctions of taxa along transform faults in New Zealand, New Guinea, California and Indonesia. Major biogeographic patterns – whether centres of diversity, boundaries of allopatric taxa or disjunctions – all include taxa with many different degrees of differentiation. Studies using a clock model of evolution will therefore interpret a biogeographic pattern as the result of many disparate events. However, this line of reasoning reaches the untenable conclusion that biogeographic patterns, including normal allopatry, are always caused by chance dispersal, never by vicariance. A more productive approach, avoiding the pitfalls of a fossil-based molecular clock, involves a close examination of molecular clades, comparative biogeography and tectonics. The New Caledonia example documented here shows that this can lead to novel, testable predictions. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 470–488. [ABSTRACT FROM AUTHOR]

Abstract: In Sawmill Canyon, located near the eastern margin of the Tuolumne batholith, central Sierra Nevada, California, a series of petrologically and structurally complex, magmatic sheeted zones intrude older granodioritic units (Kuna Crest and equigranular Half Dome) and in one case truncate these units along a sharp contact. These sheeted zones (a) consist of numerous batches of (now frozen) magma, (b) display clear outward growth directions, (c) were actively deforming during and after emplacement resulting in magmatic folds, faults and multiple magmatic mineral fabrics, and (d) are the location of numerous, but localized magma flow structures (schlieren-bounded tubes, troughs, megacryst-rich pipes) and instabilities (load casts, flame structures, slumps, diapirs, ridge and pillar structures). Geochemical data indicate that the sheeted zones largely consist of magmas derived from the Half Dome granodiorite with some late Cathedral Peak granodiorite pulses, and with fractionation and flow sorting forming widespread layering in the above structures. We interpret these sheeted zones to record the pulsing of magma during propagation and expansion of opening-mode (Mode I), submagmatic fractures at the margins of large blocks of older, fairly solidified magmatic pulses that were subsequently removed from the present crustal level. Elsewhere in the Tuolumne batholith we see similar features suggesting that a “recycling” process, i.e., the breaking off of older parts of the magma chamber and incorporation into younger intrusive units, occurred in this batholith. This recycling removed a significant portion of older units and resulted in the formation of sheeted zones and local instabilities in this batholith. Finally this recycling is one process responsible for transfer of zircon crystals between units and for obscuring whole-rock geochemical signatures. [Copyright &y& Elsevier]

Deterministic earthquake scenario simulations are playing an increasingly important role in seismic hazard and risk estimation. Our aim is to calculate a substantial number of different finite-source scenarios, embedded in a 3-D structure for a particular fault or fault system, by pre-calculating numerical Green's functions (NGFs). A large seismic fault is divided into subfaults of appropriate size for which synthetic Green's functions at the surface are calculated and stored. Consequently, ground motions from arbitrary kinematic sources can be simulated for the whole fault or parts of it, by superposition. To illustrate this approach and its functionalities we simulate M 7 (up to 0.5 Hz) scenario earthquakes for a simplified model of the Newport-Inglewood (NI) fault in the Los Angeles (LA) Basin. We quantify the variations of surface ground motion (e.g. peak ground velocity PGV and synthetic seismograms) due to source parameters (e.g. hypocentre location and corresponding slip history). The results show a complex behaviour, with dependence of absolute PGV and its variation on asperity location, source directionality and local structure and demonstrate the necessity to combine 3-D structural and finite-source effects to quantify ground motion characteristics and their variations. [ABSTRACT FROM AUTHOR]

Seismic velocity changes and nonvolcanic tremor activity in the Parkfield area in California reveal that large earthquakes induce long-term perturbations of crustal properties in the San Andreas fault zone. The 2003 San Simeon and 2004 Parkfield earthquakes both reduced seismic velocities that were measured from correlations of the ambient seismic noise and induced an increased nonvolcanic tremor activity along the San Andreas fault. After the Parkfield earthquake, velocity reduction and nonvolcanic tremor activity remained elevated for more than 3 years and decayed over time, similarly to afterslip derived from GPS (Global Positioning System) measurements. These observations suggest that the seismic velocity changes are related to co-seismic damage in the shallow layers and to deep co-seismic stress change and postseismic stress relaxation within the San Andreas fault zone. [ABSTRACT FROM AUTHOR]

We used submerged paleoshorelines as strain markers to investigate Holocene and late Pleistocene vertical tectonic movement at the intersection of the offshore Santa Cruz--Catalina Ridge with the southern boundary of the Western Transverse Ranges, within the California Continental Borderland. Past submerged shoreline positions were identified using high-resolution multibeam bathymetry, side-scan sonar, submersible observations, and the presence of intertidal and subtidal invertebrate fossils. Numerous accelerator mass spectrometry (AMS) 14C ages of shells from these paleoshorelines were found to be between ~27,000 yr radiocarbon (RC) and 11,500 yr before present, indicative of shoreline colonization during and following the Last Glacial Maximum (LGM), establishing these paleoshorelines as a usable datum for measuring vertical change since this time. Removal of the nontectonic component of vertical change using an ice-volume-equivalent eustatic sea-level compilation indicates between 20 and 45 m of uplift of the eastern part of the Northern Channel Islands block since the LGM lowstand, resulting in an uplift rate of 1.50 ± 0.59 mm/yr over the last 23 k.y. This rate closely matches uplift predicted by published slip rates for the Channel Islands thrust, which underlies the Northern Channel Islands platform. Results from post-LGM shoreline features on Pilgrim Banks are somewhat more ambiguous. Submarine paleoshoreline uplift, together with the extensive upper-crustal fold-thrust style of deformation, illustrates the transpressional interaction of the Borderland and the Western Transverse Ranges blocks where the Santa Cruz-Catalina Ridge and northern Channel Islands intersect. [ABSTRACT FROM AUTHOR]

To assess the control of fault geometry and mechanical interactions on fault-slip distributions in a complex natural system, we present results from three-dimensional mechanical models incorporating both nonplanar and rectangular planar representations of active faults within the Ventura basin region of southern California. We find that the incorporation of geologically constrained nonplanar fault surfaces into numerical models of active deformation results in a better match to available geologic slip-rate data than models utilizing rectangular planar fault surfaces. The model results demonstrate that nonplanar fault geometry and mechanical interactions exert a strong control on resultant slip distributions. Additionally, we find that slip rates at most locations along the surface trace of Ventura faults are not likely to represent average values for the entire fault surface. We propose that results from threedimensional mechanical models using realistic (i.e., nonplanar) fault geometry can be used to both predict slip rates at specific locations and determine whether existing sitespecific slip-rate estimates are representative of average fault-slip rates. Although geometric irregularities along-fault surfaces should resist slip, planar faults can have lesser slip than nonplanar faults due to the differing mechanical interactions among nearby faults in the two representations. This suggests that models using simplified or planar fault geometry are likely to inaccurately simulate regional deformation. We assert that detailed knowledge of three-dimensional fault shape as well as the geometry and configuration of deep fault intersections is essential for accurate seismic hazard characterization of regions of complex faulting such as the Ventura basin of southern California. [ABSTRACT FROM AUTHOR]

The Mesozoic-Cenozoic Cordilleran orogen of California includes multiple accretionary belts incorporated sequentially into the continental margin since Middle Triassic time. Accreted tectonic elements include subduction complexes assembled along the Cordilleran margin, intraoceanic island arcs attached to the continental margin by Jurassic arc-continent collision, and subduction complexes associated with the flanks of the exotic island arcs. Systematic analysis of areal relations and geochronological data displayed on subregional geologic maps and summary chronostratigraphic diagrams allows the punctuated but quasi-continuous pattern of tectonic accretion to be discerned. Stitching plutons and sedimentary overlap successions constrain the times that successive accretionary belts were juxtaposed and amalgamated into the edge of the continental block. In the Klamath Mountains and Sierra Nevada, a continental-margin magmatic arc of Triassic-Jurassic age includes volcanic and plutonic components built upon and intruded into deformed Paleozoic assemblages that were accreted to the Laurentian margin before Middle Triassic time. The native arc assemblage is separated from intraoceanic Triassic-Jurassic arc assemblages exposed farther west by a compound suture belt of mélange and broken formation derived from the remnant ocean basin that separated the east-facing intraoceanic arc system from the Cordilleran margin. Polarity reversal after Middle to Late Jurassic arc-continent collision was followed by accretion of a disrupted ophiolitic belt forming mafic basement in the subsurface of the Great Valley forearc basin. Subsequent forearc sedimentation accompanied the assembly of multiple belts of mélange and broken formation that form the Mesozoic-Cenozoic Franciscan subduction complex of the California Coast Ranges. Franciscan subduction was largely coeval with intrusion of the dominantly Cretaceous Sierra Nevada batholith into the roots of the Cordilleran magmatic arc, but Franciscan accretion was just a late phase of continuing tectonic expansion that spanned more than 200 m.y. along the California continental margin. [ABSTRACT FROM AUTHOR]

We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a significant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, ~280 m above the canyon floor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclic-step bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overflowing the canyon at this bend, assuming steady-uniform flow, full conversion of the failed mass into a turbidity current, and a range of friction coefficients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which suggest that 2-3 m of material fails on decadal time scales. Our calculations show that the overflowing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents. [ABSTRACT FROM AUTHOR]

High-resolution seismic-reflection and seismic-refraction imaging, combined with existing borehole, earthquake, and paleoseismic trenching data, suggest that the Santa Monica fault zone in Los Angeles consists of multiple strands from several kilometers depth to the near surface. We interpret our seismic data as showing two shallow-depth low-angle fault strands and multiple near-vertical (~85°) faults in the upper 100 m. One of the low-angle faults dips northward at about 28° and approaches the surface at the base of a topographic scarp on the grounds of the Wadsworth VA Hospital (WVAH). The other principal low-angle fault dips northward at about 20° and projects toward the surface about 200 m south of the topographic scarp, near the northernmost areas of the Los Angeles Basin that experienced strong shaking during the 1994 Northridge earthquake. The 20° north-dipping low-angle fault is also apparent on a previously published seismic-reflection image by Pratt et al. (1998) and appears to extend northward to at least Wilshire Boulevard, where the fault may be about 450 m below the surface. Slip rates determined at the WVAH site could be significantly underestimated if it is assumed that slip occurs only on a single strand of the Santa Monica fault or if it is assumed that the near-surface faults dip at angles greater than 20-28°. At the WVAH, tomographic velocity modeling shows a significant decrease in velocity across near-surface strands of the Santa Monica fault. P-wave velocities range from about 500 m/sec at the surface to about 4500 m/sec within the upper 50 m on the north side of the fault zone at WVAH, but maximum measured velocities on the south side of the low-angle fault zone at WVAH are about 3500 m/sec. These refraction velocities compare favorably with velocities measured in nearby boreholes by Gibbs et al. (2000). This study illustrates the utility of combined seismic-reflection and seismic-refraction methods, which allow more accurate reflection imaging and compositional estimations across areas with highly variable velocities, a property that is characteristic of most fault zones. [ABSTRACT FROM AUTHOR]

Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic flow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group (Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation) is a useful stratigraphic marker for understanding the post-9-Ma major faulting of the easternmost Sierra Nevada, uplift of the mountain range, and transtensional tectonics within the central Walker Lane. The Table Mountain Latite has a distinctively shallow reversed-polarity direction (I = -26.1°, D = 163.1°, and α95 = 2.7°) at sampling sites in the foothills and western slope of the Sierra Nevada. In ascending order, the Eureka Valley Tuff comprises the Tollhouse Flat Member (I = -62.8°, D = 159.9°, α95 = 2.6°), By-Day Member (I = 52.4°, D = 8.6°, α95 = 7.2°), and Upper Member (I = 27.9°, D = 358.0°, α95 = 10.4°). The Dardanelles Formation has normal polarity. From the magnetization directions of the Eureka Valley Tuff in the central Walker Lane north of Mono Lake and in the Anchorite Hills, we infer clockwise, vertical-axis rotations of ∼10° to 26° to be a consequence of dextral shear. The AMS results from 19 sites generally show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, although several indicators are transverse to radial flow. AMS-derived flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane. [ABSTRACT FROM AUTHOR]

We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strikeslip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehensive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment. [ABSTRACT FROM AUTHOR]

K2O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K2O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K2O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K2O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K2O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K2O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K2O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy. [ABSTRACT FROM AUTHOR]

The San Andreas Fault (SAF) is the Pacific-North American plate boundary, yet in southern California a significant portion of the relative plate motion is accommodated by the San Jacinto Fault (SJF). Here we investigate the initiation of the SJF and its interaction with the SAF in a three-dimensional visco-elasto-plastic finite-element model. The model results show that the restraining bend of the southern SAF causes strain localization along the SJF, thus may have contributed to its initiation. Slip on the SJF tends to reduce slip rate on the SAF and enhance deformation in the Eastern California Shear Zone. The initiation of the SJF and its interaction with the SAF reflect the evolving plate boundary zone as it continuously seeks the most efficient way to accommodate the relative plate motion. [ABSTRACT FROM AUTHOR]

This paper describes revisions to the Horizontal Time Dependent Positioning (HTDP) software carried out to address the motion associated with two recent California earthquakes. NOAA's National Geodetic Survey (NGS) developed HTDP to enable its users to compensate survey measurements for the differential movement associated with tectonic deformation, so that surveys conducted at different epochs may be compared. We focus on the process of selecting and validating models for two recent earthquakes in California, prior to their inclusion in version 2.9 of this software (HTDP v2.9)--the Parkfield earthquake with a magnitude of 6.0, which occurred on September 28, 2004, and the San Simeon, California earthquake with a magnitude of 6.5, occurring on December 22, 2003. The process of modeling and correcting for deformation can be well illustrated by utilizing data collected in the vicinity of these earthquakes, because they represent contrasting styles of deformation--Parkfield exhibited strike-slip motion and San Simeon was predominantly a blind-thrust earthquake. Also, while both earthquakes were relatively small, they produced significant displacements (though restricted to a relatively small area in central California). The Parkfield earthquake, in particular, produced an unusually high proportion of post-seismic deformation. This deformation can accumulate during a period of a few months after the earthquake, whereas co-seismic deformation occurs suddenly during an earthquake. Quantifying and characterizing deformation at these disparate time scales represents a major challenge to developing the accurate models of deformation that are required to support modern, high-accuracy surveying in tectonically active areas such as California. This study also confirms the importance of including separate dislocation models of the co-seismic and post-seismic deformation, because, as was the case for Parkfield, post-seismic deformation can be comparable in magnitude to co-seismic deformation. [ABSTRACT FROM AUTHOR]

Paleoseismological data suggest the occurrence of four bursts of seismic moment release in the Los Angeles region during the past 12,000 yr. The historic period appears to be part of an ongoing lull that has persisted for about the past 1000 yr. These periods of rapid seismic displacement in the Los Angeles region have occurred during the lulls between similar bursts of activity observed on the eastern California shear zone in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the greater San Andreas system suppress activity on faults in the eastern California shear zone, and vice versa, can explain the apparent switching of activity between the two fault networks. Combined with the observation that short-term geodetic and longer-term geologic rates co-vary on major southern California fault systems, this suggests that either (1) a temporal cluster of seismic displacements on upper-crustal faults increases ductile deformation on their downward extensions, or (2) rapid ductile slip in the lower crust beneath faults loads the upper crust, driving a seismic cluster. We suggest that alternating periods of rapid seismic displacement may be the expected mode of seismicity when two fault systems accommodate the same plate-boundary motion, and slip on one system suppresses slip on the other. [ABSTRACT FROM AUTHOR]

Abstract: Synthetic waveform modeling of the anomalous receiver functions for two stations in the eastern San Gabriel Mountains, California, suggests that a flat-topped notch structure exists on the Moho. North of the San Andreas Fault (SAF), the Moho depth is 36–38 km and is 33–35 km south of the San Gabriel Fault (SGF), but in-between under the Mt. Baldy block, it is only ∼29 km. The inferred notch structure is also supported by the Pms arrival times along the SCSN and the LARSE I profiles. The shallow Moho block correlates well with the surficial exposure of the lower plate Pelona Schist or where it resides at very shallow depth. The large Moho offsets across the two major faults can be either related to differential uplifting reflected by the difference in the denudation rate and the exposure of the lower plate Pelona Schist, or it can be the result of the juxtaposition of the distinctive terranes with initially different Moho depth due to the strike-slip movements along the two major faults. [Copyright &y& Elsevier]

The Peñon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Peñon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ~900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Peñon Blanco Formation, which is made up of ~700 m of crystal-lithic basaltic tuff, 1--3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Peñon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Peñon Blanco arc against the inboard Calaveras Complex. New geochemical data from the Peñon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Peñon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30--60 km depth by low to moderate amounts of partial melting (~3%--5% for Jasper Point and 5%--7.5% for Peñon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Peñon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America. [ABSTRACT FROM AUTHOR]

Evaluation of microstructures in unlithified near-surface sediments provides promising results for differentiating between earthquake rupture-related (coseismic) and creep-related (aseismic) structures formed by the San Andreas fault. Paleoseismic trenches excavated at two sites, Flook Ranch on the creeping section, and Alder Creek on the 1906 rupture trace, show contrasting fault-zone structures and microstructures in near-surface, late Holocene sand. At Alder Creek, the 1–2-m-wide fault zone consists of both faults and 2-10-mm-thick deformation bands. Deformation bands have preferred grain orientations ∼30° counterclockwise from the fault (viewed in the slip-parallel direction), broken and disaggregated grains, smaller than average grain size, and lower porosity than control samples. In contrast, at Flook Ranch, two 4–6-m-wide fault zones consist of multiple faults but lack deformation bands. Silty sand in the fault zone at Flook Ranch has preferred grain orientations ∼10° clockwise from the fault, lacks broken grains, and has comparable grain size but lower porosity than control samples. These microstructures record different deformation mechanisms in near-surface sediment: cataclasis at Alder Creek, and distributed deformation at Flook Ranch. Deformation bands on the 1906 rupture trace of the San Andreas fault at Alder Creek demonstrate that these structures, with their grain rotation, grain breakage, and localized porosity variations, can form coseismically in unlithified sediment. The grain bridge model accounts for fundamental microstructural characteristics of Alder Creek deformation bands, and it provides a connection between these microstructures and laboratory studies of stickslip instability. Deformation bands are easily recognizable in field and trench exposures and may be a useful indicator of coseismic slip. [ABSTRACT FROM AUTHOR]

The mechanical feasibility of normal-sense slip on low-angle faults remains a conundrum in extensional tectonics. The rarity of demonstrably active low-angle normal faults may imply that very specific criteria must be satisfied for significant fault displacement. We present new geologic observations, geomorphic mapping, and structural analysis for a low-angle fault zone along the eastern margin of Searles Valley, California. Our observations indicate that Pleistocene displacement along the range-front fault scarps is the near-surface expression of slip on a bedrock-rooted low-angle normal fault. Along the central portion of the range front in Searles Valley, high-angle faults offset late Pleistocene alluvial and lacustrine surfaces. These faults merge downward into a west-dipping, low-angle fault, but do not displace the low-angle surface. These geometric relations are satisfied only when displacement on the high-angle faults is accommodated by slip on the basal low-angle fault. We use displaced alluvial fan surfaces to determine slip rates across the fault system over late Pleistocene to Holocene time. Combining radiocarbon ages of lacustrine tufa deposits with high-precision topographic surveys of fault scarps yields average slip rates of 0.21–0.35 m/k.y. Additional mapping of faults within the Slate Range at the northern end of Searles Valley suggests that slip is transferred northward to the Manly Pass fault, a bedrock normal fault that trends northeast into Panamint Valley. Thus, although displacement along the range-front fault system dies out northward, we infer that active deformation occurs within the range and likely links extension in Searles Valley with deformation in Panamint Valley. [ABSTRACT FROM AUTHOR]

Abstract: Deformation associated with the Rinconada fault, one of the major strands of the San Andreas fault system in central California, is accommodated by both discrete fault offset (∼18km) and distributed off-fault deformation. En echelon folds adjacent to the Rinconada fault were studied in detail at two locations, near Williams Hill and Lake San Antonio, to characterize the magnitude and style of distributed deformation. The obliquity between fold hinges and the local strike of the fault was 27° and 14° at these two sites, respectively. Systematic outcrop-scale fault displacement measurements along roadcuts indicate that the maximum horizontal elongation occurs parallel to local fold hinges and ranges from 4 to 9%. We used the orientation and stretch of fold hinges to construct a transpressional kinematic model for distributed deformation. This modeling suggests a 20–50° angle of oblique convergence, 5km of fault-parallel wrench deformation, and 2–4km of fault-perpendicular shortening. Between 3° and 16° of clockwise rotation is also predicted by our model. This rotation is independently confirmed by a 14±7° vertical axis rotation from regional paleomagnetic analyses. Integrating the regional discrete and distributed components of deformation suggests that the Rinconada fault system is 80% strike-slip partitioned. [Copyright &y& Elsevier]

A high-resolution Compressed High-Intensity Radar Pulse (CHTRP) survey reveals shore-parallel variations in the Holocene sediment thickness offshore La Jolla, California. Sediment thicknesses decrease from >20 m in the south near Scripps Canyon to zero in the north approaching Torrey Pines. In addition to the south-to-north variation in sediment thickness, the transgressive surface observed in seismic lines shoals from Scripps Canyon to the north. Despite these dramatic shore-parallel subsurface changes, the nearshore bathymetry exhibits little to no change along strike. A left jog (i.e., a constraining bend) along the Rose Canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface, the decrease in relief on the transgressive surface away from the left jog, and the Holocene sediment thickness variation. This tectonic deformation is shore parallel, and thus the accommodation can be separated into its tectonic and eustatic components. [ABSTRACT FROM AUTHOR]

Late Miocene to early Pliocene deposits at Split Mountain Gorge, California, preserve a record of basinal response to changes in regional tectonics, paleogeography, and evolution of the Colorado River. The base of the Elephant Trees Formation, magnetostratigraphically dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of extension in the southwestern Salton Trough. The oldest marine sediments are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in the Salton Trough and northern Gulf of California region supports a model for localization of Pacific-North America plate motion in the Gulf ca. 6 Ma. The first appearance of Colorado River sand at the Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypothesis for initiation of the lower Colorado River. [ABSTRACT FROM AUTHOR]

We designed new laboratory exercises for undergraduate igneous petrology that teach the fundamentals of petrography and petrology within the framework of tectonic environments. Our exercises are designed to promote the use of the scientific method by requiring students to collect and use data to test hypotheses and petrogenetic concepts presented in lecture. The integration of fundamental concepts of petrology with geologic and tectonic settings, in our case drawing on the diversity found in California, allows for the introduction of more advanced material in both the laboratory and lecture portions of the course. Student written feedback and performance on exams, laboratory exercises, and final projects reflect an increased understanding of the subject matter and confidence in problem solving. This type of class structure is transportable to programs in any location, can be used for classes of reasonable size, and is an effective method to teach science classes at either an introductory or advanced level. [ABSTRACT FROM AUTHOR]

Recent, compelling arguments regarding the overwhelming influence of drainage basin lithology on strath formation (Montgomery, 2004) require reevaluation of the role other factors play in this process. Pancho Rico Creek (‘PRC’) is a bedrock- eroding stream in the tectonically active central Coast Ranges of California. Late-Quaternary time stream piracy of the upper San Lorenzo Creek catchment by PRC caused ‘instantaneous’ (at graded timescales) catchment-area expansion (from 99.5–156.6 km²) and a significant change in the lithologic composition of PRC channel bedload. This naturally occurring ‘experiment’ in the southern Gabilan Range provides insight regarding the influences of catchment area and channel-bedload composition on formation of fluvial straths in the context of newly recognized controls on bedrock-channel erosion (Skiar and Dietrich, 2001; Stock and others, 2005). Distinct compositional differences in pre- and post-capture PRC alluvium facilitates deducing from alluvial stratigraphy pre- and post-capture fluvial processes. The pre-capture drainage basin of PRC, known as Pancho Rico Valley, is entirely underlain by predominantly fine-grained, friable, clastic marine sediments of the Pancho Rico Formation. Pre-capture PRC alluvium consists only of Pancho Rico Formation clasts, but post-capture PRC alluvium also includes relatively competent clasts of Franciscan Complex rocks. Pre- and post-capture straths formed in Pancho Rico Valley only along reaches of PRC where drainage area was/is greater than ca. 60 km². This indicates that a minimum catchment area is necessary for PRC to carve straths. Spatial and temporal patterns of strath formation in Pancho Rico Valley support the hypothesis that distribution of straths within a drainage basin underlain by mechanically weak rock is largely dependent on catchment area. Two hypotheses are proposed to explain differences in the character of pre- and post-capture straths as well as the processes leading to their formation and/or preservation. Pre-capture straths are paired, major straths, which indicate that lateral incision prevailed over vertical incision during a given interval of graded time (for example, Bull, 1990). Post-capture straths are unpaired, minor straths, which indicate simultaneous vertical and lateral incision during a given interval of graded time (for example, Bull, 1990). One hypothesis proposes that PRC was able to simultaneously (over graded timescales) lower and widen its channel in post-capture time because the presence of relatively competent bedload enhanced abrasion and accelerated channel denudation. Relatively incompetent bediload precluded simultaneous channel lowering and widening in pre-capture time. An alternative, favored hypothesis is that post-capture minor straths are present along PRC because they are buried under Franciscan-Complex derived gravelly alluvium, which is more resistant to weathering than Pancho-Rico-Formation derived gravelly alluvium. Thin deposits typical of minor erosional straths, and composed of readily weathered Pancho Rico Formation bedload, were insufficient to protect straths from weathering and erosion in pre-capture lime. Pre-capture siraths survived only when buried by substantial amounts of alluvium, as may have occurred during an episode of strath formation and alluviation caused by climate change. [ABSTRACT FROM AUTHOR]

A regional synthesis of new and existing geologic and thermochronologic data document late Cretaceous—early Cenozoic regional erosion, Oligocene—Miocene volcanism, and subsequent late Miocene extension of the Basin and Range Province in northwestern Nevada and northeastern California. Across an ∼220-km-wide region between the Santa Rosa and Warner Ranges, conformable sequences of 35 to 15 Ma volcanic rocks are cut by only a single generation of high-angle normal faults that accommodated ∼23 km of total east-west extension (∼12%). Fission-track, (U-Th)/He, geologic, and structural data from the Pine Forest Range show that faulting there began at 11 to 12 Ma, progressed at a relatively constant rate until at least 3 Ma, and has continued until near the present time. Extension in the Santa Rosa Range to the east took place during the same interval, although the post-6 Ma part of this history is less well constrained. Less complete constraints from adjacent ranges permit a similar timing for faulting, and we infer that extensional faulting in northwestern Nevada began everywhere at 12 Ma and has continued up to the present. Faulting in the Warner Range in northeastern California can only be constrained to have begun between 14 and 3 Ma, but may represent westward migration of Basin and Range extension during the Pliocene. Compared to the many parts of the Basin and Range in central and southern Nevada, extension in northwestern Nevada began more recently, is of lesser total magnitude, and was accommodated entirely by high-angle normal faults. Fission-track data document Late Cretaceous unroofing of Cretaceous (115–100 Ma) gramtic basement rocks in northwestern Nevada, followed by a long period of relative tectonic quiescence that persisted through Oligocene and Miocene volcanism until the onset of Basin and Range extension at ∼12 Ma. The low magnitude of extension (12%) and early Tertiary stability suggest that the modern ∼31 km thick crust in northwestern Nevada was only slightly thicker (∼35 km) prior to extension at 12 Ma, and was no thicker than ∼38 km in the Late Cretaceous. This stands in contrast to other parts of the Basin and Range, where the crust was thickened to at least 45 to 50 km by Cretaceous thrusting and subsequently thinned to ∼30 km by large magnitude (>50%) extension. [ABSTRACT FROM AUTHOR]

High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote ± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite (∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry and THERMOCALC average- P– T calculations yield peak eclogite-facies P– T conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average- P– T of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin P– T trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, P– T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism. Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of ∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes. [ABSTRACT FROM AUTHOR]

Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity-current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high-resolution seismic-reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr−1, with increases at times of extreme relative sea-level lowstand. Coarser-grained mid-fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea-level fall. These pulses of coarse-grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr−1 on the mid- and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr−1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer-term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level. [ABSTRACT FROM AUTHOR]

Abstract: Finite strain analysis in the northern (Piute Meadow pendant (PMP)) and eastern (Saddlebag Lake pendant (SLP)) host rock pendants of the Tuolumne Intrusive Suite (TIS) reveals plane strain geometries in the pendants and a steep strain gradient in the SLP. Average regional z-axis shortening in both pendants is ca. 43% and increases to ca. 70 and 85% in the SLP within ca. 100m to the pluton contact. Microstructures in the SLP adjust to increasing temperatures and partial melting near the margin. Beyond the pluton margin, weakly elongate quartz grains with lobate grain boundaries, patchy to sweeping undulose extinction, deformation bands, and incipient subgrains are consistent with low-temperature dislocation creep. Towards the margin, quartz grain boundaries become increasingly irregular and display “chessboard” patterns indicating high-temperature dislocation creep. Grain size is markedly smaller near the margin and despite large finite strain grains do not show evidence for crystal-plastic deformation. Cuspate strain-free interstices are interpreted to mimic former anatectic melt. Outcrop-scale migmatization, coincidence with abruptly increasing finite strain, and microfracturing towards the margin implies that fracture-induced mechanical grain size reduction may have triggered the transition to melt-assisted granular flow, ultimately resulting in drastic rheological weakening. [Copyright &y& Elsevier]

A cloud rift is characterized as a large-scale, persistent area of broken, low-reflectivity stratocumulus clouds usually surrounded by a solid deck of stratocumulus. A rift observed off the coast of California was investigated using an instrumented aircraft to compare the aerosol, cloud microphysical, and thermodynamic properties in the rift with those of the surrounding solid stratocumulus deck. The microphysical characteristics in the solid stratocumulus deck differ substantially from those of a broken, cellular rift where cloud droplet concentrations are a factor of 2 lower than those in the solid cloud. Furthermore, cloud condensation nuclei (CCN) concentrations were found to be about 3 times greater in the solid-cloud area compared with those in the rift. Although drizzle was observed near cloud top in parts of the solid stratocumulus cloud, the largest drizzle rates were associated with the broken clouds within the rift area and with extremely large effective droplet sizes retrieved from satellite data. Minimal thermodynamic differences between the rift and solid cloud deck were observed. In addition to marked differences in particle concentrations, evidence of a mesoscale circulation near the solid cloud–rift boundary is presented. This mesoscale circulation may provide a mechanism for maintaining a rift, but further study is required to understand the initiation of a rift and the conditions that may cause it to fill. A review of results from previous studies indicates similar microphysical characteristics in rift features sampled serendipitously. These observations indicate that cloud rifts are depleted of aerosols through the cleansing associated with drizzle and are a manifestation of natural processes occurring in marine stratocumulus. [ABSTRACT FROM AUTHOR]

Using a combination of GPS-measured horizontal velocities of 200 sites and 115,487 range-change rates determined with the permanent scatterer interferometric synthetic aperture radar (InSAR) method in the San Francisco Bay Area, we resolve vertical motions in the region at sub-mm/yr precision. The highest displacement rates are due to nontectonic processes, such as active landslides, subsidence and rebound over aquifers, and rapid settling of unconsolidated sediments along the bay margins. Residual displacement rates are determined by removing the contribution of the GPS-derived horizontal velocity field from the InSAR range-change rates. To isolate vertical tectonic rates, we use only those InSAR measurements made on material that was not Quaternary substrate, which is susceptible to nontectonic and seasonally varying ground motions. The InSAR residuals indicate significant uplift over the southern foothills of the active Mount Diablo anticlinorium, the Mission Hills stepover region of the Hayward and Calaveras faults, and the central and southern Santa Cruz Mountains located along a restraining bend of the San Andreas fault. [ABSTRACT FROM AUTHOR]

Abstract: Observations indicate that earthquake faults occur in topologically complex, multi-scale networks driven by plate tectonic forces. We present realistic numerical simulations, involving data-mining, pattern recognition, theoretical analyses and ensemble forecasting techniques, to understand how the observable space–time earthquake patterns are related to the fundamentally inaccessible and unobservable dynamics. Numerical simulations can also help us to understand how the different scales involved in earthquake physics interact and influence the resulting dynamics. Our simulations indicate that elastic interactions (stress transfer) combined with the nonlinearity in the frictional failure threshold law lead to the self-organization of the statistical dynamics, producing 1) statistical distributions for magnitudes and frequencies of earthquakes that have characteristics similar to those possessed by the Gutenberg–Richter magnitude–frequency distributions observed in nature; and 2) clear examples of stress transfer among fault activity described by stress shadows, in which an earthquake on one group of faults reduces the Coulomb failure stress on other faults, thereby delaying activity on those faults. In this paper, we describe the current state of modeling and simulation efforts for Virtual California, a model for all the major active strike slip faults in California. Noting that the Working Group on California Earthquake Probabilities (WGCEP) uses statistical distributions to produce earthquake forecast probabilities, we demonstrate that Virtual California provides a powerful tool for testing the applicability and reliability of the WGCEP statistical methods. Furthermore, we show how the simulations can be used to develop statistical earthquake forecasting techniques that are complementary to the methods used by the WGCEP, but improve upon those methods in a number of important ways. In doing so, we distinguish between the “official” forecasts of the WGCEP, and the “research-quality” forecasts that we discuss here. Finally, we provide a brief discussion of future problems and issues related to the development of ensemble earthquake hazard estimation and forecasting techniques. [Copyright &y& Elsevier]

We present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the San Andreas, San Jacinto, and Punchbowl faults in southern California. The observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters, a fault zone scale of meters to tens of meters and related geomorphologic features. The observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters. In all of those scales, more damage is found on the side of the fault with faster seismic velocities at seismogenic depths. The observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks. The data are consistent with a preferred northwestward propagation direction for ruptures on all three faults. If our results are supported by additional observations, asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section. The property of a preferred rupture direction can explain anomalous behavior of historic rupture events, and may have profound implications for many aspects of earthquake physics on large faults. [ABSTRACT FROM AUTHOR]