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Interpretation of industry seismic lines indicates that the eastern margin of the northern Gulf of California contains the inactive Adair-Tepoca and Upper Tiburón basins. The western margin is active and includes the Wagner, Consag, Upper Delfín, and Lower Delfín basins. These basin systems are separated by a wide basement high across which the upper strata in the inactive basins correspond to the middle and lower strata in the active basins, recording the westward migration of strain and subsidence during late Pliocene time. Our results illustrate the formation of an abandoned rift margin along the eastern Gulf of California. [ABSTRACT FROM AUTHOR]

Understanding uplift and exhumation in orogenesis requires integrating the effects of both climatic and tectonic forcing. Punctuated thrust activity is widely documented from field studies in a range of mountain belts. The potential for climatic forcing of thrust activity has been a subject of recent debate. Here we use a modeling approach to analyze the behavior of individual thrust units in the context of asymmetric doubly vergent thrust wedges. The model predicts that rates of surface uplift, frontal accretion, and exhumation should be punctuated on a time scale linked to thrust sheet geometry and convergence rates. This time scale ranges from 0.1 to 5 m.y. for various settings, and should be calculated before external forcings such as climate are invoked. [ABSTRACT FROM AUTHOR]

Seven localities distributed along a 180-km-long outcrop within the UK sector of the Laurentian margin were sampled, and all showed two phases of authigenic K-feldspar. The first phase was precipitated from a cool fluid of moderate salinity, and the second phase was precipitated from a hotter, more saline fluid.Ar-Ar ages for the two episodes of authigenesis are ca. 470 Ma and ca. 432 Ma. The data can be interpreted to represent two phases of regional-scale fluid migration related to Ordovician arc accretion and Silurian continent-continent convergence, corresponding to the Grampian and Scandian episodes of the Caledonian orogeny, respectively. [ABSTRACT FROM AUTHOR]

We explore canyon incision history of the western margin of the Andean (Altiplano-Puna) plateau in the central Andes as a proxy for surface uplift. (U-Th)fHe apatite data show rapid cooling beginning at Ca. 9 Ma and continuing to Ca. 5.1 Ma in response to incision. A minimum of 1.0 km of incision took place during that interval. The youngest apatite date and a volcanic flow perched 125 m above the present valley floor dated at 2.261 ± 0.046 Ma (40Ar/39Ar) show that an additional ∼1.4 km of incision occurred between Ca. 5.1 and 2.3 Ma. Thus, we infer that a total of at least 2.4 km, or 75% of the present canyon depth was incised after en. 9 Ma. (U-Th)/He zircon data collected along the same transect imply that the western margin of the plateau was warped upward into its present monoclinal form, rather than uplift being accommodated on major surface-breaking faults. [ABSTRACT FROM AUTHOR]

We present a new method for the structural analysis of folded surfaces. The method requires a generalized definition of a fold as a patch consisting of contiguous points with Gaussian curvature and mean curvature of consistent sign. Four classes of folds result from this definition: antiforms and synforms each of synclastic and anticlastic forms. This new analysis is illustrated with reference to several examples. [ABSTRACT FROM AUTHOR]

Alluvial fan evolution and morphology are often considered to respond primarily to external forcing (e.g., tectonics, climate, and base-level change). Here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing. This process is driven by positive autogenic feed-backs between flow width, sediment transport, and rate of fan aggradation. Entrenchment is initiated where sediment accommodation space limits continued fan growth. Our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change. [ABSTRACT FROM AUTHOR]

The 40Ar/39Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by Ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician. [ABSTRACT FROM AUTHOR]

The Kenya rift valley is the classic example of an active continental rift zone. We report the rift flank cooling history based on a combination of previous apatite fission track (AFT) and new (U-Th)/He (AHe) data. Our results corroborate the Late Cretaceous rapid cooling episode of continent-wide significance revealed previously by AFT dating. Post-Cretaceous cooling of the eastern rift flank was slow with net cooling of <20°C through much of the Cenozoic. We interpret this cooling style in terms of the absence of significant relief. Samples from the western rift flank and from low elevations of the eastern rift flank reveal a late Neogene cooling episode associated with net cooling of ~38 °C, indicating that this flank was eroded to a deeper level than that to the east. The late Neogene cooling episode is interpreted as the time of uplift and shaping of the present-day relief of the graben shoulders, which attain elevations of >3400 m in central Kenya. This timing also largely coincides with the uplift of the Western Rift flanks in Uganda and Congo and with the change toward drier conditions and grassland- dominated vegetation in East Africa. We propose that the regional morphotectonic evolution of the Kenyan rift flanks contributed to late Cenozoic environmental change in East Africa, thus superimposing a pronounced local effect on global climate change at that time. [ABSTRACT FROM AUTHOR]

We present a new, near-comprehensive survey of the variations in seismic structure across the West Australian craton at the scale of the main terrane groups. Analyzing data from distant earthquakes recorded at temporary and permanent stations located across the region, we found the best-fitting structure by modeling the conversions from P- to S-wave motion (the receiver function) that take place as the seismic energy travels upward through the lithosphere. Such methods can be used to delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited, and they provide an effective alternative to active-source seismic techniques for deep crustal targets. The seismic structure is consistent within several of the individual Archean terranes, most notably the Pilbara, Murchison, and Southern Cross. These terranes are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. The structure shows significant contrasts between neighboring terranes; thus, major tectonic units have a velocity profile that is a signature of that terrane or terrane group. We infer that the seismic structure of the Archean crust and upper mantle was fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units that formed the West Australian craton. [ABSTRACT FROM AUTHOR]

Theories that propose feedbacks among climate, tectonics, and surface processes commonly assume that erosion is enhanced by glacial activity. Indeed, studies have shown that glaciers appear to limit the elevation of mountain ranges; however, comparisons between rates of glacial and nonglacial erosion are difficult to make. Ideally, such comparisons must hold precipitation and lithology constant, while only varying the erosional regime. Located in a climatic transition zone during the Pleistocene, the east-west-trending valleys of the Bitterroot Range present an opportunity for a quantitative analysis of glacial and nonglacial erosion because the north-facing sides of the valleys were glaciated, whereas the south-facing slopes were not. The different erosional regimes operating on either side of the valleys created strongly asymmetric ridges. Ridgelines separating the east-west-trending valleys have been pushed southward by glacial headwall retreat such that ridge-to-valley distances are ∼50% greater on the north-facing slopes than on the south-facing slopes. In addition, mean hillslope angles are 6° lower on the glaciated slopes than on the unglaciated slopes, and calculations of geophysical relief suggest that, on average, glaciers have removed nearly twice as much rock as nonglacial processes. Finally, we conclude that, although rates of vertical incision by glacial processes in the Bitterroot Range were more rapid than nonglacial processes, the dominant geomorphological impact of glaciers was lateral erosion by headwall retreat. [ABSTRACT FROM AUTHOR]

It is widely believed that cyclic sedimentation in active tectonic basins is caused by external forcing such as sea-level cycles, episodic tectonic events, or variation in sediment supply due to climate change. Here we present a case of cyclic sedimentation in an experiment subject to constant fault slip rate and sediment discharge with no base-level fluctuation. The experiment was designed to study sedimentation in a simplified extensional relay ramp system. Reorganization of the fluvial channel network in a cyclic manner caused local variation in sediment supply to the hanging-wall basin, where subsidence was maximized, and resulted in autogenic appearance and disappearance of a lake associated with 90° rotation of paleocurrent direction and delta formation. Comparing an early phase of the experiment using no external forcing (i.e., no tectonic movement or base-level change) with the following main phase of the experiment (i.e., including subsidence in an extensional relay ramp pattern) shows that the characteristic time scale for channel mobility was five times longer during the phase with subsidence than during the previous phase without subsidence. Active fault slip imposed a new, longer time scale for autogenic dynamics by establishing a linked tectonic-sedimentation system. Scaling the experimental results to field length and time scales suggests that comparable autogenic cycles would produce 10–20-m-thick strata on time scales of 105 yr, comparable to observed cases attributed to allogenic effects. [ABSTRACT FROM AUTHOR]

Exposures of gabbros and mantle-derived peridotites at slow-spreading oceanic ridges have been attributed to extension on long-lived, low-angle detachment faults, similar to those described in continental metamorphic core complexes. In continental settings, such detachments have been interpreted as having originated and remained active at shallow dips. Alternatively, currently shallow dipping fault surfaces may have originated at moderate to steep dips and been flattened by subsequent flexure and isostatic uplift. While the latter interpretation would be more consistent with Andersonian faulting theory, it predicts large footwall tilts that have not been observed in continental detachment faults. Here we use the magnetization of oceanic gabbro and peridotite samples exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge to demonstrate that substantial footwall rotations have occurred. Widespread rotations ranging from 50° to 80° indicate that original fault orientations dipped steeply toward the spreading axis. [ABSTRACT FROM AUTHOR]

Principal slip surfaces in fault zones accommodate most of the displacement during earthquakes. The topography of these surfaces is integral to earthquake and fault mechanics, but is practically unknown at the scale of earthquake slip. We use new laser-based methods to map exposed fault surfaces over scales of 10 pm to 120 m. These data provide the first quantitative evidence that fault-surface roughness evolves with increasing slip. Thousands of profiles ranging from 10 pm to >100 m in length show that small-slip faults (slip <1 m) are rougher than large-slip faults (slip 10-100 m or more) parallel to the slip direction. Surfaces of small-slip faults have asperities over the entire range of observed scales, while large-slip fault surfaces are polished, with RMS values of <3 mm on profiles as long as 1-2 m. The large-slip surfaces show smooth, elongate, quasi-elliptical bumps that are meters long and as high as -1 m. We infer that these bumps evolve during fault maturation. This difference in geometry implies that the nucleation, growth, and termination of earthquakes on evolved faults are fundamentally different than on new ones. [ABSTRACT FROM AUTHOR]

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]

The high elevation and deep incision of the Alps have traditionally been used as an argument for recent tectonic activity that has elevated the belt and increased erosion rates. Normal faulting and horizontal extension, however, dominate current tectonic activity, and isostatic compensation of thinning crust should lead not to increased but to decreased mean elevations. Here we test the idea that enhanced Quaternary erosion of the Alps and isostatic compensation of the mass removed can account for the distribution of present-day geodetically measured rates of vertical movement in the western Alps. Using geophysical relief and Kuhlemann's estimated average erosion rate for the Alps, we quantify the spatial distribution of erosion and the volume of eroded rock, respectively. From these, we obtain a map of rock eroded within a given time span. The calculated isostatic response of the Alpine lithosphere to erosional unloading for a variety of values of the flexural rigidity of the Alpine lithosphere reaches a maximum of ~500 m since 1 Ma in the inner Swiss Alps, and vertical movement extends across the entire belt, including peri-Alpine basins. Assuming a steady erosion rate since 1 Ma, this rebound accounts for half of the measured vertical motion of 1.1 mm/yr in the southern Valais. Thus, a significant fraction (~50%) of the present-day vertical movement results from the isostatic response to enhanced erosion during Plio-Quaternary time. [ABSTRACT FROM AUTHOR]

The late Miocene-Holocene deep-water fold-and-thrust belt of offshore northwestern Borneo displays one of the most acute angles recorded for a submarine critical-taper wedge. The very low surface (1°-5°) and basal detachment (2°-5°) dips require basal pore-fluid pressures near lithostatic pressure for the wedge to propagate at such angles. In the deep-water environment, sedimentary processes act against the wedge attaining critical taper. In places of lower sedimentation rates, the fold-and-thrust belt extends to the base of the slope and dips ~1.5°-2.5°. In areas of the highest sedimentation rates (Baram Delta), the slope has a lower angle (1°) and the slope continues beyond the limit of the fold-and-thrust belt. Slope morphology changes along the margin from concave up (relatively low sedimentation rate, low shortening rate), to convex up (relatively high sedimentation rate, low shortening rate), to stepped (relatively low sedimentation rate, high shortening rate). [ABSTRACT FROM AUTHOR]

We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from drainage area upstream of the fault. Unit stream powers are ~4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins. [ABSTRACT FROM AUTHOR]

Recent observations of cleavage patterns, strain histories, and kinematics across the Taiwan mountain belt depict systematic orogen-scale variations with respect to the synorogenic divide and suggest that the pattern of cleavage development is a predictable consequence of orogen stresses and kinematics. In Taiwan, continental crust within the collision is accreted in the prowedge facing Asia, but is advected eastward into the east-verging retrowedge, where the most deeply exhumed rocks are exposed. Wedge mechanics predict a reversal in the direction of plunge of the principal compressive stress at the topographic divide between the opposing wedges. The observation of a single cleavage in western Taiwan suggests that the cleavage in the prowedge remains stable with respect to the stress orientation. In contrast, the existence of a second crenulation cleavage in the retrowedge is evidence for an abrupt change in stress orientation and unstable buckling of preexisting prowedge fabrics. Advection of a fabric across a topographic divide in a doubly vergent wedge provides an explanation for the occurrence of cleavage fronts and fans in natural systems such as Taiwan. [ABSTRACT FROM AUTHOR]

The collision of the Indian subcontinent with Asia drives the growth and evolution of the greater Tibetan Plateau region. Fault slip rates resulting from the relative motion between crustal blocks can provide a kinematic description of the distribution of present-day deformation. I construct a three-dimensional, regional-scale elastic block model of the India-Asia collision zone that is consistent with geodetic observations of interseismic deformation, mapped fault system geometry, historical seismicity, and the mechanics of the earthquake cycle. This mechanical model of the elastic upper crust yields a set of kinematically consistent fault slip rates and block motions that may serve to constrain dynamic models of continental crustal dynamics. [ABSTRACT FROM AUTHOR]

The deformation in the Asal Rift (Djibouti) is characterized by magmatic inflation, diking, distributed extension, fissure opening, and normal faulting. An 8 yr time line of surface displacement maps covering the rift, constructed using radar interferometry data acquired by the Canadian satellite Radarsat between 1997 and 2005, reveals the aseismic behavior of faults and its relation with bursts of microseismicity. The observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults. Fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the Fieale volcanic center. Slip distribution along fault strike shows triangular sections, a pattern not explained by simple elastic dislocation theory. These observations suggest that the Asal Rift faults are in a critical failure state and respond instantly to small pressure changes in fluid-filled fractures connected to the faults, reducing the effective normal stress on their locked section at depth. [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]

Concurrent changes in seawater chemistry, sea level, and climate since the mid-Cretaceous are thought to result from an ongoing decrease in the global rate of lithosphere production at ridges. The present-day area distribution of seafloor ages, however, is most easily explained if lithosphere production rates were nearly constant during the past 180 m.y. We examined spatial gradients of present-day seafloor ages and inferred ages for the subducted Farallon plate to construct a history of spreading rates in each major ocean basin since ca. 140 Ma, revealing dramatic Cenozoic events. Globally, seafloor spreading rates increased by ∼20% during the early Cenozoic due to an increase in plate speeds in the Pacific basin. Since then, subduction of the fast-spreading Pacific-Farallon ridge system has led to a 12% decrease in average global spreading rate and an 18% or more decrease in the total rate of lithosphere production by the most conservative estimates. These rapid changes during the Cenozoic defy models of steady-state seafloor formation, and demonstrate the time-dependent and evolving nature of plate tectonics on Earth. [ABSTRACT FROM AUTHOR]

Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U-Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40Ar/39Ar dates, but can be determined relatively easily on multiple replicate grain aliquots. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, British Columbia (157 ± 3.5 [2.2%] ka weighted 95% confidence interval [CI], mean square of weighted deviates [MSWD] = 1.7) and Prindle Volcano, Alaska (176 ± 16 [8.9%] ka, MSWD = 13), in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%] ka, MSWD = 12), in the Big Pine Volcanic Field, California. All ZHe ages are either equivalent to or younger than previously determined K/Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U-Th zonation or <99.99% He degassing (assuming ca. 100 Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% CI, MSWD = 4.8) and Oak Creek to 3.4% (95% CI, MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete He degassing would require entrainment times <1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive He diffusion characteristics. [ABSTRACT FROM AUTHOR]

The kinematic evolution of the enigmatic arc-shaped southernmost Andes of Patagonia and Tierra del Fuego has been a subject of debate for most of the past century. We compared the results from analog sandbox experiments with the tectonic evolution and actual configuration of the mountain chain in order to elucidate whether oroclinal bending took place during the Tertiary, or if the southernmost Andes have been a curved orogen since at least Late Cretaceous time. Experiments simulating oroclinal rotation produced strong along-strike variations in shortening and failed to account for structural data compiled from the Fuegian Andes. Results from experiments simulating an L-shaped, concave-to-foreland indenter were in agreement with the known Tertiary structural evolution of the southernmost Andes. The diachronicity of principal shortening events previously recognized in Patagonia and Tierra del Fuego could only be reproduced by moving the indenter in two successive orthogonal directions: first approximately northward to form the Fuegian fold-and-thrust belt, and then approximately eastward to propagate thrusting in the Patagonian Andes. This two-phase evolution is consistent with a recorded change in the convergence direction of the Farallon-Nazca plate that occurred at ca. 27 Ma. [ABSTRACT FROM AUTHOR]

The geometry of foreland basins is controlled by a dynamic balance between thrust-belt migration, flexural subsidence, and fluvial deposition. To improve our ability to quantify the relationships between tectonics, climate, and foreland basin geometry, I developed analytic solutions for basin topography and stratigraphy using a classic two-dimensional mathematical model of foreland basin evolution. Model predictions for basin topography are successfully tested against observed profiles along a humid-to-arid climatic gradient in the central Andes. Using published estimates for the thrust-belt migration rate, flexural parameter, and thrust-front basin depth in this region, I show that basin topographic profiles can be used to estimate the upstream erosion rate at any point along a foreland basin. Basin-averaged erosion rates estimated in this way vary from 0.025 to 0.045 mm/yr in the central Andes, increasing from semiarid to humid climates. [ABSTRACT FROM AUTHOR]

The article discusses how plate tectonics can be used as a framework through which we understand the large-scale Phanerozoic history of Earth. Topics discussed include studies of Spherule beds are preserved within Archean age rocks in the Barberton greenstone belt, South Africa and the Pilbara craton, Australia; and use of Geodynamic calculations as an important hypothesis-testing tool, when tool when combined with the Precambrian geological record.

Axel Heiberg Island (northern Nunavut) contains the thickest Mesozoic section in the Sverdrup Basin. The ~370-km-long island is second only to Iran in its concentration of exposed evaporite diapirs. However, the tectonics of this remote region is not well known. A polar desert on the island provides excellent exposure of 46 diapirs of Carboniferous evaporites and associated minibasins. Paleogene (Eurekan) anticlines trend roughly north on a regular ~20 km wavelength and probably detach on autochthonous Carboniferous Otto Fiord evaporites. In contrast, a 60-km-wide area, known as the wall-and-basin structure (WABS) province, has bimodal fold trends and irregular wavelengths of <10 km. Here, crooked walls of diapiric anhydrite crop out in the cores of tight anticlines. Wider synclinal minibasins separate the diapiric walls. We interpret the WABS province to detach on a shallow evaporite canopy. This comprises an allochthonous coalescence of evaporite diapirs that spread extrusively during the Hauterivian (Early Cretaceous, Ca. 130 Ma), close to the onset of seafloor spreading in the Arctic Ocean and of flood basalt volcanism associated with Alpha Ridge. Since then, the evaporite canopy has yielded second-generation diapirs, now exhumed and exposed by modest Eurekan shortening. Strata record minibasin evolution and diapirism since at least the Late Triassic. Stratigraphic thinning against diapirs and spectacular angular unconformities at four levels are present between the Jurassic and Paleocene. The most widespread is the Early Cretaceous event marking the time of canopy emplacement. Outcrops record the later fate of the canopy, including exposure, onlap of diapirs, and off-diapir debris flows. [ABSTRACT FROM AUTHOR]

New Lu-Hf garnet growth ages from high-pressure rocks from the structurally lowest terrane (Nevado-Fiidbride Complex) in the Betic Cordillera of southern Spain indicate early Miocene subduction of continental crust, requiring significant revision of the timing and processes of orogeny in the western part of the Alpine orogenic system. Mafic eclogite and pelitic schist give Lu-Hf ages in the range 18–14 Ma, and fission-track data show that the terrane was exhumed by 12–8 Ma. The overlying terrane (Alpujdrride Complex) shows early Tertiary high pressure-temperature metamorphism; this was followed by a thermal event that accompanied rapid crustal extension at 22–18 Ma, leading to the formation of the Aiboran Sea backarc basin. The evidence therefore suggests two phases of continental subduction separated by a phase of large-scale intraorogenic extension. The two terranes are now separated by a mid-late Miocene low-angle extensional fault. The timing and paleogeographic setting suggest that the Nevado-Fildbride terrane originally was close to the rifted South Iberian margin, and was subducted during westward motion of the extending upper terrane between the converging African and Iberian plates. [ABSTRACT FROM AUTHOR]

The anatomy of the Southern Apennines is a matter of compelling debate. Both thick- and thin-skinned tectonic models are proposed due to paucity of deep seismic constraints. We present new and well-defined information on the velocity structure of the mountain range by using an innovative nonlinear tomographic technique applied to refraction-wide-angle reflection data. The velocity-interface model shows a high-velocity region, with values exceeding 7 km/s, at 8–9 km depth. Such large velocities cannot be related to sedimentary rocks, and imply the presence of shallow crystalline rocks, i.e., the basement involvement in the thrust belt. The integration of our results with other geophysical data (local earthquake tomography, Bouguer and magnetic anomalies) confirms this interpretation, which strongly supports thick-skinned tectonics and has significant implications for the seismotectonics and hydrocarbon potential of the Southern Apennines. Our inversion approach can be effective in other orogenic wedges, when seismic reflection profiling fails to uniquely resolve deep complex structures. [ABSTRACT FROM AUTHOR]

Several basins are developing near bends on strands of the North Anatolian transform fault in northwest Turkey. Oblique slip on these faults, rather than strain partitioning, accounts for trans-tension and subsidence. These basins are asymmetric, and tilt and subside most rapidly at their narrow ends near the bends. The turbidite surface marking the floor of the Cinarcik Basin (eastern Marmara Sea) was mostly abandoned at a sudden drop in sedimentation, which was likely coincident with the 14 ka lake-sea transition, and is now a warped reference surface from which we can measure strain and sedimentation. Subsidence and tilt are rapid, but do not require late Quaternary changes in regime. They are linked to transcurrent motion by slip parallel to an oblique bend on the North Anatolian fault and suggest tsunamogenic vertical motion in large Marmara Sea earthquakes. [ABSTRACT FROM AUTHOR]

Oceanic arcs and deep-sea trenches are the surface expressions of oceanic plates sub-ducting into the Earth's mantle. We use a new numerical technique for simulating the dynamical evolution of the lithosphere-mantle interaction in order to assess the causes of arc curvature. We group the possible causes into two classes, external feedback between the migrating lithosphere and the secondary induced mantle flow, and internal heterogeneities within the lithosphere, e.g., owing to differences in cooling ages of the plate at the trench. We statistically assess that almost all arcs on the Earth can be described by these hypotheses. The method is also directly applied to the Tonga and Aleutian arcs, bringing new insights on the origin of their shapes. [ABSTRACT FROM AUTHOR]

New structural, geochronological, and geochemical data from the Acatlán Complex of southern Mexico show that it preserves a complete history of Pangea, from assembly to breakup. Previously interpreted to be a vestige of the lapetus suture, the Acatlán Complex records a history that can be sequentially linked to the Rheic Ocean, the paleo-Pacific, and the Gulf of Mexico. This record is interpreted to reflect: (1) the development of a rift-passive margin on the southern flank of the Rheic Ocean in the Cambrian—Ordovician; (2) the formation of an extensional regime along the formerly active northern margin of Gondwana throughout the Ordovician; (3) closure of the Rheic Ocean documented by subduction-related eclogite facies metamorphism and exhumation during the Late Devonian—Mississippian; (4) Permian—Triassic convergent tectonics on the paleo-Pacific margin of Pangea; and (5) interaction with a Jurassic mantle plume coeval with the opening of the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0–1.8 mm/yr) than inside of it in the east (0.55–0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region's recent tectonic evolution. [ABSTRACT FROM AUTHOR]

The relative importance of tectonism in the stratigraphic record should be more clearly expressed during greenhouse times of Earth history due to the lack of overmasking effects of high-frequency and high-amplitude sea-level changes typical for icehouse periods. Establishment of the importance of tectonism, especially at temporal scales comparable to durations of typical transgressive-regressive cycles, has been plagued by poor temporal resolution. Our outcrop and subsurface-based study of a Cenomanian shallow-marine siliciclastic interval, constrained by bentonite-based geochronology and detailed biostratigraphy, examines this problem. In a 2.2 m.y. interval, we identified four tectonically driven erosional surfaces that dominate preserved stratigraphy. Biostratigraphic correlation to a sea-level curve for the Cenomanian—where coeval high-frequency lowamplitude eustatic cyclicity has been demonstrated—allows the first direct comparison of the effects of eustasy and tectonics at temporal scales of hundreds of thousands of years during a global greenhouse time. We suggest that minor tectonic pulses locally overshadow the effects of eustasy and exert the dominant control over preserved stratigraphy. While subtle tectonic control on sedimentation has been documented throughout the Cretaceous Western Interior, the results of this study suggest that much of that deformation occurred at sub-million-year frequencies and at time scales comparable to eustatic transgressiveregressive cycles. [ABSTRACT FROM AUTHOR]

Understanding the marine magnetic anomaly record is critical for constructing realistic geodynamo models of global geomagnetic field, polarity reversal mechanisms, and longterm geomagnetic field behavior. One of the least understood portions of the marine magnetic anomaly record is also the oldest part of the record, the Jurassic quiet zone (JQZ), where anomalies become weak and difficult to correlate. The reason for the existence of the JQZ is unclear. It has been suggested that the JQZ is a true polarity superchron, similar to the Cretaceous normal superchron. Continental magnetostratigraphic studies have suggested that the JQZ is a period of rapid polarity reversal, of low field intensity, or both. We show results of a deep-tow survey of Pacific Jurassic crust that confirms the existence of magnetic anomalies within the JQZ. We tie Ocean Drilling Program Hole 801C (167.4 Ma) into the record and show that seafloor-spreading magnetic anomalies are present around the hole and extend to 170 Ma crust. We find a rise in reversal rate with increasing age with reversal rates over 10 rev/m.y. at 160 Ma and at 167 Ma. Anomaly amplitudes decrease in the record from 155 Ma until 162 Ma, where low-amplitude anomalies are difficult to correlate. Prior to 167 Ma, anomalies regain amplitude and remain strong until the end of our record at 170 Ma. The JQZ thus appears to be a combination of low-amplitude magnetic anomalies combined with rapid field fluctuations, which could be due to either intensity or polarity changes. [ABSTRACT FROM AUTHOR]

The historical superstructure-infrastructure concept (S-I) expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels. Two-dimensional thermalmechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening (phase 1), thermal relaxation and rheological weakening (phase 2), and ductile flow at depth (phase 3). Results predict an upper-crustal superstructure, dominated by early steep structures, separated across a subhorizontal high-strain zone from a ductile infrastructure with late gently dipping structures; this is consistent with observations from the western Superior Province. These models can account for contrasts in structural style, metamorphic grade, seismic reflectivity, and age between upperand lower-crustal levels. In contrast to conventional thrust-tectonics models, the revived S-I model shows how young structures can form beneath older ones during progressive convergence, thereby encouraging reassessment of standard seismic reflection interpretations. [ABSTRACT FROM AUTHOR]

The proposed national high-level nuclear waste repository at Yucca Mountain is close to Quaternary cinder cones and faults with Quaternary slip. Volcano eruption and earth. quake frequencies are low, with indications of spatial and temporal clustering, making probabilistic assessments difficult. In an effort to identify the most likely intrusion sites, we based a three-dimensional finite-element model on the expectation that faulting and basalt intrusions are sensitive to the magnitude and orientation of the least principal stress in extensional terranes. We found that in the absence of fault slip, variation in overburden pressure caused a stress state that preferentially favored intrusions at Crater Flat. However, when we allowed central Yucca Mountain faults to slip in the model, we found that magmatic clustering was not favored at Crater Flat or in the central Yucca Mountain block. Instead, we calculated that the stress field was most encouraging to intrusions near fault terminations, consistent with the location of the most recent volcanism at Yucca Mountain, the Lathrop Wells cone. We found this linked fault and magmatic system to be mutually reinforcing in the model in that Lathrop Wells feeder dike inflation favored renewed fault slip. [ABSTRACT FROM AUTHOR]

Dynamic topography provides a measure of stresses within the Earth's interior. Dense slabs induce an upper mantle flow that deflects the surface of the Earth downward above them. By combining a simple theoretical (Stokeslet) model of subduction, gravity modeling, and seismic tomography, I show that a significant fraction (as much as 2000 m) of the topographic variations observed above the Scotia, Mariana, and Hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction. [ABSTRACT FROM AUTHOR]

New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ± 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ± 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. 685-750 Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the "Sturtian" ice age was markedly diachronous, and/or there was more than one "Sturtian"-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than "snowball" or "slushball" Earth ice ages. [ABSTRACT FROM AUTHOR]

Spectral analysis of both shipborne and airborne magnetic maps of the southern Tyrrhenian Sea reveals seven subparallel positive-negative magnetic anomaly stripes over the flat-lying deep floor of the Marsili oceanic basin. This represents the first evidence of oceanic magnetic anomalies in the Tyrrhenian Sea. The central positive stripe is along the Marsili seamount, a superinflated spreading ridge located at the basin axis. The stratigraphy of Ocean Drilling Program Site 650 and K/Ar ages from the Marsili seamount suggest that the Marsili Basin opened at the remarkable full-spreading rate of ∼19 cm/yr between ca. 1.6 and 2.1 Ma about the Olduvai subchron. This is the highest spreading rate ever documented, including that observed at the Cocos-Pacific plate boundary. Renewed but slow spreading during the Brunhes chron (after 0.78 Ma), coupled with huge magmatic inflation, gave rise to the Marsili volcano. Our new data and interpretation show that backarc spreading of the Tyrrhenian Sea was episodic, with sudden rapid pulses punctuating relatively long periods of tectonic quiescence. [ABSTRACT FROM AUTHOR]

This study examines the response of a forearc to the increase in sediment flux to the trench caused by the onset of glacial denudation in the Patagonian Andes. We investigated shelf-coastal basins in south-central Chile, which generally comprise Eocene-early Miocene nearshore facies overlain by late Miocene-early Pliocene deep-water siltstones and by late Pliocene-Quaternary nearshore deposits. Seismic profiles and coastal exposures reveal Eocene-early Pliocene extension followed by ongoing late Pliocene compression evidenced from growth strata adjacent to seismically active reverse faults. The onset of major global cooling ca. 6 Ma triggered glacial denudation in the uplifted high Andes. Exhumed material transported along the steep and humid Andean western slope increased trench sedimentation rates and caused continuous accretion and subduction of terrigenous material. We interpret forearc basin inversion as a response to a decrease in slope and basal friction of the wedge caused by frontal accretion and subduction of water-rich material, respectively, in order to reach a critical taper. This process lifted the shelf ∼1.5 km during the middle Pliocene. The Juan Fernández Ridge and Chile Rise confined >2 km trench fill between 45 and 34°S, limiting accretion and basin inversion. Glacial age trench fill and the steady decrease in plate convergence rate shifted this segment of the margin from erosive to accretionary during the Pliocene. [ABSTRACT FROM AUTHOR]

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina's devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only ∼0.1 mm yr-1. We compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata. [ABSTRACT FROM AUTHOR]

Fissure eruptions may provide important information on the shallow propagation of dikes at volcanoes. Somma-Vesuvius (Italy) consists of the active Vesuvius cone, bordered to the north by the remnants of the older Somma edifice. Historical chronicles are considered to define the development of the 37 fissure eruptions between A.D. 1631 and 1944. The 1631 fissure, which reopened the magmatic conduit, migrated upward and was the only one triggered by the subvertical propagation of a dike. The other 25 fissure eruptions migrated downward, when the conduit was open, through the lateral propagation of radial dikes. We suggest two scenarios for the development of the fissures. When the summit conduit is closed, the fissures are fed by vertically propagating dikes. When the summit conduit is open, the fissures are fed by laterally propagating dikes along the volcano slopes. Consistent behaviors are found at other composite volcanoes, suggesting a general application to our model, independent of the tectonic setting and composition of magma. At Vesuvius, the historical data set and our scenarios are used to predict the consequences of the emplacement of fissures after the opening of the conduit. The results suggest that, even though the probability of opening of vents within the inhabited south and west slopes is negligible, the possibility that these are reached by a lava flow remains significant. [ABSTRACT FROM AUTHOR]

Seamount subduction along subduction-zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate. However, tectonic erosion processes have not been examined in detail with high-quality three-dimensional (3-D) seismic reflection imaging. In 1999 we acquired 3-D seismic reflection data from the Nankai Trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure. Fortuitously, these data reveal a small (to 1 km high) basement ridge that has subducted to 7 km subseafloor. Updip from the basement ridge, a 1-km-thick sequence of sediment from the base of the accretionary wedge appears to be missing. We interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction. Tectonic erosion has removed more than 25 km³ from the updip edge of the seismogenic zone and carried it down into the seismogenic zone. The tectonically eroded sediments are presumed to enhance fault-zone fluid content, potentially reducing fault-zone effective stress, and may temporarily inhibit earthquake rupture potential. After the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating. [ABSTRACT FROM AUTHOR]

New 40Ar/39Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized isotopic age resetting at 1.18–1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilândia belt between these two cratons, new 40Ar/39Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40Ar/39Ar, and Rb/Sr thermochronologic data are used to establish post-Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15–30 km) exposed in North America to shallower levels (5–15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crustal thickness. This interpretation explains the widespread preservation of both pre-Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical orogenic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed. [ABSTRACT FROM AUTHOR]

Space is used as a proxy for time in determining the role of glaciation in shaping valley cross profiles in the Two Thumb Range (Southern Alps), New Zealand. The range is undergoing rapid tectonic transport and uplift as it is advected toward the Alpine fault. Repeated cycles of glacial erosion during the Quaternary have fashioned U-shaped valleys in the north of the range, close to the Main Divide, while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles. Tectonic transport and uplift rates, coupled with an offshore oxygen isotope (δ18O) record and glacial-geological reconstructions, allow time constraints to be applied to valley development along the range. Valley width and depth measurements and power-law equations were used to quantify the shape of cross-profile transects in 37 valleys. Valleys evolved into a characteristic U-shaped cross profile formed over ≥400 k.y. of glacial occupancy. This, regardless of other models of glacial flattening of valley long profiles, suggests that alpine glaciation excavates greater volumes of rock, indicating that glaciers are a more effective erosional agent than fluvial activity in alpine mountain belts. [ABSTRACT FROM AUTHOR]

At the end of the Miocene, the European Alps ceased outward expansion, and tectonic uplift and exhumation shifted into the orogen interior. This shift is consistent with a change from orogenic construction to orogenic destruction, reflecting an increase in the ratio of erosional flux to accretionary flux. The coincidence of this change with an increase in sediment yield from the Alps suggests a climate-driven increase in erosional flux. The timing of deformation and sediment release from the southern Alps indicates that the tectonic change occurred synchronous with the last phase of the Messinian salinity crisis. We attribute the increase in erosional flux to a climatic shift to wetter conditions throughout Europe, likely augmented by the base-level fall that occurred during the Mediterranean dessication. This climate change is represented in the stratigraphic record by the Lago Mare deposits of the Mediterranean salinity crisis. [ABSTRACT FROM AUTHOR]

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years. [ABSTRACT FROM AUTHOR]

A simple algorithm based on the transformation equations of homogeneous deformation allows application of computer-graphic software for estimation of strain and restoration of shape of distorted fossils in a single operation. This paper gives a computer-aided digital method for the estimation of strain in situations where a single fossil, or a set of fossils, is found embedded in rocks. Because the digital method deciphers reciprocal strain ellipse by removing the effect of tectonic deformation, it is also useful in precise taxonomic identification. Several natural examples of trilobites and brachiopods are tested by the digital method, as well as by other conventional methods of strain analysis. The digital method is easy to use, rapid, and accurate in comparison to other existing methods of strain analysis and retrodeformation. [ABSTRACT FROM AUTHOR]

Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill. [ABSTRACT FROM AUTHOR]