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Virtual reality concepts have been widely adapted to teach geoscientific content, most notably in virtual field trips--with increased developments due to recent travel restrictions and challenges of field access. On the spectrum between real and fully virtual environments are also combinations of digital and real content in mixed-reality environments. In this category, augmented-reality (AR) sandboxes have been used as a valuable tool for science outreach and teaching due to their intuitive and haptic interaction-enhancing operation. Most of the common AR-sandboxes are limited to the visualization of topography with contour lines and colors, as well as water simulations on the digital terrain surface. We show here how we can get beyond this limitation, through an open-source implementation of an AR-sandbox system with a versatile interface written in the free and cross-platform programming language Python. This implementation allows for creative and novel applications in geosciences education and outreach in general. With a link to a 3-D geo modelling system, we show how we can display geologic subsurface information such as the outcropping lithology, creating an interactive geological map for structural geology classes. The relations of subsurface structures, topography, and outcrop can be explored in a playful and comprehensible way. Additional examples include the visualizations of geophysical fields and the propagation of seismic waves, as well as simulations of Earth surface processes. We further extended the functionality with ArUco-marker detection to enable more precise and flexible interaction with the projected content. In combination, with these developments, we aim to make AR-sandbox systems, with the additional dimension of haptic interactions, accessible to a wider range of geoscientific applications for education and outreach. [ABSTRACT FROM AUTHOR]

The article presents a reply to the previous article "Tectonic evolution of the Mesozoic South Anyui suture zone, eastern Russia: A critical component of paleogeographic reconstructions of the Arctic region". It cites on South Anyui suture zone (SASZ) as a key tectonic boundary of the Arctic region. The article also discusses the goals of the paper such as to provide concise overview of this geologically complex region and to publish new data on the depositional age.

Previous work at the Grayback normal fault block in Arizona suggested that a low geothermal gradient (~17 °C/km) existed just prior to crustal extension in the late Oligocene to early Miocene. We present new structural data and (U-Th)/He apatite (AHe) thermo-chronometry that robustly support this conclusion. New 40Ar/39Ar geochronology and structural analysis of dikes support a uniform tilting across the width of the block of ~80° east as a result of extension, removing an important uncertainty in calculating the paleodepth. New AHe results document decreasing ages at deeper paleodepths until ~4-6 km, below which preferred AHe ages are essentially invariant between 22 and 17 Ma. We interpret this inflection in the age-paleodepth trend as the base of the fossil Oligocene AHe partial retention zone (~70 °C), which yields an estimated paleogeothermal gradient of 14 ± 7 °C/km at the inception of extension, strongly confirming earlier estimates. Recent studies suggest that the crust across much of the Basin and Range was quite cold prior to major extension, which has important implications for estimating the degree of fault block tilt from thermochronologic data and for assessing the role of crustal heating in triggering continental extension. [ABSTRACT FROM AUTHOR]

We investigated the recent evolution of the Po Plain-Alps system by integrating subsurface geophysical data from the Po Plain with new stratigraphic and structural observations from the Southern Alps margin. Inversion of structural data and chronology provided by stratigraphic constraints led to the definition of three tectonic events since the Pliocene, namely, the intra-Zanclean, the Gelasian, and the Middle Pleistocene, driven by an axis of maximum compression formerly oriented NE (intra-Zanclean) and then to the NNW (Gelasian and Middle Pleistocene). The associated deformation has been accommodated by two sets of faults consisting of NNE-trending thrust faults, mostly represented in the western sector of Lake Garda, and NW-trending strike-slip faults, observed in the southern and eastern sectors. The interplay between these two sets of faults is interpreted to produce short (<10 km length) thrust ramps activated in left transpression, bounded by longer (30-60 km) transfer faults activated in a right-lateral strike-slip motion. Based on this structural model, we infer moderate seismicity (Mw < 6) associated with the NNE-directed thrusts and stronger earthquakes (also Mw > 6.5) along the NW-trending strike-slip faults. In this framework, the newly defined Nogara fault and the Sant'Ambrogio fault, all pertaining to the NW-trending system, are regarded as potential candidates for the seismogenic source of the January A.D. 1117 event, the most destructive earthquake in the Po Plain. [ABSTRACT FROM AUTHOR]

Inversion tectonics represent a key process in many orogens worldwide. The related mechanisms of fault reactivation and the effects of an articulated preshortening setting on thrust and fold development are still challenging questions. Modes and geometries of inversion have been the object of several analogue models. In this work, we analyzed the influence of an articulated high-angle preexisting discontinuity in the development of thrusts using sandbox modeling. The model geometry is based on the architecture of the major faults in the Central-Northern Apennines of Italy, where differently oriented Mesozoic-Cenozoic inherited extensional structures are clearly detectable and display contrasting styles of positive inversion tectonics. Quartz-sand is the analogue material adopted to model Mesozoic-Cenozoic sedimentary successions, and glass microbeads represent preexisting fault rocks. The geometry of the segmented preexisting structure is composed of two segments with the same dip (~60°): one oblique and another orthogonal to the shortening direction. Our results show that different styles of positive inversion tectonics can coexist and that the obliquity angle between inherited structures and the shortening direction is a leading factor controlling the degree of inversion: The oblique segment of the discontinuity exhibits a complete reactivation, whereas along the orthogonal segment, shortcut is the prevalent mechanism. The oblique element, moreover, represents a cross-strike discontinuity that guides the localization and curved geometry of the thrusts, compartmentalizing the deformation. Our findings can be applied to fold-and-thrust belts characterized by the presence of cross-strike discontinuities. [ABSTRACT FROM AUTHOR]

Despite being the largest active collisional orogen on Earth, the growth mechanism of the Himalaya remains uncertain. Current debate has focused on the role of dynamic inter action between tectonics and climate and mass exchanges between the Himalayan and Tibetan crust during Cenozoic India-Asia collision. A major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the Himalayas from 91°E to 97°E, which makes up about one-quarter of the mountain belt. To address this issue, we conducted detailed field mapping, U-Pb zircon age dating, and 40Ar/39Ar thermo chronology along two geologic traverses at longitudes of 92°E and 94°E across the eastern Himalaya. Our dating indicates the region experienced magmatic events at 1745-1760 Ma, 825-878 Ma, 480-520 Ma, and 28-20 Ma. The first three events also occurred in the northeastern Indian craton, while the last is unique to the Himalaya. Correlation of magmatic events and age-equivalent lithologic units suggests that the eastern segment of the Himalaya was constructed in situ by basement-involved thrusting, which is inconsistent with the hypothesis of high-grade Himalaya rocks derived from Tibet via channel flow. The Main Central thrust in the eastern Himalaya forms the roof of a major thrust duplex; its northern part was initiated at ca. 13 Ma, while the southern part was initiated at ca. 10 Ma, as indicated by 40Ar/39Ar thermochronometry. Crustal thickening of the Main Central thrust hanging wall was expressed by discrete ductile thrusting between 12 Ma and 7 Ma, overlapping in time with motion on the Main Central thrust below. Restoration of two possible geologic cross sections from one of our geologic traverses, where one assumes the existence of pre-Cenozoic deformation below the Himalaya and the other assumes flat-lying strata prior to the India-Asia collision, leads to estimated shortening of 775 km (~76% strain) and 515 km (~70% strain), respectively. We favor the presence of significant basement topography below the eastern Himalaya based on projections of early Paleozoic structures from the Shillong Plateau (i.e., the Central Shillong thrust) located ~50 km south of our study area. Since northeastern India and possibly the eastern Himalaya both experienced early Paleozoic contraction, the estimated shortening from this study may have resulted from a combined effect of early Paleozoic and Cenozoic deformation. [ABSTRACT FROM AUTHOR]

The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere, where shortening between the Indian and Capricorn plates is manifest as reverse faulting (5-10-km-spaced faults) and long-wavelength (100-300 km) folding. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. There has been disagreement concerning the likely timing for the onset of deformation between plate-motion inversions and seismic reflection-based studies. In the present study, fault displacement data from seismic-reflection profiles within the central Indian Ocean demonstrate that compressional activity started much earlier than previously thought, at around 15.4-13.9 Ma. From reconstructions of fault activity histories, we shows that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0-7.5 Ma. There is no evidence for any regional unconformity before 8.0-7.5 Ma; early shortening was accommodated by activity on single isolated fault blocks. Total strain estimates derived are more variable and complex than those predicted from plate inversion, and they do not show simple west to east increase. [ABSTRACT FROM AUTHOR]

The Olympus Mons volcano on Mars is notable not only for its immense height and width, but also for substantial asymmetries in its structure. The gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank. Furthermore, the northwest flank exhibits lower-flank extensional faults, whereas the southeast shows upper-flank compressional terraces and lower-flank upthrust blocks. However, both the northwest and southeast flanks exhibit characteristic concave-upward profiles and steep bounding scarps, in contrast to other sectors. The NW-SE asymmetries are aligned with the regional slope from the Tharsis rise, but an understanding of the underlying causes has remained elusive. We use particle dynamics models of growing, spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying Olympus Mons. We find that basal slopes alone are insufficient to produce the observed concave-upward slopes and asymmetries in flank extent and deformation style that are observed at Olympus Mons; instead, lateral variations in basal friction are required. These variations are most likely related to the presence of sediments, transported and preferentially accumulated downslope from the Tharsis rise. Such sediments likely correspond to ancient phyllosilicates (clays) recently discovered by the Mars Express mission. [ABSTRACT FROM AUTHOR]

The article presents the discussion on New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid. Topics include marine red beds of the Oligocene Kulapis Formation showing mafic conglomerates and chloritic shales deriving from nearby ophiolitic sources; and ultramafic rocks normally devoiding of primary zircons but zircon crystallizing in ophiolitic rocks during hydrothermal alteration.

After 120 years of unsuccessful endeavor, a paradigm shift is required before earthquakes can be predicted. The most sensitive diagnostic of low-level changes of stress in in situ rock, variations in microcrack geometry, can be monitored by analyzing shear-wave splitting. The suggested paradigm shift is that, instead of investigating the source zone, we monitor stress accumulation before earthquakes at, possibly, substantial distances from the source. Characteristic temporal variations of shear-wave time delays have been observed in retrospect before 14 earthquakes worldwide. On one occasion, when changes were recognized early enough, the time, magnitude, and fault break of an M = 5 earthquake in southwest Iceland were successfully stress-forecast in a narrow time-magnitude window. Such stress accumulation can be theoretically modeled and is believed to be at least partially understood. When sufficient shear-wave source earthquakes are available, increasing time delays also show an abrupt decrease shortly before the impending earthquake occurs. This is not fully understood but is thought to be caused by stress relaxation as microcracks coalesce onto the eventual fault break. The new result confirming these ideas, and justifying the paradigm shift, is that logarithms of the durations of both increases and decreases in time delays are found to be proportional (self-similar) to the magnitudes of impending earthquakes. [ABSTRACT FROM AUTHOR]

Pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6-1.3 MPa and at seismic slip rates of 1.3-2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO2 gas, as confirmed by CO2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records. [ABSTRACT FROM AUTHOR]

The strengths of mountain belts and major faults have been notoriously difficult to constrain and there is ongoing debate over the controlling mechanisms and stress magnitudes. Here we show that the strengths of active thrust-belt wedges and their basal detachments can be directly determined from the covariation of surface slope a with detachment dip β, without strong assumptions about the specific strength-controlling mechanisms. Even a single taper measurement (α,β) can strongly constrain the set of possible wedge and detachment strengths. This theory is tested with dry sand wedges and then applied to the Niger delta thrust belt, the active Taiwan mountain belt, and the thrust that slipped in the M = 7.6 Chi-Chi earthquake. Their basal detachments are shown to be exceedingly weak, with effective coefficients of friction (0.04-0.1) that are an order of magnitude less than most laboratory friction coefficients (0.6-0.85). In contrast, these wedges are moderately strong internally, within the range of pressure-dependent strengths in deep boreholes. These results confirm the existence of exceedingly weak faults and strong crust, which raises important causal questions. [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]

Sediment delivery to stream channels in mountainous basins is strongly episodic, with large pulses of sediment typically delivered by infrequent landslides and debris flows. Identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins. In July 2001, intense rainfall triggered numerous debris flows in a severely burnt watershed in the Sapphire Mountains of Montana. Ten large debris flow fans were deposited on the valley floor, and investigations focused on the channel response to these sediment pulses. The channel has aggraded immediately upstream of each fan, and braided in reaches immediately downstream. Channel incision through the fans has created sets of coarse-grained terraces. The deposition upstream of the pulses consists almost exclusively of fine material, resulting in a median bed material size (D50) 1-2 orders of magnitude lower than the ambient channel material. The volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans, with 1-2 m of sand deposited across the entire valley floor. Along a 10 km study reach, cross section surveys, longitudinal profiles, and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal. [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]

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]

We present analytical results from four dredge locations across the eastern Zealandia continental margin and adjacent ocean crust. The 115 Ma dacites dredged from the West Wishbone Ridge (WWR) are isotopically primitive, weakly adakitic, slab-derived lavas. The 97 Ma A-type granites and a basalt from the easternmost Chatham Rise enlarge the known area of postsubduction Gondwana magmatism. Amphibolite-grade schists from a fault block south of the Chatham Rise provide a critical bridge between the Zealandia and West Antarctica belts of Jurassic- Cretaceous accretionary prism rocks. The new recognition of the WWR as a remnant of a 115 Ma intraoceanic subduction system means that previous hypotheses of the WWR as a fracture zone or spreading ridge require modification. The dacite ages constrain the start of Osbourn Trough spreading, which caused breakup of the Hikurangi-Manihiki igneous plateau, to before 115 Ma. We speculate that, after 115 Ma, the WWR was rifted by an intraoceanic spreading center that developed along its southeast side. Impingement of this spreading center against the Gondwana margin led to widespread 95-100 Ma postsubduction magmatism, variable lithospheric stretching, and ultimately continental splitting of Zealandia and West Antarctica across basement trends. [ABSTRACT FROM AUTHOR]

We report a magnetostratigraphic and rock magnetic study of the Yaha section, located on the southern flank of the central Tian Shan mountains, Asia. Our results show a two-fold increase in sedimentation rate as well as marked changes in rock magnetic characteristics ca. 11 Ma. After 11 Ma, sedimentation rate remained remarkably constant until at least 5.2 Ma. These findings are consistent with sedimentary records from other sections surrounding the Tian Shan. We conclude that uplift and erosion of the Tian Shan accelerated ca. 11 Ma, long after the onset of the collision between India and Asia, and that the range rapidly evolved toward a steady-state geometry via a balance between tectonic and erosion processes. [ABSTRACT FROM AUTHOR]

Unusual compositions of some oceanic basalts have been attributed to their sources containing continental lithosphere detached during the breakup of Gondwana. However, the processes of how such continental lithospheric material is detached and transported into the ocean basin have not been constrained. Here we identify Walvis Ridge, where it has been argued that Deep Sea Drilling Project (DSDP) Site 525A contains continental material, as a unique location to constrain these processes. Absolute plate motion (relative to the Tristan mantle plume) and relative plate motion (between Africa and South America) of the African plate are oblique to one another, such that tectonic detachment versus hotspot-related thermal erosion should sample spatially separated continental units of different age. We present isotopic compositions of xenoliths representing the neo-Proterozoic lithosphere at the inferred site for tectonic detachment during continental breakup and show that this process does not explain the Walvis Ridge DSDP Site 525A mantle source. Rather, thermal erosion of ancient cratonic mantle by the Tristan mantle plume is indicated. A convective return flow is required to transport the eroded subcontinental lithospheric mantle to the site of plume activity some ∼50 m.y. later and provides constraints on the direction and velocity of mantle flow in the upper mantle. [ABSTRACT FROM AUTHOR]

The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracratonic basin formation and associated tectonics kinematically compatible with protracted ‘Grenville-age’ NW-directed shortening. New U/Pb age determinations from an air-fall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and 40Ar/39Ar K-feldspar thermochronology in the Grand Canyon indicates that basement rocks cooled through 150 °C between ca. 1300 and 12511 Ma, refining exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation (lower Unkar Group) associated with NE-striking monoclinal flexures indicate NW-directed synsedimentary contraction at ca. 1250Ma. A large disconformity (∼75 m.y. duration) is inferred between the lower and upper Unkar Group and is located below the upper Hakatai Shale, as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and full grabens that record NE-SW extension on NW-striking, high-angle normal faults. Several observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, mafic magmatism, and early Nankoweap deposition: (1) intraformational faulting in the Bass Formation, (2) intraformational faulting in the 1070 Ma (old Rb/Sr date) Cardenas Basalt and lower Nan-koweap Formation, (3) syntectonic relationships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between Unkar Group and Nankoweap strata. The two tectonic phases affecting the Unkar Group (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate an Unkar Group source in the Grenville-age highlands of southwestern Laurentia during development of NW-striking extensional basins. We conclude that far-field stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma. [ABSTRACT FROM AUTHOR]

The Early Ordovician Annieopsquotch ophiolite belt occurs immediately west of the main lapetus suture zone, and imposes important constraints on the tectonic processes associated with closure of the peri-Laurentian portion of lapetus. The Annieopsquotch ophiolite, the most prominent ophiolite within the Annieopsquotch ophiolite belt, exposes a 5.5-km-thick section of gabbros, sheeted dikes, and pillow basalts, in which three magmatic episodes have been recognized based on field and geochemical data. The first phase is composed of layered troctolites, which are preserved as enclaves within the gabbro zone. Trace element modeling suggests the troctolites crystallized from boninitic melts. The troctolite substrate was intruded by the dominant, second, tholeiitic magmatic phase, which formed a gabbro-sheeted dike-basalt sequence. All tholeiites have suprasubduction zone chemical characteristics, but the suprasubduction zone signature decreases toward the top of the basalt sequence. The third magmatic episode is composed of primitive dikes, which are interpreted as off-axis intrusions. Other ophiolites within the Annieopsquotch ophiolite belt have comparable geochemical signatures, suggesting they may have constituted a single piece of oceanic lithosphere. Based on geochemical and regional tectonic constraints, the Annieopsquotch ophiolite belt is interpreted to have formed during initiation of west-directed subduction. Fast rollback of the subducting slab would have induced volatile-fluxed decompression melting of previously depleted mantle, yielding boninitic melts. The suprasubduction zone tholeiite sequence would have formed from ascending fertile mantle fluxed with subduction-related fluids as rollback continued. This suggests that the Annieopsquotch ophiolite belt does not represent the remnants of normal oceanic crust or backarc basin crust, as previously thought. Our model constrains the initiation and early evolution of a west-dipping peri-Laurentian subduction zone that was responsible for formation of several arc-backarc complexes currently preserved in the Annieopsquotch Accretionary Tract. [ABSTRACT FROM AUTHOR]

This study addresses multiple controls on foreland-basin accommodation and contributes to enhanced understanding of the evolution of the northern Andes. The Middle Magdalena Valley Basin (MMVB), Eastern Cordillera, and Llanos Basin are part of a Late Cretaceous-Cenozoic foreland-basin system, east of the Colombian Central Cordillera. Mechanical modeling indicates that the primary control on complex distributions of sedimentary thicknesses, facies, and unconformities was lithospheric flexure in response to crustal loads from the Central and Eastern Cordilleras. Shorter-wavelength folding and paleoaltitude determined the local character of strata. Our mechanical modeling consists of the application of orogenic and sedimentary loads extracted from geologic data on a continuous elastic lithosphere. The results validate two major basin configurations. The first configuration was a Maastrichtian-early Eocene foreland basin coupled with Central Cordillera uplift. Growth strata record continuous sedimentation in the Eastern Cordillera, whereas regional unconformities in the Llanos Basin (distal foreland basin) reflect isostatic adjustments of the basin's amplitude and wavelength to Central Cordillera episodic uplift and tectonic quiescence. The second major basin configuration was characterized by Central Cordillera erosion since middle Eocene times recorded by a regional pediment surface. In the absence of Central Cordillera effective loading, loads from onlappiang sediments and Eastern Cordillera piggyback sub-basins provoked post-middle Eocene accommodation in the MMVB and Llanos Basin. Intensified Eastern Cordillera uplift during the Neogene produced basinal tilting recorded by unconformities in the MMVB. This study highlights the importance of assessing the causes of tectonic accommodation as a foundation for interpretation of the evolution of large foreland and intermontane basins. [ABSTRACT FROM AUTHOR]

New 40Ar/39Ar data reveal ages and thermal discontinuities that identity mapped and unmapped fault boundaries in the Potomac terrane in northern Virginia, thus confirming previous interpretations that it is a composite terrane. The rocks of the Potomac terrane were examined along the Potomac River, where it has been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, at least two metamorphic thermal domains were identified, the Blockhouse Point and Bear Island domains, separated by a fault active in the late Devonian. Early Ordovician (ca. 475 Ma) cooling ages of amphibole in the Bear Island domain reflect cooling from Taconic metamorphism, whereas the Blockhouse Point domain was first metamorphosed in the Devonian. The 40Ar/39Ar data from muscovites in a third (eastern) domain within the Mather Gorge Formation, the Stubblefield Falls domain, record thrusting of the Sykesville Formation over the Mather Gorge Formation on the Plummers Island fault in the Devonian. The existence of two distinctly different thermal domains separated by a tectonic boundary within the Mather Gorge argues against its status as a formation. Hornblende cooling ages in the Sykesville Formation are Early Devonian (ca. 400 Ma), reflecting cooling from Taconic and Acadian metamorphism. The ages of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian), marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Our data indicate that net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummets Island fault is east-side-up. Zircon fission-track cooling ages demonstrate thermal equilibrium across the Potomac terrane in the early Permian, and apatite fission-track cooling ages record tilting of the Potomac terrane in the Cretaceous or later with the west side up at least 1 km. [ABSTRACT FROM AUTHOR]

To better constrain the probable timing of formation and evolution of the Altyn Tagh sinistral strike-slip system in the Mesozoic, a 40Ar/39Ar thermochronological study has been carried out in the north-central segment of the Altyn Tagh fault system, the northern margin of the Qaidam Basin, and the eastern Kunlun orogenic belt. Muscovite, biotite, and K-feldspar separated from mylonite, granite, pegmatite, and metamorphic rocks have been analyzed. The range of 40Ar/39Ar data and structural evidence indicate that a peak metamorphic event in terranes bordering the Altyn Tagh fault system occurred between 450 and 420 Ma. At ca. 250–230 Ma there is evidence for initial sinistral strike-slip shearing. Sinistral strike-slip deformation occurred later along the Altyn high fault system at 165–160 Ma and 100–89 Ma, respectively. Cooling histories in the northern margin of the Qaidam Basin and the eastern Kunlun orogenic belt show that these areas also experienced rapid cooling ca. 250–230 Ma, as was the case for the early Altyn Tagh fault system. This regional tectonic and cooling process indicates that the initial formation of the Altyn Tagh sinistral slip fault system occurred in latest Permian Early Triassic time and was coupled with, or related to, suturing in the northern margin of the Qaidam Basin and the Kunlun orogenic belt. Cooling events along the Altyn Tagh fault system between 165 and 160 Ma and 100-89 Ma were accompanied by differential closure along the Bangong Lake-Nujiang suture zone in its eastern and western sectors during the Middle-Late Jurassic and Early Cretaceous, respectively (Zhao et al., 2001; Wang et al., 2002). [ABSTRACT FROM AUTHOR]

Intramontane basins may act as important sediment storage areas, serve as recorders of the history of deformation, record syntectonic deposition, and document the evolution of climatic conditions during deposition. We document the timing, cyclicity, and processes that led to the filling and reexcavation of the intramontane Quebrada del Toro basin in NW Argentina. Geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last ∼8 m.y. The last filling and excavation cycle occurred within the last 0.98 m.y. and has led to the deposition and removal of ∼61.4 km³ of material from the basin, leading to a basin- wide averaged minimum denudation rate of 0.16 mm/yr. Aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland. This portion of the fluvial system is actively incising through an uplifting bedrock zone. We use observations within the Toro to test a quasiphysically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening. Our work shows that the bedrock incision rate constant required to reproduce conditions observed within the Toro basin is consistent with values measured independently in similar rock types. Therefore, in intramontane basins that experience similar processes of filling and evacuation, this model may be used to assess the relative importance of tectonic rock uplift, bedrock resistance to fluvial incision, and climate in determining the geomorphic and sedimentologic history of these basins. [ABSTRACT FROM AUTHOR]

In order to better understand mechanisms of active faults, we studied relationships between fault behavior and rock units along the Hayward fault using a three-dimensional geologic map. The three-dimensional map—constructed from hypocenters, potential field data, and surface map data—provided a geologic map of each fault surface, showing rock units on either side of the fault truncated by the fault. The two fault-surface maps were superimposed to create a rock-rock juxtaposition map. The three maps were compared with seismicity, including aseismic patches, surface creep, and fault dip along the fault, by using visualization software to explore three-dimensional relationships. Fault behavior appears to be correlated to the fault-surface maps, but not to the rock-rock juxtaposition map, suggesting that properties of individual wall-rock units, including rock strength, play an important role in fault behavior. Although preliminary, these results suggest that any attempt to understand the detailed distribution of earthquakes or creep along a fault should include consideration of the rock types that abut the fault surface, including the incorporation of observations of physical properties of the rock bodies that intersect the fault at depth. [ABSTRACT FROM AUTHOR]

We use three large normal fault arrays in the northeastern Basin and Range Province, western United States, to document catchment development and relief production during fault growth. Fault slip and slip rates increase systematically along strike from zero at the fault tips. Catchment relief and across-strike range width both increase as slip accumulates but reach maximum values at a distance of ∼15 km from the fault tips and remain uniform along strike over much of the footwalls. Catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5–6 along strike. We infer that catchments first elongate in the across-strike direction as slip accumulates and the range half-width increases. Once the half-width reaches its maximum value, continued catchment growth is possible only by along-strike capture, which increases outlet spacing but not relief. The close correspondence between catchment relief and range half-width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone. Thus, the limiting factor in footwall development is the width of the range, which is controlled by two external agents: the geometry and spacing of the major faults, and the elevations of base level on both flanks. [ABSTRACT FROM AUTHOR]

An intriguing paradox in European tectonics is that present intracontinental seismicity seems to be broadly distributed, whereas past deformation was restricted to sedimentary basin areas. These basins were created by repeated Mesozoic rifting and later affected by pervasive Late Cretaceous—early Tertiary inversion. We propose that the change from localized deformation to distributed seismicity reflects changes in strength due to an interplay between Neogene thermal perturbations by mantle plumes and stress-induced intraplate deformation. As a result, inversion tectonics in the North Sea Basin area ceased and gave way to lithaspheric folding, which is most pronounced in onshore Paleozoic massif areas. Thermomechanical models demonstrate that this important transition in the mode and distribution of intraplate deformation can be explained by a strength reversal resulting from the combined effects of changes in Moho depth due to the transition from rifting to inversion and heating by mantle plumes. [ABSTRACT FROM AUTHOR]

We quantitatively investigate the relation between nappe stacking and subduction in the Aegean region. If nappe stacking is the result of the decoupling of upper-crustal parts (5–10 kin thick) from subducting lithosphere, then the amount of convergence estimated from balancing the nappe stack provides a lower limit to the amount of convergence accommodated by subduction. The balanced nappe stack combined with the estimated amount of completely subducted lithosphere indicates 700 km of Jurassic and 2400 km of post-Jurassic convergence. From seismic tomographic images of the underlying mantle, we estimate 2100–2500 km of post-Jurassic convergence. We conclude that (1) the imaged slab represents the subducted lithosphere that originally underlay the nappes, (2) since the Early Cretaceous, subduction in the Aegean has occurred in one single subduction zone, and (3) the composition of the original basement of the nappes indicates that at least 900 km of sub-upper-crust continental lithosphere subducted in the Aegean. [ABSTRACT FROM AUTHOR]

It is widely accepted that the thickened oceanic crust of the Caribbean plate, its basaltic accreted margins, and accreted mafic terranes in northwestern South America represent the remnants of a single ca. 90 Ma oceanic plateau. We review geologic, geochemical, and paleomagnetic evidence that suggests that the Caribbean-Colombian oceanic plateau in fact represents the remnants of two different oceanic plateaus, both dated as ca. 90 Ma. The first of these plateaus, the Caribbean Plateau, formed ca. 90 Ma in the vicinity of the present-day Galapagos hotspot. Northeastward movement of the Farallon plate meant that this plateau collided with the proto-Caribbean arc and northwestern South America <10 m.y. after the platcau's main phase of formation. Paleomagnetic evidence suggests that the second of these plateaus, the Gorgona Plateau, formed at 26°–30°S, possibly at the site of the present-day Sala y Gomez hotspot. Over the next ∼45 m.y., this plateau was carried progressively northeastward on the Farallon plate and collided in the middle Eocene with the proto-Andean subduction zone in northwestern South America. The recognition of a second ca. 90 Ma Pacific oceanic plateau strengthens the link between plateau formation and global oceanic anoxic events. [ABSTRACT FROM AUTHOR]

Oxygen, carbon, and strontium isotope variations in vein-filling calcite and quartz Cements and their host rocks are used to elucidate the origin, spatial and temporal evolution, and migration pathways of fluids in the detachment Nuncios fold complex, northeastern Mexico. The folded Mesozoic sedimentary sequence contains two regional paleohydrostratigraphic units separated by a unit of low permeability. Two main generations of cements are present in both paleohydrostratigraphic units. Distinct differences exist between δ18O, δ13C, 87Sr/86Sr, and fluid-inclusion temperatures of early vein-filling cements in the lower and the upper units. These differences, together with a strong correspondence between early cement and host-rock δ18O and δ13C values, suggest that early diagenetic fluids were compartmentalized between the two units. Late vein-filling cements have isotopic compositions and fluid-inclusion temperatures that converge to similar values, indicating a change to open fluid flow between the lower and upper units. We hypothesize that the fluid history of the Nuncios fold complex evolved in two main stages: (1) burial diagenesis and early folding, during which fluids were confined within individual units, and (2) late-stage folding, during which increased deformation associated with fold tightening caused the expulsion of fluid from the lower unit into the upper unit. [ABSTRACT FROM AUTHOR]