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Volcano seismicity is an important tool used to monitor volcanic hazards, as seismic signals are commonly associated with fracturing and the movement of volcanic fluids. In investigating the relation between fluids and seismic signals, we record acoustic emissions (AEs) in the laboratory that result from fracturing and fluid depressurization through the fractured rock. We vary the fluid viscosity to create field analogues of hydrothermal and magmatic fluids. While fracturing causes high-frequency volcano-tectonic (VT)-type AE signals, fluid venting results in VT and long-period AEs with variable but overall lower dominant frequencies. The viscosity of the vented fluid correlates (1) inversely with the peak dominant frequency, (2) inversely with the number of induced AEs, (3) proportionally to the onset time of AEs, and (4) inversely with the initial rate of AE generation. Tremor was observed only in the early stages of low-viscosity fluid venting and thus may be more associated with gases and hydrothermal fluids than with magma. [ABSTRACT FROM AUTHOR]

Understanding the thermal evolution of the Alleghanian orogen in the southernmost Appalachians (southeastern United States) is key to understanding the orogenic collapse and extensional tectonics that led to the breakup of Pangea. Cooling-age contours derived from 40Ar/39Ar ages of hornblende, muscovite, and biotite from granitic plutons and their country rocks show a regional pattern of younging toward the southeast. This cooling-age gradient, along with the distribution of normal faults, discontinuities in metamorphic grade, and rifting of the South Georgia basin suggest that the southernmost Appalachian crystalline rocks between the Brevard fault and the Coastal Plain unconformity were exhumed as the lower plate of an extensional detachment fault system. The Suwannee and Charleston terranes likely preserve part of the upper plate. These features represent the postorogenic collapse of the southernmost Appalachians probably along an extensive detachment fault system in the Permian-Middle Triassic. This episode of extension may have played a crucial role in thinning the continental crust to its current thickness and likely was the first stage of extension that eventually led to the breakup of Pangea and formation of the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

The concave-inboard (concave toward the overriding plate) geometry of most convergent margins is considered a natural consequence of the depression of the edge of a thin spherical cap, whereas concave-outboard margin segments commonly form around indenters on the subducting plate. At the Cascadia subduction zone, the apex of a >500-km-long concave-outboard bend in the trench presently shows no obvious subduction of an indenter, but does coincide with the axis of an outboard-facing concavity in upper-plate rocks arched around the Olympic Peninsula in northwestern Washington State, USA. Here we synthesize paleomagnetic and structural data together with new analyses of Global Navigation Satellite System data to show that the upper plate at Cascadia has been folded from the Miocene to the present into an orocline with an axial trace that bisects the Olympic Peninsula. The processes that accommodate bending, which we suggest include (1) folding by flexural slip on the orocline limbs and (2) shortening, uplift, and escape within the core of the fold at the Olympic Mountains, have the combined result of relative motion of the forearc towards the arc at the core of the orocline, and sustained opposing rotations of the upper plate on the orocline limbs. We propose that oroclinal bending is promoted and maintained by along-strike variations in plate-boundary tractions resulting from the geometry of the plate interface at depth and suggest that these processes can contribute to the development of concave-outboard margins without the need for a subducting indenter. [ABSTRACT FROM AUTHOR]

The late Miocene Messinian salinity crisis (MSC) was a significant oceanographic event that caused widespread evaporitic accumulation throughout the Mediterranean Basin. Although multiple hypotheses exist regarding the origin of evaporitic and post-evaporitic deposits, researchers remain divided on the magnitude of base-level fall, and on whether these accumulations record deep-water or non-marine conditions. Here, we introduce a previously unknown, upper Messinian fluvial deposit comparable in size to the late Miocene Nile River fluvial valley fill and show that near-complete desiccation of the eastern Mediterranean was responsible for its development. The basin-wide accumulation, which is located offshore Cyprus, Syria, Lebanon, and Israel, lies directly atop deep-basin evaporites and related erosional surfaces, and is one of the largest known riverine deposits associated with the terminal MSC. From marked onshore incision and basinward thinning trends, the source of the accumulation is presumed to be a formerly unidentified drainage basin in southern Turkey and western Syria; the deposit extends >500 km into the western Levant Basin, where its depositional sink is marked by six well-developed backstepping lobes. Based on the deposit's seismic stratigraphy and morphology, which provide clear evidence of subaerial exposure, we question current hypotheses proposing a deep-water origin for late Messinian accumulations. We also draw specific attention to the development of extensive circum-Mediterranean non-marine conditions prior to Zanclean marine transgression, and to the previously overlooked role of fluvial systems in diluting hypersaline lakes in evaporitic basins. [ABSTRACT FROM AUTHOR]

Models of orogens identify subduction of oceanic crust as the key mechanism leading to continental collision. Such models, based inter alia on thermobarometric and geochronological evidence preserved in high-pressure metamorphic rocks and subduction-related magmatism, have been used to explain the convergence of Europe and Adria in the Cretaceous-Cenozoic and the subsequent Alpine orogen. Here, we review the metamorphic, igneous, and sedimentary record of the past 300 Ma of the Alpine orogen to show that there is no evidence of igneous activity during subduction initiation and prograde high-pressure metamorphism, leading to an ~50 Ma hiatus in magmatism, or "arc gap." The closure of rift basins forming the Piemont-Liguria ocean did not follow a classical Wadati-Benioff-type subduction. Instead, subduction initiation at passive margins allowed for the accretion of the hydrated portion of the subducting plate within an orogenic wedge as subduction of dry subcontinental lithosphere inhibited magmatism during subduction initiation and ocean closure. [ABSTRACT FROM AUTHOR]

Thrust zones in a deep-water fold-thrust belt, offshore northwestern Borneo, display prominent reflections that can be mapped through a three-dimensional seismic volume. Unlike faultplane reflections obtained from thrusts in other systems, these have positive polarity. Well data show that the reflections in the Borneo data set originate from fault-bound sandstone slices, with porosity-occluding calcite cement, entrained along the thrust zone. The thrust zone can support elevated fluid pressures beneath the fault with the cemented sandstones. Multiple sandstone slices indicate complex patterns of thrust zone localization, perhaps a common feature for deformation in sedimentary multilayers typical of many deep-water depositional successions. [ABSTRACT FROM AUTHOR]

Initiation of subduction is rarely encountered in modern tectonic environments due to its ephemeral and destructive nature. We report the geological and geophysical evidence indicating a transitional phase from buckling to embryonic subduction along the eastern Korean margin. The transition appears to be caused by compressional reactivation of the strike-slip boundary between the continental (Korean Peninsula) and oceanic (Ulleung Basin) crusts since the Early Pliocene. Evidence for compressional reactivation includes (1) a west-dipping major thrust and coincident crustal buckling of the Ulleung Basin; (2) an east-west structural asymmetry inferred from the gravity anomaly and P-wave tomography; and (3) ongoing crustal uplift and high-angle faults along the eastern Korean margin. The juxtaposition of underthrusting and buckling of the crust in the Ulleung Basin, and its associated ubiquitous reverse faulting on the eastern Korean margin, imply the potential development of a new subduction system along the western margin of the East Sea (Japan Sea). We propose that the East Sea comprises two incipient subduction margins (i.e., the Korean and Japanese sides), which are now competing to reach a self-sustaining subduction stage. [ABSTRACT FROM AUTHOR]

A change in tectonics affects erosion rates across a mountain belt, leading to a period of non-steady sediment flux delivery downstream. The nonlinear relationship between tectonics and transient sediment delivery from an erosional catchment makes extraction of paleo-tectonic signals from stratigraphy challenging. We use a numerical landscape evolution model to explore how sediment flux from an erosional watershed responds to non-steady rock uplift. We focus on the time lag between the onset of a rock uplift change and the onset of a corresponding change in the sediment flux and the magnitude of the sediment flux relative to the steady rate. We observe that (1) sediment flux does not always record changes in the rock uplift rate when the duration of a rock uplift interval is less than 25% of landscape response time, or time for a landscape to transition from one steady state to another after a perturbation; (2) sediment flux response to variable rock uplift is positively correlated with the duration of rock uplift intervals; and (3) a nonlinear response between erosion rates and tectonic perturbations can result in increasing sediment flux through time even after rock uplift rate decreases. How quickly the sediment flux signal responds to a perturbation depends on how close the landscape was to steady state before the perturbation. These results illustrate conditions under which tectonic signals have the potential to be stored in the stratigraphic record or lost in an erosional system, and the importance of network dynamics for understanding signal propagation. [ABSTRACT FROM AUTHOR]

Lateral changes in seismic velocity 100-300 km beneath the Appalachian orogen (eastern North America) do not follow the pattern of its major terranes, suggesting that more recent, and possibly ongoing, geodynamic processes are taking place in the sub-lithospheric mantle. One prominent, sharply delineated, seismically slow feature underlying parts of New England (USA) likely reflects a volume of significantly elevated temperatures in the asthenosphere. Using numerous new observations of splitting in seismic shear waves from distant earthquakes, we show that this upper mantle volume also lacks the systematic directional dependence (anisotropy) of seismic wave speed that is ubiquitous beneath most of northeastern North America. This regional anisotropic fabric, which likely forms as the asthenosphere is sheared by North American plate motion, appears to be locally erased beneath central New England, with changes in its strength occurring over distances on the order of 50 km. Highly localized variation in the strength of seismic anisotropy in a region of strongly elevated asthenospheric temperature suggests the presence of a narrow thermal upwelling in the upper mantle beneath New England. The lack of obvious surface expressions (volcanism or uplift) and the small lateral scale of the hypothesized upwelling suggest a geologically recent phenomenon. [ABSTRACT FROM AUTHOR]

Hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes, including the Namibian Damara belt. Here, subduction-related deformation occurred at temperatures σ550 °C, and vein geometry is consistent with plate interface shear, low effective normal stresses, and mixed-mode deformation. Quartz vein δ18O values relative to Standard Mean Ocean Water (SMOW) range from 9.4‰ to 17.9‰ (n = 30), consistent with precipitation from metamorphic fluids. A dominant subset of quartz veins away from long-lived high-strain zones and basaltic slivers have δ18O values in a smaller range of 14.9‰ ± 1‰, requiring precipitation from a fluid with δ18O of 12‰ ± 1‰ at 470-550 °C. This uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime. In active subduction zones, brittle deformation within the plastic regime is inferred from observations of tectonic tremor, a noise-like seismic signal including overlapping low- and very low-frequency earthquakes, which occurs below the seismogenic zone. Both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure, could represent a mixture of shear and dilatant failure, and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime. [ABSTRACT FROM AUTHOR]

The 3000-km-long Transantarctic Mountains (TAMs), which separate cratonic East Antarctica from tectonically active West Antarctica, remain one of the least understood of Earth's major mountain ranges. The tectonic mechanism that generates the high elevation, as well as the processes that produce major differences between various sectors of the TAMs, are still uncertain. Here we present newly constructed seismic images of the crust and uppermost mantle beneath central Antarctica derived from recently acquired seismic data, indicating ongoing lithospheric foundering beneath the southern TAMs. These images reveal an absence of thick, cold cratonic lithosphere beneath the southern TAMs. Instead, an uppermost-mantle slow seismic anomaly extends across the mountain front and 350 km into East Antarctica, beneath a high plateau near the South Pole. Under the slow anomaly, a relatively high-wavespeed root is found at ~200 km depth, connected with the East Antarctic lithosphere, suggesting that sinking lithosphere has been replaced at shallow depths by warm, slow-velocity asthenosphere. A mantle lithosphere foundering model is proposed to interpret these images, which best explains the present large area of high elevation and the uplift of the TAMs, as well as Miocene-age volcanism in the Mount Early region. [ABSTRACT FROM AUTHOR]

Where short-term (10²-104 yr) tectonic deformation rates derived from paleoseismology and tectonic geomorphology differ from long-term (105-106 yr) rates derived from geologic and thermochronometric methods, opportunities arise to assess the causes of spatio-temporal variations in fault slip rates. Such discrepancies are germane to understanding seismic hazard and fault interactions in regions such as southern California, USA, that are underlain by complex fault systems prone to large earthquakes on blind thrust faults. We present thermochronometric results from two transects in the hanging wall of the Malibu Coast fault in the Santa Monica Mountains, on which limited evidence for recent earthquakes has been found. Apatite (U-Th-Sm)/He ages from these transects range from late Miocene to Pleistocene but are dominantly Pliocene. Zircon (U-Th)/He ages from Cretaceous-Paleocene strata are partially reset. Inverse thermal modeling of these data indicates that the most deeply buried samples reached peak temperatures of ~150 °C (~5 km depth) at 5-7 Ma, and were then exhumed at rates of ~1 mm/yr. These long-term exhumation rates are faster than short-term fault slip rates on which current hazard assessments are based. [ABSTRACT FROM AUTHOR]

The tectonic origin of the Pennsylvanian-Permian Ancestral Rocky Mountain system is debated. Most models invoke stress from collision along the Ouachita-Marathon belt as the primary driver for Ancestral Rocky Mountain (ARM) (western USA) tectonism, but no existing model has convincingly explained the timing, northeastsouthwest contractional kinematics, and amagmatic nature of this system. We propose that stress generated by transpressional deformation along the southwestern (Sonora) margin of Laurentia combined with that from collision along the Ouachita-Marathon and Nevada margins drove uplift and basin development of the ARM. This model explains ARM timing, uplift kinematics, and lack of magmatism, and is supported by a new generation of late Paleozoic global plate reconstructions. [ABSTRACT FROM AUTHOR]

Serpentinization is a widespread process that affects large-scale geodynamic processes along plate boundaries, including continental breakup, seafloor spreading, and subduction. Documenting the timing of serpentinization is critical for our understanding of these processes, but direct dating of serpentinites has been challenging or impossible. We present the first application of magnetite (U-Th)/He chronometry to date stages of alteration and cooling in ultramafic rocks. In order to demonstrate the viability of magnetite He dating in these lithologies, magnetite ages were obtained from two ultramafic lithologies of the Kampos mélange belt, a high-pressure-low-temperature subduction complex on the island of Syros, Greece. Magnetite (U-Th)/He measurements of internal fragments from large grains within a chlorite schist and a serpentinite record Miocene exhumation-related cooling ages, whereas smaller grains from the serpentinite record mineral growth associated with hydrothermal fluid flow along Pliocene normal faults. These age results with magnetite trace element geochemistry reveal evidence for multiple episodes of fluid-rock alteration, which has implications for the cooling history and local geochemical exchanges of this high-pressure-low-temperature terrane. This method provides a new tool that may be expanded to investigate the processes and time scales of serpentinization from a variety of tectonic settings. [ABSTRACT FROM AUTHOR]

Proxy temperature reconstructions indicate a dramatic cooling from the Cenomanian to Maastrichtian. However, the spatial extent of and mechanisms responsible for this cooling remain uncertain, given simultaneous climatic influences of tectonic and greenhouse gas changes through the Late Cretaceous. Here we compare several climate simulations of the Cretaceous using two different Earth system models with a compilation of sea surface temperature proxies from the Cenomanian and Maastrichtian to better understand Late Cretaceous climate change. In general, surface temperature responses are consistent between models, lending confidence to our findings. Our comparison of proxies and models confirms that Late Cretaceous cooling was a widespread phenomenon and likely due to a reduction in greenhouse gas concentrations in excess of a halving of CO2, not changes in paleogeography. [ABSTRACT FROM AUTHOR]

Terrestrial water isotope records preserve a history of hydrological cycling that is influenced by past climate and surface topography. δ18O and δD records from authigenic minerals of the western United States display a long-term increase during the Neogene in the vicinity of the Sierra Nevada and the central Rocky Mountains (Rockies), but a smaller increase or decrease in the northern Great Basin. Interpretations of these isotopic trends require quantitative estimates of the influence of climatic and environmental changes on δ18O and δD of soil water. Here we use a coupled atmosphere-land model with water-isotopologue tracking capabilities, ECHAM5-JSBACH-wiso, to simulate precipitation and δ18O responses to elevation-independent changes in Neogene geography, equator to pole temperature gradient (EPGRAD), grassland expansion, and tropical Pacific sea surface temperatures. Both precipitation and soil water δ18O (δ18Osw) respond strongly to Neogene strengthening of the EPGRAD, but weakly to other forcings. An increase in EPGRAD leads to significant drying and 18O enrichment (3‰-5‰) of soil water over the northern Sierra Nevada and central Rockies as a result of Hadley circulation strengthening and enhanced coastal subtropical subsidence. These large-scale circulation changes reduce inland moisture transport from the Pacific Ocean and Gulf of Mexico. Our simulated δ18Osw responses could explain 50%-100% of the proxy δ18O increases over the Sierra Nevada and central Rockies, suggesting that climate change rather than surface subsidence may have been the dominant climate signal in δ18O records in these regions. On the contrary, δ18O responses to climate changes are small in the Great Basin, indicating that the observed δ18O increase over this region was likely a direct response to surface subsidence with elevation losses of 1-1.5 km. Adding this elevation loss to current Great Basin elevations reveals the former existence of a uniformly high plateau extending from the Sierra Nevada to the central Rockies prior to Neogene extension. This revised elevation history brings Neogene δ18O and δD paleoaltimetry of the western United States in accordance with independent lines of structural evidence and early Cenozoic elevation reconstructions. [ABSTRACT FROM AUTHOR]

The Weber Deep--a 7.2-km-deep forearc basin within the tightly curved Banda arc of eastern Indonesia--is the deepest point of the Earth's oceans not within a trench. Several models have been proposed to explain the tectonic evolution of the Banda arc in the context of the ongoing (ca. 23 Ma-present) Australia-Southeast Asia collision, but no model explicitly accounts for how the Weber Deep achieved its anomalous depth. Here we propose that the Weber Deep formed by forearc extension driven by eastward subduction rollback. Substantial lithospheric extension in the upper plate was accommodated by a major, previously unidentified, low-angle normal fault system we name the "Banda detachment." High-resolution bathymetry data reveal that the Banda detachment is exposed underwater over much of its 120 km down-dip and 450 km lateral extent, having produced the largest bathymetric expression of any fault discernable in the world's oceans. The Banda arc is a modern analogue for highly extended terranes preserved in the many regions that may similarly have "rolled open" behind migrating subduction zones. [ABSTRACT FROM AUTHOR]

Using silicone slabs as a model analogue for lithospheric plates subducting into a box of glucose syrup, as an analogue of the mantle, we explore the subduction of continental lithosphere in a context of intercontinental collision. The continental indenter pushed by a piston, reproducing the collision, attached to a dense oceanic plate, subducts to two-thirds of the depth of the mantle box. We show that, surprisingly, the continental plate attached to the back wall of the box subducts, even if not attached to a dense oceanic slab. The engine of this subduction is not the weight of the slab, because the slab is lighter than the mantle, but the motion of the piston, which generates horizontal tectonic forces. These are transmitted to the back wall plate through the indenter and the upper plate at the surface, and by the advancing indenter slab through the mantle at shallow depth. We define this process as collisional subduction occurring in a compressional context. The collisional subduction absorbs between 14% and 20% of the convergence, and represents an unexplored component of collisional mass balance. The transmission of tectonic forces far from the collision front favors the formation of a wide plateau. Our experiments reproduce adequately the amount and geometry of the Asian lithosphere subduction episodes inferred during the collision, leading us to conclude that it reproduces adequately the physics of such process. [ABSTRACT FROM AUTHOR]

Submarine and fluvial channels exhibit qualitatively similar geomorphic patterns, yet produce very different stratigraphic records. We reconcile these seemingly contradictory observations by focusing on the channel belt scale and quantifying the time-integrated stratigraphic record of the belt as a function of the scale and trajectory of the geomorphic channel, applying the concept of stratigraphic mobility. By comparing 297 submarine and fluvial channel belts from a range of tectonic settings and time intervals, we identify channel kinematics (trajectory) rather than channel morphology (scale) as the primary control on stratigraphic architecture and show that seemingly similar channel forms (in terms of scaling) have the potential to produce markedly different stratigraphy. Submarine channel belt architecture is dominated by vertical accretion (aggradational channel fill deposits), in contrast to fluvial systems that are dominated by lateral accretion (point bar deposits). This difference is best described with the channel belt aspect ratio, which is 9 for submarine systems and 72 for fluvial systems. Differences in channel kinematics and thus stratigraphic architecture between the two environments appear to result from markedly different coupling between channel aggradation and overbank deposition. The methodology and results presented here are also applicable to interpreting channelized stratigraphy on other planets and moons. [ABSTRACT FROM AUTHOR]

Evidence for a long-lived, high-elevation plateau contradicts the hypothesis that uplift of the southern Tibetan Plateau was a driver of late Miocene-Pliocene changes in ecology and monsoon strength. We illuminate the mechanisms underlying late Miocene-Pliocene environmental changes using a multi-proxy record from the Zhada Basin, southwestern Tibetan Plateau, between ca. 9.2 and 2.3 Ma. An increase in mean carbonate d18Oc and d13Cc values, decrease in grain size, and onset of lacustrine deposition at 6.0 Ma is attributed to local tectonic damming and transition from a through-flowing fluvial system to a terminal lake. This is followed at 3.5 Ma by a decrease in lake size indicated by synchronous increase in grain size, progradation of lake-margin depositional systems, increase in d18Oc values at lake-central locations, and divergence in d18Oc values between inflowing and lake water. We attribute lake shrinking to decreasing Indian summer monsoon precipitation due to the combined effects of decreased southern Tibetan Plateau elevations and cooler Indian Ocean temperatures. We attribute increased grain size and d18Oc value variability to changes in fluvial discharge due to increased sensitivity to orbital forcing, itself possibly coupled with onset of glacial conditions at high elevations. These mechanisms link regional tectonic events to Pliocene changes in climate and environmental conditions, including erosion, granularity, sediment accumulation rates, and potentially biological turnover on and around the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

There remains substantial debate concerning the relative roles of tectonics and global climate in driving the evolution of climate in Central Asia. Today, interior Asia--including the Taklamakan, Gobi, and Ordos Deserts--is exceptionally arid and is surrounded by distinct rainfall boundaries, such as those generated by the Asian monsoon systems to the east and south and those generated by high topography to the north and west. Determining how and why these boundaries have varied over the Neogene is hindered by the lack of a single proxy that can be broadly applied through space and time. We construct isoscapes of pedogenic carbonate δ13C (δ13Cc) over the Neogene in Asia by combining a compilation of 2236 published measurements with new data from three localities in northern Central Asia. Pedogenic carbonate δ13C records local aridity--excepting localities impacted by C4 grasslands and during large changes in atmospheric pCO2--through variations in soil respiration, depth of carbonate formation, and the effect of water stress on plant δ13C. Together, these effects reflect changes in both primary productivity and mean annual precipitation. Throughout the Neogene, we find consistent and exceptionally high δ13Cc in interior Asia with a ring of lower δ13Cc that demarcates higher precipitation. This persistent ring of lower δ13Cc corresponds in the south and east with the influence of the Asian monsoon systems; in the west and north, it reflects both orographic rainfall due to uplift of the Tian Shan and to moisture delivery by the mid-latitude westerlies. Finally, δ13Cc and, hence, aridity increases regionally in the latest Neogene, reflecting the effects of Northern Hemisphere glaciation and cooling. This widespread "de-greening" would have increased regional albedo and modified basin-scale water balances, resulting in greater dust fluxes due to reduced vegetative cover and precipitation. [ABSTRACT FROM AUTHOR]

We present a regional model of plate geometry and kinematics of Southeast Asia since the Late Cretaceous, embedded in a global plate model. The model involves subduction polarity reversals and sheds new light on the origin of the subduction polarity reversal currently observed in Taiwan. We show that this subduction zone reversal is inherited from subduction of the proto-South China Sea plate and owes its current location to triple junction migration and slab rollback. This analysis sheds new light on the plate tectonic context of the Taiwan orogeny and questions the hypothesis that northern Taiwan can be considered an older, more mature equivalent of southern Taiwan. [ABSTRACT FROM AUTHOR]

In contrast to the long narrow volcanic chains in the Pacific Ocean, Atlantic hotspot tracks, in particular in the South Atlantic (e.g., Tristan-Gough, Discovery, Shona, and Bouvet), are irregular and, in some cases, diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure, and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ), and Meteor Rise. Six samples yielded ages of 83-72 Ma and are 10-30 m.y. younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with seafloor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions range from compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49°-55°S), indicating the existence of a Shona hotspot as much as 84 m.y. ago and its derivation from a source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages, and geochemistry indicate that the Richardson, Meteor, and Orcadas seamounts originally formed as a single volcano that was dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin. [ABSTRACT FROM AUTHOR]

Although the Himalayan range is classically presented as cylindrical along strike, segmentation of the range in terms of structure, topography, precipitation, and erosion patterns is becoming widely recognized. The potential climatic or tectonic controls on these lateral variations remain controversial. Thermokinematic models predict that the geometry of the main Himalayan detachment controls the kinematics, exhumation, and topography of the orogen: where the detachment includes a major crustal ramp, the topography shows a steep gradient that focuses orographic precipitation and exhumation, whereas the topography is gentler and exhumation less focused above a flatter detachment. We test this prediction by comparing the patterns of river incision (specific stream power) and long-term exhumation (from apatite fission track thermochronology) in central Nepal, where a major crustal ramp has been imaged by geophysical methods, with new exploratory data from the remote Karnali River transect in western Nepal, where a ramp is predicted to be absent or minor. Our results show that both exhumation rates and river incision capacity are significantly higher and focused on the crustal ramp in central Nepal, whereas they are lower and the pattern is more diffuse in western Nepal. These differences support a model in which lateral variations in topography and exhumation are controlled by variations in the geometry of the detachment, and imply that along-strike climatic variations in the Himalaya respond to tectonics rather than driving it. [ABSTRACT FROM AUTHOR]

The current view regarding the timing of regionally developed penetrative tectonic fabrics in sedimentary rocks is that their development postdates lithification of those rocks. In this case, fabric development is achieved by a number of deformation mechanisms, including grain rigid body rotation, crystal-plastic deformation, and pressure solution. The latter is believed to be the primary mechanism responsible for the domainal structure of cleavage in low-grade metamorphic rocks. In this study we combine field observations with strain studies to characterize considerable (>50%) Acadian crustal shortening in a Devonian clastic sedimentary sequence from southwest Ireland. Despite these high levels of shortening there is a marked absence of the domainal cleavage structure and intraclast deformation that are expected with this level of deformation. Fabrics in these rocks are predominantly a product of rigid body rotation and repacking of extraformational clasts during deformation of a clastic sedimentary sequence before lithification was complete. [ABSTRACT FROM AUTHOR]

The sedimentary cover of the North American craton preserved little evidence of the Paleozoic tectonic events that shaped the Appalachian orogen on its eastern side. A notable exception is the NNWtrending Hudson Bay central high, which corresponds to a normalfault array extending for a minimum length of 500 km. A working hypothesis is proposed in which stresses applied to the continental margin during the Silurian-earliest Devonian Salinian orogeny were transmitted over a distance of >1400 km in the continental interior, where they induced the normal-fault reactivation of older structural discontinuities. The shutdown of tectonic activity along the Hudson Bay central high during the latest Early Devonian to earliest Middle Devonian is interpreted as resulting from a change in the direction of plate convergence during the Acadian orogeny. [ABSTRACT FROM AUTHOR]

Accretionary and collisional orogenesis in the Tarim and North China cratons during Paleozoic time can be correlated with events associated with the assembly and subsequent incipient dispersal of Gondwana. Zircon U-Pb and Hf isotopic data from the northern margins of the two cratons and neighbors have revealed comparable ∊Hf(t)-time patterns. Zircons with magmatic ages of 500-400 Ma display a large spread and decreasing ∊Hf(t) values with time, whereas 400-310 Ma zircons have dominantly positive ∊Hf(t) values and an overall increasing trend. The marked shift of the zircon Hf array at ca. 400 Ma was most likely related to a major tectonic switch from advancing to retreating accretionary orogenesis, corresponding to the development of regional extension. The commencement of subduction at 500 Ma and establishment of an advancing accretionary orogen along the preexisting passive margin was synchronous with early Paleozoic continental collisional events along the southern margins of the two cratons. The temporal agreement of these events, and their accordance with collision and/or accretion events during Gondwana assembly, suggest that the Tarim and North China cratons likely collided with the northern Australia margin of East Gondwana at ca. 500 Ma. They subsequently dispersed from Gondwana in the Early Devonian, coinciding with switching accretionary tectonics along the northern margins of the two cratons that were possibly induced by the slab rollback of the subducting paleo- Asian Ocean plate. [ABSTRACT FROM AUTHOR]

Kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones. The appropriate plane in which to interpret shear sense indicators is the vorticity normal surface, which is often inferred using fabric (foliation, lineation) orientation. Strain modeling, however, suggests that such fabrics can be unreliable for determining a kinematic framework. We demonstrate the application of a new quantitative method, crystallographic vorticity axis (CVA) analysis, that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale. We apply CVA analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries. In all cases, the aggregate of calculated grain-scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis. This new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information, and without assumptions about the kinematics of deformation. [ABSTRACT FROM AUTHOR]

Identifying topographic and erosion rate response to tectonic and climatic forcing remains challenging. This is in part because of the difficulty in isolating the respective roles of climate and tectonics. Here we exploit 2500 thermochronometric data points collected over several decades of research, using a new inverse technique, to image the space-time evolution of erosion rate across the European Alps over the past 35 m.y. The most striking feature of our results is a two- to three-fold increase in erosion rate over the past 2 m.y. exclusively within the Western and Central Alps. This increase appears to be controlled by the inferred high rock uplift rate due to the progressive detachment of the European slab under the Western Alps. The similarity in mean elevation between the Western and Eastern Alps indicates a surprisingly low topographic response to this differential tectonic forcing, and points to the role of enhanced glacial erosion in response to surface uplift. [ABSTRACT FROM AUTHOR]

We suggest a new explanation for the presence of crustally derived zircons in the upper-mantle rocks of ophiolitic complexes, as an alternative to subduction-related models. Integrated isotopic (U-Pb, Hf, and O isotopes) and trace-element data for zircons from the Tumut ophiolitic complex (southeast Australia) indicate that these grains are related to granitic magmatism and were introduced into the mantle rocks after their emplacement into the crust. These observations emphasize that a clear understanding of the origin of individual zircon populations and their relationship to the host rock is essential to interpretations of the tectonic history of upper-mantle rocks and the dynamics of crust-mantle interactions. [ABSTRACT FROM AUTHOR]

Reverse faults frequently generate large and destructive earthquakes, yet their seismic hazard remains difficult to assess with traditional paleoseismic tools because their surface expressions are often complex and subtle. This contribution assesses the utility of millennial-scale denudation rates derived from in-situ cosmogenic 10Be for revealing the spatial patterns and magnitudes of rock uplift produced by slip along reverse faults. We present seventeen basin-averaged denudation rates from rivers draining the Santa Cruz Mountains along the San Andreas fault (California, USA) which closely reproduce known uplift rate patterns associated with a restraining bend along the fault. An additional component of vertical deformation appears to be superposed on the uplift due to the restraining bend; this may result from regional transpression, further irregularities in the fault trace, or interactions with neighboring faults. Our results indicate that 10Be-derived denudation rates can reveal patterns of rock uplift adjacent to reverse faults over length-scales relevant for characterizing their seismic hazard potential. [ABSTRACT FROM AUTHOR]

In the year following the end of the 2009 eruption of Redoubt Volcano, Alaska, four significant swarms of low-frequency, low-magnitude (ML < 0.1) earthquakes occurred at shallow depths beneath the summit. Because swarms of low-frequency (LF) earthquakes preceded eruptions in 1989 and 2009, the posteruption swarms caused considerable concern and prompted the Alaska Volcano Observatory to raise the monitoring levels on three occasions. None of these swarms led to eruptions, however, and most observers (including us) initially concluded that the swarms had been caused by minor stress adjustments in the new lava dome or in the surrounding summit glaciers. New observations reveal that the shallow LF swarms were accompanied by 2 families of repeating earthquakes at depths between 3 km and 6 km below sea level, where the magma storage region is thought to be. These mid-crustal volcano-tectonic (VT) type earthquakes were identical to earthquakes recorded during the 2009 Redoubt eruption more than 6 months earlier. Focal mechanisms demonstrate that these earthquakes have thrust mechanisms inconsistent with the strike-slip nature of regional faulting. Based on these observations, we conclude that they are generated through processes occurring within the magma storage region. The concurrence of the repeating VT earthquakes with the shallow LF swarms indicates that the shallow LF earthquakes were also mag-matically driven. Our results emphasize that even brief episodes of low-amplitude earthquake activity, such as the LF swarms observed at Redoubt following the 2009 eruption, can be indicative of mag-matic activity. Perhaps more significant, however, is the demonstration that the conduit system at Redoubt remained active, intact, and capable of transporting heat and fluids to the surface months after the eruption was considered over. [ABSTRACT FROM AUTHOR]

Rivers are a major component of sediment routing systems that control the transfer of terrigenous sediments from source to sink. Although it is widely accepted that rivers are perturbed by millennial-scale climatic variability, the extent to which these signals are buffered or transferred down river systems to be recorded in sediments at or beyond the river mouth remains debated. Here, we employ a physically based numerical model to address this outstanding issue. Our model shows that river transport strongly amplifies high-frequency sediment flux variations arising from changing water discharge, due to positive feedback between discharge and the channel gradient. This behavior is distinctly different from short-period sediment flux signals (with constant water discharge) where the output sediment flux is strongly dampened within the river, due to negative feedback between the channel gradient and sediment concentration. We conclude that marine sedimentary basins may record sediment flux cycles resulting from discharge (and ultimately climate) variability, whereas they may be relatively insensitive to pure sediment flux perturbations (such as for example those induced by tectonics). [ABSTRACT FROM AUTHOR]

The extensional South Tibetan fault system (STFS) crops out for more than 2000 km along the length of the Himalayan range crest. Interpretations of the signifi cance of this structure in Himalayan tectonics vary. While many regard it as rooted northward beneath Tibet, with offset estimates ranging as high as >200 km, others regard it as a relatively minor, rootless structure. Such inconsistent interpretations are possible because, except for the Everest region, where glacial erosion has exposed a cross section of the STFS, there are few direct constraints on the amount of slip on this feature. Arguments for very large displacements have been based on indirect lines of evidence (e.g., geothermobarometric data) that are multiply interpretable. However, new fi eld work and remote sensing data reported here directly support the petrologic arguments for large displacements, at least in the eastern Himalaya. Near the western boundary of Bhutan, the trace of the STFS is apparently offset ~70 km across a NNE-SSW--trending graben, the Yadong cross structure. This offset has been attributed by previous authors to left-lateral slip along the Jomolhari fault system, which bounds the eastern edge of the graben. In fact, the basal structure of the STFS is not offset, but forms a regionally continuous low-angle detachment surface. The outcrop pattern of the structure, kinematic indicators in fault-related tectonites, and the lack of matchable correlative hanging wall and footwall units require a minimum displacement of ~65 km for the detachment. Such displacements are comparable with estimates for major contractional structures in the eastern Himalaya, such as the Main Central thrust system, and confirm the importance of STFS extension in the evolution of the orogenic system. [ABSTRACT FROM AUTHOR]

Identifying the processes that initiate and control large explosive eruptions on time scales of human interest is challenging. We present evidence for syneruptive, subsurface interactions between rhyolite magmas derived from distinctive sources that reflect tectonic processes starting, then temporarily stopping, the 27 ka Oruanui supereruption (530 km3 magma; Taupo, New Zealand). Subordinate (3%--16%) biotite-bearing pumice lapilli occur in fall deposits from phases 1 and 2 (of 10) among the biotite-free "normal" pumices in the Oruanui deposits. The biotite-bearing magma is sourced to an independent magmatic system 10--15 km northeast of Taupo. Lateral migration of the biotite-bearing magma was initiated during a rifting episode that also triggered the Oruanui eruption. Limited physical mingling indicates that the biotite-bearing and Oruanui biotite-free magmas interacted in the conduit simultaneously with eruption. After phase 1, a time break of months elapsed before phase 2 explosive activity resumed from the same vent, again with intermixed biotite- bearing magma. A rift-related tectonic event, with its accompanying stress change, is inferred to have initiated diking from northeast to southwest, transporting biotite-bearing magma and triggering the Oruanui eruption. Stress relaxation following the rifting event terminated phase 1 before a tectonic event caused renewed diking and activity in phase 2. Tectonics may trigger large silicic eruptions through rupturing of shallow crustal magma chambers and lateral magma transport, especially in extensional regimes, without necessarily leaving any discernible geological imprint. Only the presence of a "foreign" magma in the Oruanui deposits allows the tectonic control to be identified. [ABSTRACT FROM AUTHOR]

The distribution and character of Venus's impact craters led to the widely accepted idea that Venus underwent global catastrophic resurfacing ca. 500 Ma, and thus Venus records only a short history, encompassing surface evolution since postulated catastrophic resurfacing. Ribbon tessera terrain (RTT), a structurally distinctive unit, represents some of Venus's oldest surfaces, and is widely accepted as forming prior to postulated global catastrophic resurfacing. We constructed a global geologic map of RTT unit exposures and structural trends using National Aeronautics and Space Administration (NASA) Magellan data. Map relations illustrate that RTT displays planet-scale patterns that, together with altimetry, record a rich geologic history that predates proposed global catastrophic resurfacing. [ABSTRACT FROM AUTHOR]

We report new geochemical data for marine sediments sampled in the frontal prism associated with the Costa Rica subduction zone during Leg 205 of the Ocean Drilling Program (ODP). We describe variation in sediment geochemistry with depth as the décollement zone, the interface between overriding and downgoing tectonic plates, is approached. This variation can be explained by three-component mixing of ash, lower plate sediments (LPS), and frontal prism or upper plate sediments (UPS). We detect in-mixing of LPS in localized sediment intervals, amounting to tens of vertical meters of LPS incorporation; no persuasive evidence of LPS transfer into the prism has been shown until this contribution. This inference of fi ne structure in the prism provides new insight into how tectonic kneading of sediments occurs in décollement zones. [ABSTRACT FROM AUTHOR]

Archean cratons are characterized by granite-greenstone belts, by abundant tonalites, trondhjemites, and granodiorites (TTGs), and by the presence of strong and buoyant subcratonic lithospheric mantle (SCLM). Individually, mechanisms for their formation remain controversial, and together they provide no clear picture of the tectonic style prevalent during the late Hadean and Early Archean. Following earlier observation that the dome-and-keel structure of granite-greenstone terranes resembles salt diapirs, we present numerical calculations that show that in pre-Archean to Middle Archean thermal regimes, mafic to ultramafic volcanics overlying a felsic basement will overturn diapirically in as few as 10 Ma, displacing as much as 60% of the volcanics to the lower crust. This suggests that diapirism may have dominated Hadean and Early Archean crustal tectonics. Furthermore, repeated cycles of bimodal volcanism and crustal recycling by diapirism provide a mechanism for making TTGs, leaving a restite that contributes to the SCLM from above. This diapiric tectonism later declined as the Earth cooled and plate tectonics became the dominant paradigm. [ABSTRACT FROM AUTHOR]

Accreted terranes of the Appalachian Iapetan and peri-Gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the Laurentian margin, indicating that during accretion, the shape of the continental margin acted as a template around which accreted terranes were molded. In North Carolina and Newfoundland, post-accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories, no longer concordant with those in Laurentia. A bend in structural trends confined to the peri-Gondwanan realm at the North Carolina-South Carolina state line is interpreted to represent the dextrally displaced outboard portion of the Virginia promontory. In Newfoundland, the Hermitage flexure is interpreted as a dextrally displaced Laurentian promontory that originated along strike to the northeast. In both places, promontories were truncated and dextrally translated for ~220-250 km by a Devonian-Mississippian orogen-parallel transcurrent system, which may well have extended for the length of the eastern Laurentian margin. South of the nearby St. Lawrence promontory, extreme thinning of Appalachian crust beneath the Maritimes Basin is consistent with extension at a releasing stepover. Estimates of extension are consistent with those obtained from promontory offsets. [ABSTRACT FROM AUTHOR]

We employ a new thermodynamic method for self-consistent computation of compositional and thermal effects on phase transition depths, density, and seismic velocities. Using these profiles, we compare theoretical and observed differential traveltimes between P410s and P (T410) and between P600s and P410s (T660-410) that are affected only by seismic structure in the upper mantle. The anticorrelation between T410 and T660-410 suggests that variations in T410 and T660-410 of ∼8 s are due to lateral temperature variations in the upper mantle transition zone of ∼400 K. If the mantle is a mechanical mixture of basaltic and harzburgitic components, our traveltime data suggest that the mantle has an average temperature of 1600 ± 50 K, in agreement with temperature estimates from magma compositions of mid-ocean ridge basalts. We infer a 100 K hotter mantle if we assume the mantle to have a homogeneous pyrolitic composition. The transition-zone temperature beneath hotspots and within subduction zones is relatively high and low, respectively. However, the largest variability in T410 and T660-410 is recorded by global stations far from subduction zones and hotspots. This indicates that the 400 K variation in upper mantle temperature is complicated by tilted upwellings, slab flattening and accumulation, ancient subduction, and processes unrelated to present-day subduction and plume ascent. [ABSTRACT FROM AUTHOR]

Recent studies suggest that as much as half of the organic carbon (OC) undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering. These estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils, freshwater sediments, and river deltas, and thus only minimally contributes to the extraneously old 14C ages of total organic matter often observed on adjacent shelves. Here we test this paradigm by measuring the Δ14C and δ13C values of individual higher plant wax fatty acids as well as the δ13C values of extractable alkanes isolated from the Eel River margin (California). The isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition. A coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated, with pre-aged terrestrial material instead composing a considerable proportion of all organic matter. If these findings are characteristic of other continental margins proximal to small mountainous rivers, then the importance of petrogenic OC burial in marine sediments may need to be reevaluated. [ABSTRACT FROM AUTHOR]

We report here the discovery of a new end-Cryogenian glacial diamictite and an overlying basal Ediacaran cap carbonate within the Tsagaan Oloom Formation of southwestern Mongolia. The identification of the Cryogenian-Ediacaran boundary, coupled with new δ13C chemostratigraphic profiles, facilitates the integration of the Neoproterozoic stratigraphy of Mongolia with records elsewhere. These correlations indicate that a previously unrecognized, -16‰ Cryogenian δ13C anomaly is present in the newly defined Tayshir member (informal) of the Tsagaan Oloom Formation. Furthermore, chemostratigraphic and lithostratigraphic relationships suggest an ~35 m.y. depositional hiatus below the phosphorite-bearing Zunne Arts member (informal) of the upper Tsagaan Oloom Formation and that subsidence renewed in the latest Ediacaran-Early Cambrian. We propose that the lower ~1500 m of the Tsagaan Oloom Formation was deposited on a thermally subsiding passive margin after Rodiniaage rifting, whereas the Zunne Arts member and the overlying ~1600 m of Early Cambrian strata were deposited in a foredeep basin that formed as the southern margin of the Dzabkhan terrane was subducted beneath the Khantayshir-Dariv arc. [ABSTRACT FROM AUTHOR]

Eastern Anatolia is the site of lithospheric thinning, plateau uplift, heating, and synconvergent extension. Using numerical geodynamic experiments, we test the hypothesis that these tectonic anomalies are all related and the consequence of delamination of the mantle lithosphere. Our findings indicate that delamination during plate convergence results in ~2-km-high plateau uplift. The removal of mantle lithosphere induces distinct regions of contraction and thickening, as well as extension and thinning of the crust. The latter occurs even within a regime of plate shortening, although it is muted with increasing plate convergence. Detachment of the delaminating slab results in minor surface topographic perturbation, but only above the delamination hinge. The plateau uplift and pattern of surface contraction and/or extension are consistent with a topographic profile at 42°E and geologically interpreted zone of synconvergent extension at eastern Anatolia. [ABSTRACT FROM AUTHOR]

The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the northwest margin of the Indo-Asian collision zone. To address the quantitative contribution of the earthquake to plate convergence, we performed paleoseismological trench excavations at Nisar Camp site near Muzaffarabad across the middle section of the 2005 surface rupture. The fault strands exposed in the trench cut late Holocene fluvial deposits and record evidence of both the 2005 and a penultimate event, supported by the presence of colluvial deposits and a downdip increase in displacement along the fault strands. The 2005 event produced a net slip of 5.4 m, and the penultimate earthquake exhibits a similar amount of slip. Radiocarbon ages and historical accounts loosely constrain the timing of the penultimate event between 500 and 2200 yr B.P.; however, the exposed section encompasses ~4 k.y. of stratigraphy, suggesting an average interevent interval of ~2 k.y. for the 2005 type events. We thus conclude that the 2005 event did not occur on the plate boundary megathrusts, but on intraplate active faults within the Sub-Himalaya. Consequently, the accumulated elastic strain around the complex northwestern margin of the Indo-Asian collision zone has not been significantly released by the 2005 earthquake. [ABSTRACT FROM AUTHOR]

Silicic magma bodies stabilize the continental margin by redistributing low-density material into the upper crust. We examined the tectonic traits of 91 young, arc-related, silicic calderas to test previous assertions that the nature of voluminous silicic arc volcanism depends on specific tectonic characteristics. We find that caldera occurrence positively correlates with convergence rate except in arcs with backarc spreading, which have few or no calderas. Calderas located on oceanic and young continental crust are predominantly dacitic, whereas rhyolitic calderas are mainly located on Mesozoic or older continental crust under extension. Occurrence does not increase with greater local subduction obliquity or duration of present arc activity. Our determination of controls on the formation and composition of silicic calderas in volcanic arcs contributes to the study of continental evolution. [ABSTRACT FROM AUTHOR]

We refine conventional continental-scale geodynamic models by including conversion of mechanical work done by deformation into heat. The intensity of the shear heating is extracted from the Brace- Goetze strength envelope without any additional model parameters or assumptions. Incorporation of this, certainly present, heating rate into a model may result in up to tenfold stress reduction, which is exceeding the effects of variation of common parameters within their uncertainty limits. Shear heating with lithospheric thickening at high integrated strength solves the puzzle of the heretofore-missing heat source recorded by metamorphism, magmatism, and heat flow in mountain building. However, the mechanism is self-limiting as the rising temperature reduces stress and thus the rate of heat production. Thus this is a self-regulating mechanism maintaining a moderate integrated lithospheric strength consistent with results of model-independent forcebalanced calculations and with surface heat flow measurements. [ABSTRACT FROM AUTHOR]

The Llano uplift of central Texas, United States, exposes the core of a Mesoproterozoic orogenic belt that formed along the southern margin of Laurentia during Grenville time. A new collisional model is proposed that reconciles differences in structural stacking, apparent tectonic transport, and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high-pressure eclogitic rocks, emplacement of ophiolitic rocks, and subsequent late-stage to postcollisional plutonism. Our model proposes that subduction with southward polarity resulted in collision of an exotic arc with Laurentia, emplacement of ophiolitic rocks, and telescoping of the intervening basinal sediments, followed by overriding of the arc and margin of Laurentia by a southern continent with transport toward Laurentia. The model further proposes that convergence led to subduction of the Laurentian margin, resulting in high-pressure metamorphism, but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from Laurentia, in a manner similar to that proposed for the Alpine orogeny. Slab breakoff resulted in upwelling of the asthenosphere, leading to intrusion of juvenile granitic plutons. Subduction along strike caused continued contraction that waned with time. The eastern uplift records continent-arc-continent collision, whereas the western uplift records continent-continent collision; the two regions also expose different crustal levels in the orogen. The striking similarity with Phanerozoic orogens, including emplacement of ophio lites and formation of high-pressure rocks, implies that plate tectonic processes including subduction were active prior to the Neoproterozoic. [ABSTRACT FROM AUTHOR]

The extensional Pannonian Basin was formed in a few million years during Miocene time synchronously with contraction in the surrounding Alpine and Carpathian orogens. This system is characteristic of a class of extensional basins that form in the midst of active orogenic (mountainforming) belts. The mechanism that causes this type of geological event is enigmatic but usually has been associated with subduction. We examine a new hypothesis for intra-orogenic extensional basin formation in which gravitational spreading of previously thickened crust triggers gravitational instability of the mantle lithosphere. A basin is formed by lithospheric extension as shortening and lithospheric downwelling occur in the surrounding mountain belts. This mechanism provides a mechanically self-consistent explanation for all of the main structural features of the Pannonian-Carpathian system and presents a plausible alternative to the popular view that subduction and slab rollback have driven the development of this basin. [ABSTRACT FROM AUTHOR]

The North Pacific Rim is a tectonically active plate boundary zone, parts of which may be characterized as a laterally moving orogenic stream. Crustal blocks are transported along large-magnitude strike-slip faults in western Canada and central Alaska toward the Aleutian-Bering Sea subduction zones. Throughout much of the Cenozoic, at and west of its Alaskan nexus, the North Pacific Rim orogenic Stream (NPRS) has undergone tectonic escape. During transport, relatively rigid blocks acquired paleomagnetic rotations and fault-juxtaposed boundaries while flowing differentially through the system, from their original point of accretion and entrainment toward the free face defined by the Aleutian-Bering Sea subduction zones. Built upon classical terrane tectonics, the NPRS model provides a new framework with which to view the mobilistic nature of the western North American plate boundary zone. [ABSTRACT FROM AUTHOR]

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