Estimated likelihood of observing a large earthquake on a continental low‐angle normal fault and implications for low‐angle normal fault activity

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106807/1/grl51557.pd

The weight of the mountains: Constraints on tectonic stress, friction, and fluid pressure in the 2008 Wenchuan earthquake from estimates of topographic loading

Though it is widely recognized that large mountain ranges produce significant stresses in the Earth's crust, these stresses are not commonly quantified. Nonetheless, near large mountains topography may affect fault activity by changing the stress balance on the faults. In this work, we calculate the stress field from topography in the Longmen Shan (Sichuan, China) and resolve those stresses on several models of the faults that ruptured in the 2008 Mw 7.9 Wenchuan earthquake. We find that the topography results in shear stresses up to 20 MPa and normal stresses up to 80 MPa on the faults, with significant variability across the faults. Topographic stresses generally load the fault in a normal and left‐lateral shear sense, opposite to the inferred coseismic slip sense, and thus inhibit the coseismic slip. We estimate the tectonic stress needed to overcome topographic and lithostatic stresses by assuming that the direction of maximum shear accumulated on the faults is roughly collinear with the inferred coseismic slip. We further estimate the static friction and pore fluid pressure assuming that the fault was, on average, at Mohr‐Coulomb failure at the time of the Wenchuan earthquake. We use a Bayesian inversion strategy, yielding posterior probability distributions for the estimated parameters. We find most likely estimates of maximum tectonic compressive stress near 0.6 ρgz and oriented ∼E‐W, and minimum tectonic stress near 0.2 ρgz. Static friction on the fault is near 0.2, and pore fluid pressure is between 0 and 0.4 of the total pressure.Key PointsTopographic stress is significant on Wenchuan faultsTopographic stresses resist tectonic slipTectonic stress, friction, and fluid pressure estimatedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111754/1/jgrb51070.pd

Evidence for constriction and Pliocene acceleration of east-west extension in the North Lunggar rift region of west central Tibet

This is the publisher's version, also available electronically from http://onlinelibrary.wiley.com/doi/10.1002/tect.20086/abstract;jsessionid=36D445F6B0A54FA5B74E359605FC0AD1.f04t02The active north trending North Lunggar rift in west central southern Tibet exposes an extensional metamorphic core complex bounded by an east dipping low-angle normal fault. Apatite and zircon (U-Th)/He thermochronology and thermal modeling of the North Lunggar rift document a minimum timing for rift inception at >10 Ma and rapid footwall exhumation between 5 and 2 Ma. Miocene footwall cooling and exhumation rates were initially slow to moderate at 400°C Ma−1 and 4–10 mm a−1. Footwall isotherms were significantly compressed during rapid exhumation resulting in an elevated transient geothermal gradient between 50 and 90°C km−1. The minimum magnitude of horizontal extension for the North Lunggar rift is 8.1–12.8 km; maximum is 15–20 km, less in the south at ~10 km. Mean Pliocene extension rate is 1.2–2.4 mm a−1 in the ~120° direction. Results for the North Lunggar rift are similar in magnitude, rate, and orientation of slip to the kinematically linked Lamu Co dextral strike-slip fault to the north. This suggests a state of constrictional strain during Pliocene time along this stretch of the Bangong-Nujiang suture from which the Lamu Co fault emanates. The onset of extension in this region may be explained by crustal thickening and gravitational orogenic collapse, followed by accelerated rifting resulting from localized crustal stretching and increased magmatic activity, potentially driven by the position and northward extent of underthrusting Indian lithosphere

Miocene initiation and acceleration of extension in the South Lunggar rift, western Tibet: Evolution of an active detachment system from structural mapping and (U-Th)/He thermochronology

This is the publisher's version, also available electronically from http://onlinelibrary.wiley.com/doi/10.1002/tect.20053/abstractOngoing extension in Tibet may have begun in the middle to late Miocene, but there are few robust estimates of the rates, timing, or magnitude of Neogene deformation within the Tibetan plateau. We present a comprehensive study of the seismically active South Lunggar rift in southwestern Tibet incorporating mapping, U-Pb geochronology and zircon (U-Th)/He thermochronology. The South Lunggar rift is the southern continuation of the North Lunggar rift and comprises a ~50 km N-S central horst bound by two major normal faults, the west-dipping South Lunggar detachment and the east-dipping Palung Co fault. The SLD dips at the rangefront ~20°W and exhumes a well-developed mylonite zone in its footwall displaying fabrics indicative of normal-sense shear. The range is composed of felsic orthogneiss, mafic amphibolite, and leucogranite intrusions dated at ~16 and 63 Ma. Zircon (U-Th)/He cooling ages are Oligocene through late Pliocene, with the youngest ages observed in the footwall of the SLD. We tested ~25,000 unique thermokinematic forward models in Pecube against the structural and (U-Th)/He data to fully bracket the allowable ranges in fault initiations, accelerations, and slip rates. We find that normal faulting in the SLR began in the middle Miocene with horizontal extension rates of ~1 mm a−1, and in the north accelerated at 8 Ma to 2.5–3.0 mm a−1 as faulting commenced on the SLD. Cumulative horizontal extension across the SLR ranges from <10 km in the south to 19–21 km in the north

Estimating fault slip rates in the Himalaya and Tibet over 10 - 10^6 year timescales

The Himalayan-Tibetan orogen is the highest on the modern Earth and an archetypal region for studying continental collisions. As such, its characteristics have been the basis for many different models for orogenesis, in spite of a lack of data on deformation styles and rates in many parts of the orogen. More data on deformation rates and histories are needed to create, modify or reject hypotheses seeking to explain aspects of this orogeny. This work comprises four studies of deformation over vastly different temporal scales, with a spatial emphasis on the western Himalaya and Tibet. The first study combines global positioning system (GPS) geodesy and structural field observations to study arc-parallel extension and translation of the Himalaya. Arc-parallel extension is estimated at ~3 cm yr-1 over the length of the Himalaya, with the highest rates in eastern Nepal. Arc-parallel translation is expressed as slip on the Karakoram fault and decreases in rate and magnitude from northwest to southeast. Results from this study indicate that a model of variably-oblique convergence between the Indian plate and the Himalaya is likely responsible for the observed deformation, while other models considered fail to match observations. The second study combines field mapping, zircon (U-Th)/He thermochronology, zircon U-Pb geochronology and thermokinematic modeling to determine the deformation history of the previously unstudied South Lunggar Rift in southwestern Tibet. Results indicate that extension started in the middle Miocene and accelerated in the late Miocene to modern horizontal extension rates of 1-3 mm yr-1. Cumulative extension in the rift varies from 3 to 21 km along strike. The third study extends the thermokinematic modeling performed in the South Lunggar Rift to the North Lunggar Rift; key results include a northward propagation in rapid extension that may result from the underthrusting of the Indian plate beneath Tibet. The fourth study is a neotectonic slip rate study on the southeastern Karakoram fault. Mapping results suggest that late Quaternary offsets of geomorphic features may be considerably lower than previously estimated. Slip rate estimates await laboratory results but are likely much lower than earlier estimates, consistent with the oblique convergence hypothesis for Himalayan deformation. These combined results provide much-needed data on deformation rate and style in the orogen and highlight the role of the Indian plate in driving orogenesis

Peruvian Altiplano Stratigraphy Highlights Along-Strike Variability in Foreland Basin Evolution of the Cenozoic Central Andes

Retroarc foreland basins in the Andean plateau contain critical information on geodynamic processes driving plateau development by providing a record of exhumation and sediment sourcing, as well as the timing, location, and magnitude of basin subsidence. However, this record is incomplete along orogenic strike and particularly limited in southern Peru. We measured similar to 6,200m of nonmarine clastic strata in the northern Peruvian Altiplano, documented through lithofacies characterization and paleocurrent analysis, conglomerate clast counts, sandstone petrography, and detrital zircon U-Pb geochronology; for the latter we employ quantitative detrital zircon interpretation methods including multidimensional scaling, mixture modeling, and quantification of zircon roundness. Results show dominant sediment sourcing from the Western Cordillera and/or western Altiplano. Sediment accumulation rates define an upward-convex Paleogene subsidence profile consistent with deposition in a northeastward-migrating flexural foreland basin system, with lithospheric loading from an increasingly proximal Western Cordilleran hinterland. Basin deposition following a 23-9Ma angular unconformity shows a marked increase in sediment accumulation rates >800m/Myr, interpreted as a departure from flexural subsidence. Results highlight along-strike variability in Andean foreland basin evolution, as foredeep deposits are thicker, and the onset of rapid sediment accumulation occurs earlier in southern Peru compared to Bolivia and Argentina. Results tentatively support models of orogenic cyclicity and reveal that episodes of high-flux magmatism in southern Peru are slightly out of phase with those documented in northwest Argentina, which may be controlled by preexisting Paleozoic-Mesozoic structural and stratigraphic fabrics and the rate of underthrusting of melt-fertile continental lower crust and mantle lithosphere.National Science Foundation [EAR-1550097]; National Geographic Society Committee for Research and Exploration; Geological Society of America Grants-in-Aid; International Association of Sedimentologists; Sigma Xi; Joe and Lucy Steward Memorial Endowment; Sam Penn Memorial Endowment; Houston Alumni Association; British Petroleum; Marathon Oil Corporation6 month embargo; published online: 29 May 2018This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]