Evans, Eileen L., et al. "Persistent slip rate discrepancies in the eastern California (USA) shear zone." Geology 44.9 (2016): 691-694.
Understanding fault slip rates in the eastern California shear zone (ECSZ) using GPS geodesy is complicated by potentially overlapping strain signals due to many sub-parallel strike-slip faults and by inconsistencies with geologic slip rates. The role of fault system geometry in describing ECSZ deformation may be investigated with total variation regularization, which algorithmically determines a best-fitting geometry from an initial model with numerous faults, constrained by a western United States GPS velocity field. The initial dense model (1) enables construction of the first geodetically constrained block model to include all ECSZ faults with geologic slip rates, allowing direct geologic-geodetic slip rate comparisons, and (2) permits fault system geometries with many active faults that are analogous to distributed interseismic deformation. Beginning with 58 ECSZ blocks, a model containing 10 ECSZ blocks is most consistent with geologic slip rates, reproducing five of 11 within their reported uncertainties. The model fits GPS observations with a mean residual velocity of 1.5 mm/yr. Persistent geologic-geodetic slip rate discrepancies occur on the Calico and Garlock faults, on which we estimate slip rates of 7.6 mm/yr and <2 mm/yr, respectively, indicating that inconsistencies between geology and geodesy may be concentrated on or near these faults and are not due to pervasive distributed deformation in the region. Discrepancies may in part be due to postseismic relaxation following the A.D. 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes. Otherwise, resolving geologic-geodetic discrepancies would require as much as 11.4 mm/yr of off-fault deformation within <10 km of the main ECSZ faults, with ∼5 mm/yr concentrated near the Calico fault.
Smit, Jeroen, Jan-Diederik van Wees, and Sierd Cloetingh. "The Thor suture zone: From subduction to intraplate basin setting." Geology 44.9 (2016): 707-710.
The crustal seismic velocity structure of northwestern Europe shows a low P-wave velocity zone (LVZ) in the lower crust along the Caledonian Thor suture zone (TSZ) that cannot be easily attributed to Avalonia or Baltica plates abutting the TSZ. The LVZ appears to correspond to a hitherto unrecognized crustal segment (accretionary complex) that separates Avalonia from Baltica, explaining well the absence of Avalonia further east. Consequently, the northern boundary of Avalonia is shifted ∼150 km southward. Our interpretation, based on analysis of deep seismic profiles, places the LVZ in a consistent crustal domain interpretation. A comparison with present-day examples of the Kuril and Cascadia subduction zones suggests that the LVZ separating Avalonia from Baltica is composed of remnants of the Caledonian accretionary complex. If so, the present-day geometry probably originates from pre-Variscan extension and eduction during Devonian–Carboniferous backarc extension. The reinterpretation of deep crustal zonation provides a crustal framework in which the northern limit of Avalonia corresponds to the southern limit of the deep North German Basin and the northern limit of prolific gas reservoirs and late Mesozoic inversion structures.
Hsu, Wei-Hao, et al. "Pleistocene onset of rapid, punctuated exhumation in the eastern Central Range of the Taiwan orogenic belt." Geology 44.9 (2016): 719-722.
The Taiwan orogenic belt is often treated as a steady, southward-propagating orogenic system with an essentially constant erosion rate of 4–6 mm/yr over the past 5 m.y. We present 4 new age-elevation transects from the Central Range based on 19 new and 86 previously published fission track and (U-Th)/He dates of completely reset detrital zircon and apatite grains. The age-elevation curves and thermal models imply slow cooling prior to ca. 2–1.5 Ma (at exhumation rates of ∼0.1 mm/yr), an increase in exhumation rates from ca. 2–1.5 Ma to ca. 0.5 Ma (2–4 mm/yr), and possibly a further acceleration in exhumation from ca. 0.5 Ma to present (4–8 mm/yr). Three transects from three different latitudes in the eastern Central Range yield similar results, each showing punctuated exhumation with progressively faster rates.
Persaud, Patricia, et al. "Fault zone characteristics and basin complexity in the southern Salton Trough, California." Geology 44.9 (2016): 747-750.
Ongoing oblique slip at the Pacific–North America plate boundary in the Salton Trough produced the Imperial Valley (California, USA), a seismically active area with deformation distributed across a complex network of exposed and buried faults. To better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments, we used data primarily from the Salton Seismic Imaging Project to construct a three-dimensional P-wave velocity model down to 8 km depth and a velocity profile to 15 km depth, both at 1 km grid spacing. A VP = 5.65–5.85 km/s layer of possibly metamorphosed sediments within, and crystalline basement outside, the valley is locally as thick as 5 km, but is thickest and deepest in fault zones and near seismicity lineaments, suggesting a causative relationship between the low velocities and faulting. Both seismicity lineaments and surface faults control the structural architecture of the western part of the larger wedge-shaped basin, where two deep subbasins are located. We estimate basement depths, and show that high velocities at shallow depths and possible basement highs characterize the geothermal areas.
AUGUST 2016
Abera, R., J. van Wijk, and G. Axen. "Formation of continental fragments: The Tamayo Bank, Gulf of California, Mexico." Geology 44.8 (2016): 595-598.
We present a model for microcontinent formation that is based on the structure of the Tamayo trough in the Gulf of California, Mexico. Potential field modeling of a transect through the Tamayo trough and Tamayo Bank suggests that the crust of the Tamayo trough is oceanic, and that the Tamayo Bank is a detached continental fragment. The oceanic crust that separates the Tamayo Bank from the mainland of Mexico is thin (5 km), so oceanic spreading was probably magma starved before it ceased. Such a thin crust has also been described on the Aegir Ridge in the North Atlantic, which became extinct after the Jan Mayen microcontinent separated from Greenland. In our model for the origin of microcontinents, the locus of plate spreading jumps to the weakened continental margin when the spreading ridge becomes amagmatic and the force required for continued extension at the ridge increases. A microcontinent is formed when the ridge jumps into a continental margin, and an asymmetric ocean basin or microplate is formed when the ridge jumps within oceanic crust.
Brantut, Nicolas, et al. "Dynamic weakening and amorphization in serpentinite during laboratory earthquakes." Geology 44.8 (2016): 607-610.
The mechanical properties of serpentinites are key factors in our understanding of the dynamics of earthquake ruptures in subduction zones, especially intermediate-depth earthquakes. Here, we performed shear rupture experiments on natural antigorite serpentinite, which showed that friction reaches near-zero values during spontaneous dynamic rupture propagation. Rapid coseismic slip (>1 m/s), although it occurs over short distances (<1 mm), induces significant overheating of microscale asperities along the sliding surface, sufficient to produce surface amorphization and likely some melting. Antigorite dehydration occurs in the fault walls, which leaves a partially amorphized material. The water generated potentially contributes to the production of a low-viscosity pressurized melt, explaining the near-zero dynamic friction levels observed in some events. The rapid and dramatic dynamic weakening in serpentinite might be a key process facilitating the propagation of earthquakes at intermediate depths in subduction zones.
Carrapa, B., et al. "Asymmetric exhumation of the Mount Everest region: Implications for the tectono-topographic evolution of the Himalaya." Geology44.8 (2016): 611-614.
The tectonic and topographic history of the Himalaya-Tibet orogenic system remains controversial, with several competing models that predict different exhumation histories. Here, we present new low-temperature thermochronological data from the Mount Everest region, which, combined with thermal-kinematic landscape evolution modeling, indicate asymmetric exhumation of Mount Everest consistent with a scenario in which the southern edge of the Tibetan Plateau was located >100 km farther south during the mid-Miocene. Northward plateau retreat was caused by erosional incision during the Pliocene. Our results suggest that the South Tibetan Detachment was a localized structure and that no coupling between precipitation and erosion is required for Miocene exhumation of Greater Himalayan Sequence rocks on Mount Everest.
Hubbard, Judith, et al. "Structural segmentation controlled the 2015 Mw 7.8 Gorkha earthquake rupture in Nepal." Geology 44.8 (2016): 639-642.
The ongoing collision of India with Asia is partly accommodated by slip on the Main Himalayan Thrust (MHT). The 25 April 2015, Mw 7.8 Gorkha earthquake is the most recent major event to rupture the MHT, which dips gently northward beneath central Nepal. Although the geology of the range has been studied for decades, fundamental aspects of its deep structure remain disputed. Here, we develop a structural cross section and a three-dimensional, geologically informed model of the MHT that are consistent with seismic observations from the Gorkha earthquake. A comparison of our model to a detailed slip inversion data set shows that the slip patch closely matches an oval-shaped, gently dipping fault surface bounded on all sides by steeper ramps. The Gorkha earthquake rupture seems to have been limited by the geometry of that fault segment. This is a significant step forward in understanding the deep geometry of the MHT and its effect on earthquake nucleation and propagation. Published models of fault locking do not correlate with the slip patch or our fault model in the vicinity of the earthquake, further suggesting that fault geometry was the primary control on this event. Our result emphasizes the importance of adequately constraining subsurface fault geometry in megathrusts in order to better assess the sizes and locations of future earthquakes.
Horton, Brian K., and Facundo Fuentes. "Sedimentary record of plate coupling and decoupling during growth of the Andes." Geology 44.8 (2016): 647-650.
Geochronologic, provenance, and sediment accumulation records from the long-lived (>100 m.y.) retroarc basin at the transition from the central to southern Andes provide improved resolution to examine the duration and controls on mixed-mode deformation and an enigmatic foreland depositional hiatus. Detrital zircon U-Pb ages for the Malargüe and Neuquén basin systems of western Argentina reveal shifts in exhumation and accumulation compatible with magmatic-arc and thrust-belt sources during unsteady Cretaceous–Neogene deformation. Fully developed foreland basin conditions were only achieved during separate periods of Late Cretaceous and Neogene shortening contemporaneous with possible episodes of enhanced coupling between a westward-advancing South American plate and the subducting Nazca slab. Separating these two contractional episodes is a 20–40 m.y. phase of reduced sedimentation and unconformity development, potentially signifying a neutral to extensional mode across the retroarc hinterland to forearc region during diminished plate coupling. We propose that the Andean orogen and its foreland and forearc basins have always been sensitive to variations in subduction dynamics, such that regional shifts in slab buoyancy and subduction geometry (particularly slab dip) superimposed on plate-scale shifts in convergence have governed mechanical coupling along the plate boundary and resulting fluctuations among contractional, extensional, and neutral tectonic regimes.
von Hagke, Christoph, et al. "Origin and time evolution of subduction polarity reversal from plate kinematics of Southeast Asia." Geology 44.8 (2016): 659-662.
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.
JULY 2016
van der Baan, Mirko, David W. Eaton, and Giona Preisig. "Stick-split mechanism for anthropogenic fluid-induced tensile rock failure." Geology(2016): G37826-1.
Fluids play a critical role in natural and human-induced rock failure. It is unclear, however, if propagation of a tensile fracture is inherently an episodic or continuous process. For example, typical average propagation speeds of hydraulic fracture tips on the order of 1–10 m/min suggest continuous crack growth, possibly at subcritical stress intensities. In contrast, using field observations and numerical and mathematical analyses, we show that fracture growth due to anthropogenic hydraulic fracturing is most likely to occur in an episodic fashion, characterized by stick-split behavior that is analogous to stick-slip motion of earthquakes. The stick-split mechanism is regulated by cyclic variations in fluid pressure near the crack tip, in which each successive failure produces a local pressure drop that temporarily halts or slows fracture propagation. A pressure drop results in partial fracture closure, producing noncontinuous fracture propagation through a process that is reminiscent of hand clapping. Rupture speeds for individual failure events are on the order of the shear-wave velocity of the medium; thus, continuous crack growth is not a likely mechanism for anthropogenic hydraulic fracturing treatments despite slow average tip propagation speeds.
Houseknecht, David W., and Christopher D. Connors. "Pre-Mississippian tectonic affinity across the Canada Basin–Arctic margins of Alaska and Canada." Geology (2016): G37862-1.
New and reprocessed seismic reflection data on the Alaskan and Canadian Arctic margins of the Canada Basin, together with geologic constraints from exploration wells and outcrops, reveal structural and stratigraphic relationships in pre-Mississippian rocks that constrain models of Canada Basin opening. Lithostratigraphic age and acoustic character indicate that the Devonian and older passive-margin to foreland-basin succession in the Canadian M’Clure Strait is also found on the central Alaska margin. This succession also displays similar structural geometry and relief as well as deformational age on both margins. Moreover, Middle Devonian to Early Mississippian tectonic vergence—north directed on the central Alaska margin and east directed in the Canadian M’Clure Strait—indicates a common direction of tectonic transport if the two margins were conjugate. All of these observations demonstrate that pre-Mississippian rocks of the Alaskan and Canadian Arctic margins share a common tectonic history of uplift and exhumation and that the two margins were conjugates prior to opening of the Canada Basin.
Ten Brink, Uri S., B. D. Andrews, and N. C. Miller. "Seismicity and sedimentation rate effects on submarine slope stability." Geology 44.7 (2016): 563-566.
We explore the effects of earthquake frequency and sedimentation rate on submarine slope stability by extracting correlations between morphological and geological parameters in 10 continental margins. Slope stability increases with increasing frequency of earthquakes and decreasing sedimentation rate. This increase in stability is nonlinear (power law with b < 0.5), accelerating with decreasing interseismic sediment accumulation. The correlation is interpreted as evidence for sediment densification and associated shear strength gain induced by repeated seismic shaking. Outliers to this correlation likely identify margins where tectonic activity leads to relatively rapid oversteepening of the slope.
Barak, Shahar, and Simon L. Klemperer. "Rapid variation in upper-mantle rheology across the San Andreas fault system and Salton Trough, southernmost California, USA." Geology 44.7 (2016): 575-578.
We present new shear-wave splitting data showing systematic lateral variations in upper-mantle anisotropy across the plate boundary in southernmost California (USA). Beneath the Peninsular Ranges batholith, fast polarization directions parallel the direction of former Farallon subduction, suggestive of a slab remnant. Near the eastern edge of the batholith, across the Elsinore fault, fast polarization directions change rapidly to align with the direction of San Andreas fault shear. We infer that the Elsinore fault penetrates the entire lithosphere and may represent a future localization of the plate boundary that is migrating west from the San Andreas fault. Beneath the Salton Trough and the Chocolate Mountains region, large splitting times, despite a very thin lithosphere, imply vertical melt pockets in the uppermost mantle aligned in the shear direction. Largest splitting times, ∼1.2 s, are seen closest to the Sand Hills fault that projects southeast from the San Andreas fault. Further east, in the southern Basin and Range province, fast directions align with North America absolute plate motion.
Roy, S. G., et al. "Dynamic links among rock damage, erosion, and strain during orogenesis." Geology 44.7 (2016): 583-586.
We provide model evidence for a previously unexplored positive feedback between tectonic strain and fluvial erosion by considering rock erodibility as a function of shear damage. Plastic shear strain permanently damages the upper crust within planar shear zones, providing a greater ease for detachment and transport by fluvial processes. The subsequent rapid erosion of exposed shear zones reforms the topographic stress field in a way that encourages continued accommodation of strain, a positive feedback response that becomes more prominent with greater shear damage. Based on model results and natural examples, rock strength heterogeneity plays a major role in the evolution of drainage network patterns and the capability for valley-scale strain localization in active orogens.
Wech, Aaron G. "Extending Alaska’s plate boundary: Tectonic tremor generated by Yakutat subduction." Geology 44.7 (2016): 587-590.
The tectonics of the eastern end of the Alaska-Aleutian subduction zone are complicated by the inclusion of the Yakutat microplate, which is colliding into and subducting beneath continental North America at near-Pacific-plate rates. The interaction among these plates at depth is not well understood, and further east, even less is known about the plate boundary or the source of Wrangell volcanism. The drop-off in Wadati-Benioff zone (WBZ) seismicity could signal the end of the plate boundary, the start of aseismic subduction, or a tear in the downgoing plate. Further compounding the issue is the possible presence of the Wrangell slab, which is faintly outlined by an anemic, eastward-dipping WBZ beneath the Wrangell volcanoes. In this study, I performed a search for tectonic tremor to map slow, plate-boundary slip in south-central Alaska. I identified ∼11,000 tremor epicenters, which continue 85 km east of the inferred Pacific plate edge marked by WBZ seismicity. The tremor zone coincides with the edges of the downgoing Yakutat terrane, and tremors transition from periodic to continuous behavior as they near the aseismic Wrangell slab. I interpret tremor to mark slow, semicontinuous slip occurring at the interface between the Yakutat and North America plates. The slow slip region lengthens the megathrust interface beyond the WBZ and may provide evidence for a connection between the Yakutat slab and the aseismic Wrangell slab.
The 2013 Lushan earthquake: Implications for seismic hazards posed by the Range Front blind thrust in the Sichuan Basin, China
Geology, October 2014, v. 42, p. 915-918, first published on August 28, 2014, doi:10.1130/G35809.1
Rethinking turbidite paleoseismology along the Cascadia subduction zone
Geology, September 2014, v. 42, p. 827-830, First published on July 29, 2014, doi:10.1130/G35902.1
The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments
Geology, May 2014, v. 42, p. 435-438, First published on March 21, 2014, doi:10.1130/G35272.1
Earthquakes and fault zone structure
Geology, April 2014, v. 42, p. 343-346, First published on February 24, 2014, doi:10.1130/G35071.1
Geology
Cassel, E. J., S. A. Graham, and C. P. Chamberlain (2009), Cenozoic tectonic and topographic evolution of the northern Sierra Nevada, California, through stable isotope paleoaltimetry in volcanic glass, Geology, 37, 547-550.
Cowgill, E., R. D. Gold, C. Xuanhua, W. Xiao-Feng, J. R. n. Arrowsmith, and J. Southon (2009), Low Quaternary slip rate reconciles geodetic and geologic rates along the Altyn Tagh fault, northwestern Tibet, Geology, 37, 647-650.
Dessa, J. X., F. Klingelhoefer, D. Graindorge, C. André, H. Permana, M. A. Gutscher, A. Chauhan, and S. C. Singh (2009), Megathrust earthquakes can nucleate in the forearc mantle: Evidence from the 2004 Sumatra event, Geology, 37, 659-662.
Griffith, W. A., A. Rosakis, D. D. Pollard, and C. W. Ko (2009), Dynamic rupture experiments elucidate tensile crack development during propagating earthquake ruptures, Geology, 37, 795-798.
Lin, Z., T. Oguchi, Y.-G. Chen, and K. Saito (2009), Constant-slope alluvial fans and source basins in Taiwan, Geology, 37, 787-790.
Mackey, B. H., J. J. Roering, and J. A. McKean (2009), Long-term kinematics and sediment flux of an active earthflow, Eel River, California, Geology, 37, 803-806.
Ouimet, W. B., K. X. Whipple, and D. E. Granger (2009), Beyond threshold hillslopes: Channel adjustment to base-level fall in tectonically active mountain ranges, Geology, 37, 579-582.
Schmalholz, S. M., B. J. P. Kaus, and J.-P. Burg (2009), Stress-strength relationship in the lithosphere during continental collision, Geology, 37, 775-778.
Strecker, M. R., R. Alonso, B. Bookhagen, B. Carrapa, I. Coutand, M. P. Hain, G. E. Hilley, E. Mortimer, L. Schoenbohm, and E. R. Sobel (2009), Does the topographic distribution of the central Andean Puna Plateau result from climatic or geodynamic processes?, Geology, 37, 643-646.
Vernon, A. J., P. A. van der Beek, and H. D. Sinclair (2009), Spatial correlation between long-term exhumation rates and present-day forcing parameters in the western European Alps, Geology, 37, 859-862.
Xu, X., X. Wen, G. Yu, G. Chen, Y. Klinger, J. Hubbard, and J. Shaw (2009), Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China, Geology, 37, 515-518.
GSA Bulletin
Dingler, J., G. Kent, N. Driscoll, J. Babcock, A. Harding, G. Seitz, B. Karlin, and C. Goldman (2009), A high-resolution seismic CHIRP investigation of active normal faulting across Lake Tahoe Basin, California-Nevada, Geological Society of America Bulletin, 121, 1089-1107.
Lamb, M. P., and W. E. Dietrich (2009), The persistence of waterfalls in fractured rock, Geological Society of America Bulletin, 121, 1123-1134.
Phillips, F. M., M. Zreda, M. A. Plummer, D. Elmore, and D. H. Clark (2009), Glacial geology and chronology of Bishop Creek and vicinity, eastern Sierra Nevada, California, Geological Society of America Bulletin, 121, 1013-1033.