Range-front Fault Research Articles

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American–Pacific plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was flooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras–Silver Creek–Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pull-apart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the Silver Creek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene–Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ∼0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

Mammoth Mountain and its mafic periphery--A late Quaternary volcanic field in eastern California

The trachydacite complex of Mammoth Mountain and an array of contemporaneous mafic volcanoes in its periphery together form a discrete late Pleistocene magmatic system that is thermally and compositionally independent of the adjacent subalkaline Long Valley system (California, USA). The Mammoth system first erupted ca. 230 ka, last erupted ca. 8 ka, and remains restless and potentially active. Magmas of the Mammoth system extruded through Mesozoic plutonic rocks of the Sierra Nevada batholith and extensive remnants of its prebatholith wall rocks. All of the many mafic and silicic vents of the Mammoth system are west or southwest of the structural boundary of Long Valley caldera; none is inboard of the caldera’s buried ring-fault zone, and only one Mammoth-related vent is within the zone. Mammoth Mountain has sometimes been called part of the Inyo volcanic chain, an ascription we regard inappropriate and misleading. The scattered vent array of the Mammoth system, 10 × 20 km wide, is unrelated to the range-front fault zone, and its broad nonlinear footprint ignores both Long Valley caldera and the younger Mono-Inyo range-front vent alignment. Moreover, the Mammoth Mountain dome complex (63%–71% SiO 2 ; 8.0%–10.5% alkalies) ended its period of eruptive activity (100–50 ka) long before Holocene inception of Inyo volcanism. Here we describe 25 silicic eruptive units that built Mammoth Mountain and 37 peripheral units, which include 13 basalts, 15 mafic andesites, 6 andesites, and 3 dacites. Chemical data are appended for nearly 900 samples, as are paleomagnetic data for ∼150 sites drilled. The 40 Ar/ 39 Ar dates (230–16 ka) are given for most units, and all exposed units are younger than ca. 190 ka. Nearly all are mildly alkaline, in contrast to the voluminous subalkaline rhyolites of the contiguous long-lived Long Valley magma system. Glaciated remnants of Neogene mafic and trachydacitic lavas (9.1–2.6 Ma) are scattered near Mammoth Mountain, but Quaternary equivalents older than ca. 230 ka are absent. The wide area of late Quaternary Mammoth magmatism remained amagmatic during the long interval (2.2–0.3 Ma) of nearby Long Valley rhyolitic eruptions.

SAR interferometry and optical remote sensing for analysis of co-seismic deformation, source characteristics and mass wasting pattern of Lushan (China, April 2013) earthquake

Co-seismic deformation associated with the Lushan (China) earthquake that occurred along the south-western segment of the Longmenshan Fault Zone (LFZ) on the 20th April 2013 has been estimated by differential interferometric SAR (DInSAR) technique using Radarsat-2 data. The Lushan earthquake resulted in the deformation of the Sichuan basin and the Longmenshan ranges in proximity to the LFZ. The line of sight (LOS) displacement values obtained from DInSAR technique mainly range between −4.0cm to +3.0cm. The western Sichuan basin shows oblique westward movement with predominant downward component in areas farther from LFZ and predominant westward component over the downward movement in areas closer to the source fault. Inversion modelling has been used to derive the seismic source characteristics from DInSAR derived deformation values using elastic dislocation source type. The linear inversion model converged at a double-fault source solution consisting of a deeper, steep, NW dipping fault plane-1 of 60km×16km dimension and a shallower, gentle, NW dipping fault plane-2 of 60km×15km dimension, with distributed slip values varying between 0 to 2.26m. These fault planes (fault planes-1 and -2) coincide with the Dachuan-Shuangshi fault and the buried Range Front Fault, respectively. The inversion model gives a moment magnitude of 6.81 and the geodetic moment of 2.07×1019Nm, comparable to those given in literature, derived using teleseismic body wave data. Thus DInSAR technique helped to quantify the co-seismic deformation and to retrieve the source characteristics from the estimated deformation values. The study also evaluated the distribution pattern of earthquake induced landslides (EIL) triggered fresh or re-activated during the Lushan earthquake and found that they show spatial association with the seismic source zone and also with various pre-conditioning factors of slope instability.

Decoupling of foreland basin subsidence from topography linked to faulting and erosion

Many mountain ranges in regions of plate convergence are flanked by foreland sedimentary basins that are usually thought to result from flexing of the elastic lithosphere under the passive weight of sediments and thrust sheets emplaced in the mountain belt. However, in some places the depth of the foreland basin bears little relation to the weight of basin sediments and neighboring mountains, implying that this model is too simple and/or that additional forces must affect subsidence. Here I investigate this problem using a numerical model that incorporates coupling between an upthrusting mountain range and a subsiding basin. I show that only part of the subsidence reflected in the shape of a foreland basin is a flexural isostatic response to crustal thickening and sediment loading. The remaining subsidence results from dragging of the basin margin downward by slip on a range-front fault and related flexural deformation of the surrounding lithosphere. The maximum basin depth is shown to increase with increasing fault displacement and can become decoupled from the elevation of the mountain range, especially as erosion becomes increasingly important. Deeply eroded model mountain belts may have foreland basins several times deeper than flexural models predict on the basis of surface loads. These results challenge the prevailing view that fault-bounded foreland basins result solely from the passive weight of mountains and sediments acting on the lithosphere and they highlight that slip on major range-front reverse faults, when repeated over many earthquake cycles, may be recorded in their geometry and stratigraphic records.

Active simultaneous uplift and margin-normal extension in a forearc high, Crete, Greece

The island of Crete occupies a forearc high in the central Hellenic subduction zone and is characterized by sustained exhumation, surface uplift and extension. The processes governing orogenesis and topographic development here remain poorly understood. Dramatic topographic relief (2–6 km) astride the southern coastline of Crete is associated with large margin-parallel faults responsible for deep bathymetric depressions known as the Hellenic troughs. These structures have been interpreted as both active and inactive with either contractional, strike-slip, or extensional movement histories. Distinguishing between these different structural styles and kinematic histories here allows us to explore more general models for improving our global understanding of the tectonic and geodynamic processes of syn-convergent extension. We present new observations from the south–central coastline of Crete that clarifies the role of these faults in the late Cenozoic evolution of the central Hellenic margin and the processes controlling Quaternary surface uplift. Pleistocene marine terraces are used in conjunction with optically stimulated luminesce dating and correlation to the Quaternary eustatic curve to document coastal uplift and identify active faults. Two south-dipping normal faults are observed, which extend offshore, offset these marine terrace deposits and indicate active N–S (margin-normal) extension. Further, marine terraces preserved in the footwall and hanging wall of both faults demonstrate that regional net uplift of Crete is occurring despite active extension. Field mapping and geometric reconstructions of an active onshore normal fault reveal that the subaqueous range-front fault of south–central Crete is synthetic to the south-dipping normal faults on shore. These findings are inconsistent with models of active horizontal shortening in the upper crust of the Hellenic forearc. Rather, they are consistent with topographic growth of the forearc in a viscous orogenic wedge, where crustal thickening and uplift are a result of basal underplating of material that is accompanied by extension in the upper portions of the wedge. Within this framework a new conceptual model is presented for the late Cenozoic vertical tectonics of the Hellenic forearc.

Mechanics of relative and absolute displacements across normal faults, and implications for uplift and subsidence along the eastern escarpment of the Sierra Nevada, California

The magnitude of late Cenozoic rock uplift of the Sierra Nevada (California, USA) remains unresolved despite more than a century of investigation, with estimates ranging from essentially zero to ∼3 km of uplift at the range crest. Two sets of two-dimensional end-member mechanical models bracket how normal faulting along the eastern escarpment of the Sierra Nevada contributed to uplift of the range over a time span of millions of years. The short-term models are based on dislocations in an elastic half-space. The long-term models involve thin elastic beams resting on an inviscid fluid. Both sets of models predict that if the regional topography were entirely a response to faulting along the eastern escarpment, then the bedrock floors immediately east of the range should consistently lie thousands of meters below sea level, instead of thousands of meters above sea level as they generally do. Both sets of analyses indicate that although faulting would lift the range crest, it would drop the rock east of the range-front faults at least as much, and perhaps much more; model results suggest that ∼66%–85% of the current escarpment relief stems from subsidence of the grabens east of the Sierra Nevada, with only ∼15%–34% resulting from crestal uplift. Our results strongly indicate that range-front faulting in the last 3–10 m.y. uplifted rock at the Sierra Nevada crest by hundreds of meters to as much as 1 km, and that this uplift was superposed on high topography that predated the origin of the eastern escarpment. These conclusions are compatible with diverse geologic observations and measurements.

Initiation of Sierra Nevada range front–Walker Lane faulting ca. 12 Ma in the Ancestral Cascades arc

The eastern escarpment of the Sierra Nevada (USA) forms one of the most prominent topographic and geologic features in the Cordillera, yet the timing and nature of fault displacements along it remain relatively poorly known. The central Sierra Nevada range front is an ideal place to determine the structural evolution of the range front because it has abundant dateable Cenozoic volcanic rocks. The Sonora Pass area of the central Sierra Nevada is particularly good for reconstructing the slip history of range-front faults, because it includes unusually widespread and distinctive high-K volcanic rocks (the ca. 11.5–9 Ma Stanislaus Group) that serve as outstanding strain markers. These include the following, from base to top. (1) The Table Mountain Latite (TML) consists of voluminous trachyandesite, trachybasaltic andesite, and basalt lava flows, erupted from fault-controlled fissures in the Sierra Crest graben-vent system. (2) The Eureka Valley Tuff consists of three trachydacite ignimbrite members erupted from the Little Walker caldera. These ignimbrites are interstratified with lava flows that continued to erupt from the Sierra Crest graben-vent system, and include silicic high-K as well as intermediate to mafic high-K lavas. The graben-vent system consists of a single ∼27-km-long, ∼8–10-km-wide approximately north-south graben that is along the modern Sierran crest between Sonora Pass and Ebbetts Pass, with a series of approximately north-south half-grabens on its western margin, and an ∼24-km-wide northeast transfer zone emanating from the northeast boundary of the graben on the modern range front south of Ebbetts Pass. In this paper we focus on the structural evolution of the Sonora Pass segment of the Sierra Nevada range front, which we do not include in the Sierra Crest graben-vent complex because we have found no vents for high-K lava flows here. However, we show that these faults localized the high-K Little Walker caldera. We demonstrate that the range-front faults at Sonora Pass were active before and during the ca. 11.5–9 Ma high-K volcanism. We show that these faults are dominantly approximately north-south down to the east normal faults, passing northward into a system of approximately northeast-southwest sinistral oblique normal faults that are on the southern end of the ∼24-km-wide northeast transfer zone in the Sierra Crest graben-vent complex. At least half the slip on the north-south normal faults on the Sonora Pass range front occurred before and during eruption of the TML, prior to development of the Little Walker caldera. It has previously been suggested that the range-front faults formed a right-stepping transtensional stepover that controlled the siting of the Little Walker caldera; we support that interpretation by showing that synvolcanic throw on the faults increases southward toward the caldera. The Sonora Pass–Little Walker caldera area is shown here to be very similar in structural style and scale to the transtensional stepover at the Quaternary Long Valley field. Furthermore, the broader structural setting of both volcanic fields is similar, because both are associated with a major approximately northeast-southwest sinistral oblique normal fault zone. This structural style is typical of central Walker Lane belt transtension. Previous models have called for westward encroachment of Basin and Range extension into the Sierra Nevada range front after arc volcanism ceased (ca. 6–3.5 Ma); we show instead that Walker Lane transtension is responsible for the formation of the range front, and that it began by ca. 12 Ma. We conclude that Sierra Nevada range-front faulting at Sonora Pass initiated during high-K arc volcanism, under a Walker Lane transtensional strain regime, and that this controlled the siting of the Little Walker caldera.

Volcanic signature of Basin and Range extension on the shrinking Cascade arc, Klamath Falls‐Keno area, Oregon

AbstractDetailed geologic mapping of the Klamath Falls‐Keno area revealed the complex relationship between subduction, crustal extension, and magmatic composition of the southern Oregon Cascade volcanic arc. Volcanism in the study area at ~7–4 Ma consisted of calc‐alkaline basaltic andesite and andesite lava flowing over a relatively flat landscape. Local angular unconformities are evidence that Basin and Range extension began at by at least ~4 Ma and continues today with fault blocks tilting at a long‐term rate of ~2°/Ma to 3°/Ma. Minimum NW‐SE extension is ~1.5 km over ~28 km (~5%). High‐alumina olivine tholeiite (HAOT) or low‐K, low‐Ti transitional high‐alumina olivine tholeiite (LKLT) erupted within and adjacent to the back edge of the calc‐alkaline arc as the edge receded westward at a rate of ~10 km/Ma at 2.7–0.45 Ma. The volcanic front migrated east much slower than the back arc migrated west: ~0 km/Ma for 6–0.4 Ma calc‐alkaline rocks; ~0.7 km/Ma, if ~6 Ma HAOT‐LKLT is included; and ~1 km/Ma, if highly differentiated 17–30 Ma volcanic rocks of the early Western Cascades are included. Declining convergence probably decreased asthenospheric corner flow, decreasing width of calc‐alkaline and HAOT‐LKLT volcanism and the associated heat flow anomaly, the margins of which focused on Basin and Range extension and leakage of HAOT‐LKLT magma to the surface. This declining corner flow combined with steepening slab dip shifted the back arc west. Compensation of extension by volcanic intrusion and extrusion allowed growth of imposing range‐front fault scarps only behind the trailing edge of the shrinking arc.

Co-seismic, geomorphic, and geologic fold growth associated with the 1978 Tabas-e-Golshan earthquake fault in eastern Iran

We describe the seismicity and late Cenozoic deformation associated with a blind thrust fault at Tabas-e-Golshan (hereafter referred to as Tabas), eastern Iran, which generated a devastating Mw 7.3 earthquake on the 16th September 1978. Measurements from a structural transect through the Sardar anticline segment indicate fault-propagation folding above a gently ~20° eastward-dipping blind thrust fault. The thrust flattens into a horizontal detachment at a depth of only ~2km. Tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes, with many of the slip surfaces showing fresh striations with a large component of right-lateral strike–slip. A steeply dipping fault zone showing almost pure strike–slip is also developed within the forelimb of the fold. Our field observations are consistent with the source parameters of the 1978 Tabas earthquake, and additional events in 1979 and 1980, which all involved slip on a shallowly-dipping thrust with a significant component of right-lateral slip. The surface of an alluvial fan, which is likely to have been abandoned at 8–10ka, has been folded as it crosses the Sardar anticline. The age constraints, combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth, provide an approximate rate of horizontal shortening of ~1.5mm/yr. Shortening at Tabas appears to result from transpressional bending at the north end of the Nayband strike–slip fault. A northward continuation of the Nayband Fault, which may be slipping at rates of >2mm/yr, is identified along the base of the Shotori Mountains ~10–20km east of the Tabas thrust. The range-front fault did not move in 1978 and constitutes an additional threat to local populations.

Discussion of rupture mechanisms on the seismogenic fault of the 2008 M s8.0 Wenchuan earthquake

The May 12, 2008, M s 8.0 Wenchuan earthquake was the outcome of a recent movement of an active intra-continental thrust fault zone. The seismogenic fault of this earthquake underwent oblique-slip faulting along the central fault and pure thrust faulting along the range-front fault of the Longmenshan fault zone. The former had a steep dip and large vertical displacement, and the latter had a gentle dip and little vertical displacement. The fault zone consisted of compressive double fault ramps rupturing with right-lateral strike-slip components resulting from strain partitioning of a deep oblique slip fault in the brittle zone of the upper crust. The kinematic pattern and rupture mechanisms are complex for the seismogenic fault, as indicated by the geometric pattern of its surface ruptures, the coseismic displacement distribution and focal mechanisms of the main shock and aftershocks. As a tear fault, the NW-trending, left-lateral, strike-slip Xiaoyudong fault zone has accommodated NE-trending displacements with different shortening amounts. However, because of intense compression on the southwestern segment of the seismogenic fault, the left-lateral, strike-slip Xiaoyudong fault also carries a clear compression component. Normal faulting with a strike-slip component controls the formation of a fault-trough along the central fault, which is characterized by thrusting with a strike-slip component and strike-slip with thrusting. The fault-troughs are the product of the interaction of slip and gravity on the seismogenic fault under specific geological and geomorphic conditions. Gravitational force exaggerated the vertical component of fault displacement, which by no means represents the actual maximum vertical displacement of the seismogenic fault.

Audiomagnetotelluric characterization of range-front faults, Snake Range, Nevada

Two controlled-source audio-magnetotelluric (CSAMT) profiles were collected on the eastern flank of the Snake Range in eastern Nevada across geologically complex terrain to investigate the suspected presence of faults along the range front. The location of the range-bounding faults is not easily determined on geologic grounds because of the presence of extensive young sedimentary cover and overall geologic complexity. Characterization of the presence, location, and structural style of the range-front faults is critical to assessment of connectivity of groundwater aquifers near the mountain front and in adjacent alluvial basins. A total of 48 CSAMT soundings were recorded along two lines that were chosen to maximize subsurface geologic information. Two generations of faults were interpreted based on the CSAMT data: an older, low-angle fault that is cut by a younger, more steeply dipping fault. Lack of deep boreholes in the region required that the subsurface interpretation rely on analogy from surface outcrops within and adjacent to the study area. The success of the CSAMT method as applied in this study hinged on near-ideal collection conditions, the relatively high contrast in electrical resistivity provided by the rock types involved, and well-developed geologic conceptual models of the region.

Structural constraints on the timing of left-lateral shear along the Red River shear zone in the Ailao Shan and Diancang Shan Ranges, Yunnan, SW China

The >1000-km-long Oligocene—Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene—active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea, has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene–Early Miocene left-lateral shear and localized partial melting (leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strike-slip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by K-feldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (∼239–243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (∼26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (∼35 Ma). Later biotite leucogranites intruded the orthogneisses and migmatite host rocks before a significant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (∼500–550 °C) for the ALS and lower temperatures (∼250–150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated final exhumation of the Red River gneisses. Using published U-Th-Pb and 40 Ar/ 39 Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed leucogranites (31.9–24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ∼17 Ma.

Microbiology and geochemistry of smith creek and grass valley hot springs: Emerging evidence for wide distribution of novel thermophilic lineages in the US Great Basin

INTRODUCTION The endorheic Great Basin (GB) region in the western US is host to a variety of non-acidic geothermal spring systems. Heating of the majority of these systems is due to their association with range-front faults, in contrast to caldera-associated hot springs in systems such as Yellowstone National Park and Kamchatka (Faulds et al., 2006). Previous characterization of two geothermal systems in the GB, Great Boiling/Mud Hot springs (Costa et al., 2009) and Little Hot Creek (Vick et al., 2010), indicated that they host several novel deep lineages of potentially abundant Bacteria and Archaea. To expand the knowledge base of the microbiology and geochemistry of GB hot springs and facilitate their comparison to other terrestrial geothermal systems worldwide, we present here the characterization of Smith Creek (SC1) and Grass Valley (GVS1) hot springs in central Nevada, US.

Cenozoic range-front faulting and development of the Transantarctic Mountains near Cape Surprise, Antarctica: Thermochronologic and geomorphologic constraints

[1] The Transantarctic Mountains (TAM) define the western flank of the West Antarctic rift system. Cape Surprise, near the Shackleton Glacier in the central TAM, is the only location along the range's >3500 km length where upper Paleozoic Beacon Supergroup strata are down-faulted to near sea level. Previous studies have inferred a range front master normal fault accommodates extension and rock uplift across the TAM front in this region. The history of rock uplift is debated, suggested as early as Mesozoic, typically Cenozoic, or even Pliocene or younger. Structural observations, apatite fission track (AFT) thermochronology, and geomorphologic mapping undertaken within the TAM front east of the Shackleton Glacier indicate extension, faulting, and denudation was mostly late Eocene–late Oligocene and likely into the early Miocene. An exhumed AFT partial annealing zone is found at the coast and traced >50 km inland. The base of that exhumed partial annealing zone indicates denudation accelerated at ∼40 Ma near the coast and at 30–35 Ma on the inland side of the TAM front. Vertically offset AFT isochrones across the TAM front reveal a step-faulted architecture rather than a single master fault. The cumulative vertical offset of the 55 Ma isochrone is 2.3–2.7 km, compared to 2.4–2.6 km offset of Beacon strata, indicating that all significant normal faulting is Cenozoic, and not related to Mesozoic extension within the West Antarctic rift system. Denudation from <26 Ma to ∼14 Ma produced a locally preserved erosion surface within the TAM front. Erosion surface remnants indicate the TAM front has undergone minimal internal deformation and only 290–790 m of surface uplift along offshore faults since the middle Miocene.

The maximum coseismic vertical surface displacement and surface deformation pattern accompanying the Ms 8.0 Wenchuan earthquake

The amount of coseismic deformation and its distribution of the Wenchuan earthquake provide important scientific bases for revealing the mechanisms of earthquake preparation and characterizing the rupture propagation of the Wenchuan earthquake. The previous studies have indicated that the earthquake ruptured the middle-to-north segment of the Longmenshan central fault and the middle segment of the Longmenshan range-front fault, which are characterized by two surface rupture zones of 240 km and 90 km in length, respectively. Based on the pre-earthquake information and photos of landforms and buildings obtained through geologic and geomorphic survey of the area around Shaba Village of Beichuan County, Sichuan Province and the extensive interview with local villagers, we measured the displacements of the major terrain features and the dislocated buildings by total station instruments and differential GPS and obtained the maximum vertical displacement of 9±0.5 m and right-lateral displacement of 2±0.5 m around the Zou’s house in Shaba Village. Though the near-surface deformation exhibits a normal faulting around Shaba Village, the dynamic environment has not changed on the whole. The NW wall of the fault uplifted but without gravity gliding as normally occurring on the hanging wall of a normal fault, which proves that the 9±0.5 m displacement should be the maximum coseismic vertical displacement of the May 12, 2008 Ms 8.0 Wenchuan earthquake.

An in-depth look at distal Sierra Nevada palaeochannel fill: drill cores through the Table Mountain Latite near Knights Ferry

The ∼10.4 Ma Table Mountain Latite (TML) consists of high-K trachyandesite lavas that likely erupted from the Little Walker Center near Sonora Pass and flowed 80 miles (130 km) through the palaeo-Stanislaus river channel to Knights Ferry in the Sierra Nevada foothills. Complete sections through the proximal facies of the flow stratigraphy are common in the high-Sierra and along range-front faults, but in much of its distal facies in the Sierra Nevada foothills, the internal features of the unit are poorly exposed. Condor Earth Technologies offered us the opportunity to study three complete vertical sections through the distal facies of the TML through access to three drill cores. These cores, spaced 1500′ (457 m) apart and oriented oblique to inferred flow direction by 30°, are referred to here as cores A, B, and C, in a down-palaeoflow direction. Previous outcrop studies of the distal TML over the past century have concluded that the distal TML consists of a single flow. This is true of core B, which is dominated by a single 144′ (44 m) thick flow, underlain by a thin (∼5′, 1.5 m) flow with a vesiculated top, inferred to represent a toe of the thick flow, inasmuch as there is no baked zone or weathered contact between them. This contrasts sharply with core C, where the 50′ (15.2 m) thick TML consists of four flows 5–12′ (1.6–3.6 m) thick, each also defined by vesiculated tops, but differing in having weathered tops 1–2′ (0.3–0.6 m) thick, inferred to record an eruptive hiatus between each flow. The third flow in core C appears to be geochemically distinct from both overlying and underlying units, although all of the core samples are petrographically similar. The TML in cores C and B overlies coarse-grained andesitic volcaniclastic debris and fluvial deposits. By contrast, in core A, the TML overlies very fine-grained siltstones interpreted to record deposition in still water, probably a small lake produced by damming of the river by the thick flow in core B. The TML in core A is 20′ (6 m) thick, and is composed of eleven 1–3′ (0.3–1.0 m) thick couplets, each consisting of a clay-altered hyaloclastite breccia passing upwards into a relatively fresh, nonbrecciated, vesiculated top. We interpret these couplets to represent thin toes of a flow that generated steam explosions when they came into contact with a standing body of water. Palaeomagnetic remanence data (inclination only) on lava flows from the cores are consistent with the Classic Table Mountain direction and inconsistent with directional results from any other Stanislaus Group lava flows published to date. Thus, by palaeomagnetic correlation, the lavas studied here were emplaced between 10.36 ± 0.06 and 10.41 ± 0.08 Ma. In addition, remanence results suggest that the three sampled flows from core C were erupted over a time period shorter than the secular variation rate (i.e. less than a few centuries) and the Knights Ferry portion of the TML was emplaced within several centuries of the average Classic TML age range spanning the Sierra Nevada. This indicates that the weathered flow tops at Knights Ferry formed very quickly.

Can footwall unloading explain late Cenozoic uplift of the Sierra Nevada crest?

Globally, normal-fault displacement bends and warps rift flanks upwards, as adjoining basins drop downwards. Perhaps the most evident manifestations are the flanks of the East African Rift, which cuts across the otherwise minimally deformed continent. Flank uplift was explained by Vening Meinesz (1950, Institut Royal Colonial Belge, Bulletin des Séances, v. 21, p. 539–552), who recognized that isostasy should cause uplift of a normal-faulted footwall and subsidence of its hanging wall. Uplift occurs because slip on a dipping normal fault creates a broader root of less-dense material beneath the footwall, and a narrowed one beneath the hanging wall. In this paper, we investigate the potential influence of this process on the latest stages of Sierra Nevada uplift. Through theoretical calculations and 3D finite element modelling, we find that cumulative slip of about 4 km on range-front faults would have produced about 1.3 km peak isostatic uplift at the ridge crest. Numerical models suggest that the zone of uplift is narrow, with the width controlled by bending resistance of the seismogenic crust. We conclude that footwall unloading cannot account for the entire elevation of the Sierran crest above sea level, but if range-front faulting initiated in an already elevated plateau like the adjacent Basin and Range Province, then a hybrid model of pre-existing regional uplift and localized footwall unloading can account for the older and newer uplift phases suggested by the geologic record.

Cenozoic palaeocanyon evolution, Ancestral Cascades arc volcanism, and structure of the Hope Valley–Carson Pass region, Sierra Nevada, California

We use new geologic mapping, geochronological, and geochemical data on Tertiary volcanic, volcaniclastic, and intrusive rocks to investigate the volcanic, stratigraphic, and structural evolution of the Carson Pass region south of Lake Tahoe in the central Sierra Nevada. Volcanic and volcaniclastic rocks were deposited in east–west-trending palaeocanyons carved into Mesozoic granitic and metamorphic basement rocks; sediments were transported westwards towards the present-day Central Valley of California. In the Carson Pass–Hope Valley area, two palaeotributaries are preserved in faulted terrane east of the present-day Sierran crest (Hope Valley area); these merge at the crest to form one large, >7 km wide palaeocanyon that is undisrupted by faults west of the crest (Carson Pass–Kirkwood area). This single, large palaeocanyon roughly coincides with the present-day Mokelumne River drainage. New 40Ar/39Ar dates and stratigraphic data east of the crest, integrated with previously published data west of the crest, constrain the ages of strata and unconformities within the Hope Valley–Carson Pass–Kirkwood palaeocanyon system. We interpret three major erosional unconformities to record uplift events at ca. 23–16, 13.5–11, and 10–7 Ma. In other parts of the central Sierra, these uplift events are inferred to correspond to range-front faulting events. We propose the term ‘Hope Valley Graben’ for the structural feature mapped immediately east of the Sierran crest at Carson Pass. It is a nearly symmetrical full graben that offsets volcanic rocks as young as 6 Ma at least 400 m (1300 ft) on each of its bounding faults – herein named the Red Lake Fault on the west and the Hope Valley Fault on the east. However, we infer that faulting began before eruption of the 6 Ma volcanic rocks for three reasons: (1) the graben-localized emplacement of one of the largest volcanic centres in the Sierra, the 6.34 ± 0.14–6.18 ± 0.14 Ma Markleeville Peak Center; (2) andesite lava flows erupted at 6.22 ± 0.14 Ma from the Red Lake Fault, and abut it within the graben; and (3) brecciated granite along the Red Lake Fault is intruded by altered andesite, indicating that the fault started slipping before magmatism ceased. Our stratigraphic and geochronologic data do not permit an estimate of the amount of pre-6 Ma displacement in the Hope Valley graben. The geometry of the palaeocanyon system indicates that a dextral component of slip demonstrated for transtensional faults in other parts of the region did not operate in the Hope Valley graben.

Miocene evolution of the western edge of the Nevadaplano in the central and northern Sierra Nevada: palaeocanyons, magmatism, and structure

The Sierra Nevada of California is the longest and tallest mountain range in the co-terminus USA, and has long been regarded as topographically very young ( < 6 Ma); however, recent work has provided evidence that the range is very old (>80 Ma), and represents the western shoulder of a Tibetan-like plateau (the Nevadaplano) that was centred over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet some of the most unambiguous constraints on the Sierran landscape evolution are derived from field studies of dated strata preserved in the palaeochannels/palaeocanyons that crossed the range in Cenozoic time. Our work in the Sierra Nevada suggests that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic palaeocanyons and palaeochannels reflect the shape of the Cretaceous orogen, but they were also affected by Miocene tectonic and magmatic events. In the central Sierra Nevada, we infer that the inherited Cretaceous landscape was modified by three Miocene tectonic events, each followed by ∼2–5 Myr of subduction-induced magmatism and sedimentation during a period of relative tectonic quiescence. The first event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada and adjacent western Nevada. We suggest that this caused thermal uplift and extension. The second event, at about 11–10 Ma, records the birth of the ‘future plate boundary’ by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third event, at about 8–7 Ma, is associated with renewed range-front faulting in the central Sierra, and rejuvenation and beheading of the palaeocanyons. Volcanic pulses closely followed all three events, and we tentatively infer that footwall uplift of the Sierra Nevada occurred during all three events. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range-front faulting at 3–3.5 Ma, which resulted in a fourth tilting and crestal uplift event. Cenozoic rocks along the western edge of the Nevadaplano record the following variation, from the central to the northern Sierra: decrease in crustal thickness (and presumably palaeoelevation), decrease in palaeorelief and attendant decrease in coarse-grained fluvial- and mass-wasting deposits, and greater degree of encroachment by Walker Lane-related faults beginning at 10–11 Ma. By mapping and dating Cenozoic strata in detail, we show that what is now the Sierra Nevada was, at least in part, shaped by the Miocene structural and magmatic events.

Late Quaternary rates of uplift and shortening at Baatar Hyarhan (Mongolian Altai) with optically stimulated luminescence

We investigate mountain building in the Altai range of western Mongolia, focusing on Baatar Hyarhan, a NW-trending massif bounded by active thrust faults. Our primary aims are to describe how thrusting has evolved over time, to calculate late Quaternary slip rates by dating offset alluvial markers with optically stimulated luminescence (OSL) and to compare these late Quaternary rates with measurements of deformation on decadal and geological timescales. Patterns of topography and drainage suggest that Baatar Hyarhan has grown in length and has propagated laterally from the SE towards the NW over time. On the NE side of the massif, the range-bounding Zereg fault appears active only along younger parts of Baatar Hyarhan; next to the oldest, SE part of the massif faulting has migrated into the adjacent Zereg Basin, where it has uplifted low, linear ridges of folded sediment, known locally as forebergs. On the SW side of the massif, only the range-bounding Tsetseg fault appears active. Using OSL, we establish ages of ∼15, ∼20 and ∼85 kyr for alluvial deposits cut by these faults. These ages are close to those of alluvial markers in the separate Gobi Altai range, suggesting that periods of fan and terrace formation may correlate over wide tracts of Mongolia, presumably under the primary control of climate. Combining our OSL ages with offsets measured with differential GPS, we calculate Late Quaternary slip rates across forebergs in the Zereg Basin and across the range-bounding Zereg and Tsetseg faults. Uncertainties in fault dip (due to lack of clear fault exposures) and burial ages (due to incomplete resetting of the luminescence clock) mean that the exact slip rates are poorly constrained. Nevertheless, the vertical displacement rates we calculate across the Zereg and Tsetseg range-front faults—0.2–0.6 and 0.1–0.4 mm yr−1, respectively—are at the lower end of long-term (∼5 Myr) estimates of 0.4–0.8 and 0.3–0.7 mm yr−1, respectively. Vertical rates of deformation may, therefore, have remained constant over the past ∼5 Myr, but equally the late Quaternary rates might be lower than the geological ones. This possible discrepancy could be accounted for if some of the shortening has shifted away from the range-front faults onto other nearby structures. The forebergs in the eastern Zereg Basin are an obvious candidate, but they show at least 10 km cumulative shortening (which would take a few Ma to accumulate at late Quaternary rates) and cannot simply be regarded as the latest stage of outward mountain growth. The total Late Quaternary shortening rate across all three areas of faulting is 0.7–2.4 mm yr−1, making up between one tenth and one third of the ∼7 mm yr−1 convergence across the whole Altai range.