Range-bounding Faults Research Articles

Recognition of crustal extension in the Basin and Range Province: A history

The Basin and Range Province of the western United States has inspired or augmented many important geological ideas but is most famous for the concept of crustal extension. Acceptance of the concept was a slow process. Prior to the recognition of plate tectonics, extension was unexpected and hard to understand; when mapping of the region began in the 1860s, mountain ranges were assumed to be the products of crustal compression due to contraction of the Earth. The first mapping suggested that ranges are anticlinal folds, but the idea of uplift along vertical range-bounding faults arose shortly thereafter. There was recognition starting in the 1880s that at least some faults were normal-sense rather than vertical, which implied extension, but there were decades of dissent by geologists who still promoted the anticlinal-folding idea, thought that inclined faults were reverse faults, or believed that the basins and ranges were created by water or wind or glacial erosion rather than tectonics. Normal faulting gradually became widely accepted, but comprehension of its meaning was hindered by apparent thrust faults in parts of the province that suggested contractional deformation. These faults would later be interpreted as low-angle normal faults, and crustal extension became widely, though not universally, accepted in the 1960s as magnetic anomalies confirmed seafloor spreading. Roughly a century elapsed between the first geologic mapping and general acceptance of crustal extension. A parallel evolution of thought occurred in Africa as a consensus arose that rift valleys were products of extension. "Even those who have more sympathy with man’s endeavor than with the affairs of Nature may take an interest in the Science of Tectonics…. as a rule, the knowledge of the structure of a mountain chain comes as the reward of glorious struggle, both physical and mental." —Edward Battersby Bailey, Tectonic Essays (1935, p. 1)

Wasatch Fault Structure from Machine Learning Arrival Times and High-Precision Earthquake Locations

Abstract On 18 March 2020, a magnitude 5.7 earthquake hit the Salt Lake valley in the state of Utah, United States. Using a dense geophone deployment and machine learning (ML), an additional several thousand events were detected and located. Currently, both the mainshock and the majority of the aftershocks are suspected to have occurred on or near a deeper portion of the Salt Lake segment of the Wasatch fault—part of a large range-bounding fault system thought to be capable of generating an Mw 7.2 earthquake. However, a small subset of aftershocks may have occurred on a portion of the more steeply, eastward dipping, and poorly understood West Valley fault. Unfortunately, the catalog locations and lack of focal mechanisms for this subset of aftershocks provide only a crude constraint on the true fault structure. To better illuminate fault structure, we relocate the ML-generated catalog with a range of magnitudes from −2 to 4.6, using: (1) NonLinLoc, a nonlinear location algorithm, (2) source-specific station terms, and (3) waveform coherence. We further compute first-motion focal mechanisms for 68 events. Results of the relocation suggest a simpler, minimally listric Wasatch fault geometry, contrary to what has been previously proposed. We also find that analysis of the focal mechanisms and waveform similarity indicates minimal event similarity throughout the Magna sequence, suggesting a highly complex and heterogeneous rupture zone, as opposed to rupture on a single plane. These findings suggest an increased seismic hazard due to the overall shallowness of the earthquake sequence and highly varied rupture mechanisms.

Late Quaternary activity of the NW Cardrona Fault, Otago, New Zealand

ABSTRACT We use new paleoseismic data and lidar to reassess late Quaternary activity of the NW (northwest) Cardrona Fault, a ∼60 km long range-bounding fault in Otago. Paleoseismic investigations of the NW Cardrona Fault were conducted in the 1980s, but findings were limited by a paucity of materials suitable for dating. Here, re-exposure and re-assessment of a 1980s trench at Macdonalds Creek provide stratigraphic evidence for two surface rupturing earthquakes on the NW Cardrona Fault. Through Optically Stimulated Luminescence dating and OxCal modelling, we estimate these earthquakes occurred at (mean ± 2 standard deviations, σ) 20.5 ± 7.8 ka and 8.7 ± 6.6 ka. The timings of these earthquakes are consistent with those proposed from re-interpretation of another 1980s trench on the NW Cardrona Fault in the Kawarau River valley, ∼25 km farther south. The slip rate and recurrence interval derived from the Macdonalds Creek trench data are 0.13 ± 0.05 mm/yr and 11,800 ± 5100 years (2σ error) respectively. These estimates imply lower slip rates and longer earthquake recurrence intervals on the NW Cardrona Fault than previously assessed.

Quaternary Map, Active Faulting, Tectonic Geomorphology, and Uplift Rate of the Humboldt Range in the Basin and Range Province of Nevada, United States

ABSTRACT Fault-truncated, uplifted, and abandoned alluvial fans are frequent along the western flank of the active normal fault bounded Humboldt range. Terrestrial cosmogenic nuclide surface exposure dating of a faulted terrace at Rocky Canyon indicates that uplift of the range has averaged about 0.12 mm/yr during the last ∼160 ka. A similar uplift rate of less than about 0.07 mm/yr over the last 35 ka or more years is estimated from offset of a younger terrace at the same site. Values of extension rate across the range are 0.04–0.07 mm/yr, if it is assumed that the range-bounding fault dips at 60°, in general accord with very low values of contemporary geodetic strain reported for the area. Geomorphology at Rocky Canyon records a period leading up to about 160 ka ago that erosion and aggradation of sediment along the rangefront were dominant over fault generated uplift, at which time faulting again became dominant, leading to uplift, incision, and preservation of ∼160-ka-old alluvial terraces here and elsewhere along the rangefront.

Crustal focal mechanisms in the NW sierras de cordoba, and connections with mio-pliocene basement thrusting of the easternmost Sierras Pampeanas, south-Central Andes

The seismicity studied in this paper is located along the back arc region, between 30°35′ and 31°52’ S latitude, in the morphostructural unit known as Eastern Sierras Pampeanas. Seventeen crustal earthquakes could be identified and located, between June 2004 and March 2018, using seismic data recorded by the broadband seismological network of National Institute for Seismic Prevention (INPRES), combined with free seismic data from the Incorporated Research Institutions for Seismology (IRIS). Focal mechanisms were calculated from localized seismicity using the P-wave first motions and interpreted as a strain style on the western portion of Sierras de Córdoba. Overall, the seismicity in this region during the study period was constrained to the upper ∼25 km of the crust and is mostly concentrated along minor structures and not along the major range-bounding faults. Although there is evidence of quaternary activity along the larger structures, the relationship between these displacements and modern deformations is not well developed due to the lack of local seismic stations in the area and the long recurrence interval between earthquakes of significant magnitudes. However, very recent morphometric studies in the Sierras Pampeanas have analyzed the main topography controls and suggested relatively higher uplift rates in the central sectors of the Sierra de Pocho, and a balance between tectonic uplift and erosion rates. Nevertheless, three seismic events showed greater depths around 36 km according to previously proposed geological and geophysical models of crustal thickness and mohorovicic depth. Although focal mechanism solutions show varied results, distributed throughout all the region, it is possible to determine an inverse deformation with an oblique component assigned to regional faults and an area where a normal deformation with an oblique component predominates, near to the Ojo de Agua Lineament. Accordingly with seismological observations, we proposed a model in which the seismogenic structures seem to have detached from deep horizontal intra-crustal interfaces that affects the faults and brittle structures, which document that the Eastern Sierras Pampeanas are actively deforming in response to compression from the convergent margin.

Fault reactivation in the Sierras Pampeanas resolved across Andean extensional and compressional regimes using thermochronologic modeling

The reactivation of existing faults has long been recognized to control the location and style of deformation in orogenic systems including the Sierras Pampeanas in the southern Central Andes; however, it can remain difficult to resolve the timing and fault kinematics across multiple orogenic phases. In the Sierras Pampeanas, basement-cored range-bounding faults mark a Paleozoic terrane boundary and are seismically active; however, the protracted history of deformation along this lithospheric-scale regional fault system - located ~400 km from the plate boundary - has been poorly constrained. In this study, we present new apatite (U–Th-Sm)/He and fission track thermochronology data and thermal history models from ten samples from the Sierra de Valle Fértil-La Huerta in the Western Sierras Pampeanas of Argentina. The spatio-temporal patterns of rock heating and cooling identified by our thermal history models throughout this fault system are consistent with four discrete phases of deformation. Thermal patterns indicative of Mesozoic extensional deformation and latest Miocene - Pliocene compressional deformation corroborate previous interpretations of regional deformation episodes in the Sierras Pampeanas. Our study also identifies a period of middle-Miocene cooling that revises the location of the Andean thrust front. This episode of deformation was previously unrecognized in the Sierras Pampeanas and is contemporaneous with thin-skinned deformation in the adjacent Precordillera thrust belt. A Paleogene phase of cooling may also support a phase of compressional deformation in the Sierras Pampeanas during a period in which neutral tectonic conditions have been proposed along the Andean margin. Our findings outline a history of fault reactivation suggesting that the long-lived fault systems in the Sierras Pampeanas are sensitive to plate-margin processes.

A Broad, Distributed Active Fault Zone Lies beneath Salt Lake City, Utah

Abstract Although the Wasatch fault is currently known to have a high-seismic hazard from motion along range-bounding faults, new seismic data reveal faulted and folded 13,000–30,000-yr-old Lake Bonneville strata beneath Salt Lake City (SLC). Coupled with previous excavation trench, borehole, and other geologic and geophysical observations, we conclude that a zone of latest Pleistocene and/or Holocene faulting and folding kinematically links the East Bench and Warm Springs faults through a 3 km wide relay structure and transfer zone. We characterize faults beneath downtown SLC as active, and these faults may displace or deform the ground surface during an earthquake. Through offset but linked faults, our observations support throughgoing ruptures across faults of the Wasatch fault zone (WFZ) and an elevated risk of earthquake-induced building damage.

Slip rates and paleoearthquakes along the east segment of the Guanggaishan–Dieshan fault zone, West Qinling Range, NW China

The WNW-trending Guanggaishan–Dieshan fault (GDF) is located in the West Qinling Range at a site of tectonic junction between the Tibetan plateau, the Sichuan basin and the Ordos block. Tectonically, this region is bounded by the East Kunlun fault to the southwest, the West Qinling fault to the north, and the Longmenshan fault to the southeast. The activity of the GDF is essentially related with the northeastward expansion of the Tibetan Plateau and associated strain partition between the East Kunlun fault and the West Qinling fault. This study reported the sense of fault movement, rate of fault slip, and the timing of three paleoseismic events on the east segment of the Guanggaishan–Dieshan fault (EGDF) with which no historical earthquake has been documented before. We focused our investigation on three branch faults in the central part of the EGDF using a range of study methods, e.g. structural mapping, paleoseismological trenching, unmanned aircraft vehicle photogrammetry, differential GPS survey, as well as radiocarbon and optically stimulated luminescence dating. Our new results highlighted: (1) the EGDF is dominated by the left-lateral sense of movement with a minor component of dip slip, (2) the range-bounding branch is characterized by a maximum left-lateral slip rate of 0.72 ± 0.34 mm/year and a maximum vertical-slip rate of 0.13 ± 0.03 mm/year, respectively. Kinematics of the EGDF is similar to many other active faults in the West Qinling Range, together they play a joint role in accommodating and transferring the residual left-lateral slip of the East Kunlun Fault. Three paleoseismic events were identified on the range-bounding fault based on two types of event markers: cut-cover relation and colluvial deposition. The ages of the most recent two events are estimated to be 2765–3320 cal. year BP and 12,775–13,005 cal. year BP and the age of the oldest event is as early as 18,495–32,950 cal. year BP. In contrast, there is no compelling evidence to show paleoseismic event along the rest two branches, and further work is needed to demonstrate this.

Plate boundary trench retreat and dextral shear drive intracontinental fault-slip histories: Neogene dextral faulting across the Gabbs Valley and Gillis Ranges, Central Walker Lane, Nevada

Abstract The spatial-temporal evolution of intracontinental faults and the forces that drive their style, orientation, and timing are central to understanding tectonic processes. Intracontinental NW-striking dextral faults in the Gabbs Valley–Gillis Ranges (hereafter referred to as the GVGR), Nevada, define a structural domain known as the eastern Central Walker Lane located east of the western margin of the North American plate. To consider how changes in boundary type along the western margin of the North American plate influenced both the initiation and continued dextral fault slip to the present day in the GVGR, we combine our new detailed geologic mapping, structural studies, and 40Ar/39Ar geochronology with published geologic maps to calculate early to middle Miocene dextral fault-slip rates. In the GVGR, Mesozoic basement is nonconformably overlain by a late Oligocene to Miocene sequence dominated by tuffs, lavas, and sedimentary rocks. These rocks are cut and offset by four primary NW-striking dextral faults, from east to west the Petrified Spring, Benton Spring, Gumdrop Hills, and Agai Pah Hills–Indian Head faults. A range of geologic markers, including tuff- and lava-filled paleovalleys, the southern extent of lava flows, and a normal fault, show average dextral offset magnitudes of 9.6 ± 1.1 km, 7.0 ± 1.7 km, 9.7 ± 1.0 km, and 4.9 ± 1.1 km across the four faults, respectively. Cumulative dextral offset across the GVGR is 31.2 ± 2.3 km. Initiation of slip along the Petrified Spring fault is tightly bracketed between 15.99 ± 0.05 Ma and 15.71 ± 0.03 Ma, whereas slip along the other faults initiated after 24.30 ± 0.05 Ma to 20.14 ± 0.26 Ma. Assuming that slip along all four faults initiated at the same time as the Petrified Spring fault yields calculated dextral fault-slip rates of 0.4 ± 0.1–0.6 ± 0.1 mm/yr, 0.4 ± 0.1–0.5 ± 0.1 mm/yr, 0.6 ± 0.1 mm/yr, and 0.3 ± 0.1 mm/yr on the four faults, respectively. Middle Miocene initiation of dextral fault slip across the GVGR overlaps with the onset of normal slip along range-bounding faults in the western Basin and Range to the north and the northern Eastern California shear zone to the south. Based on this spatial-temporal relationship, we propose that dextral fault slip across the GVGR defines a kinematic link or accommodation zone between the two regions of extension. At the time of initiation of dextral slip across the GVGR, the plate-boundary setting to the west was characterized by subduction of the Farallon plate beneath the North American plate. To account for the middle Miocene onset of extension across the Basin and Range and dextral slip in the GVGR, we hypothesize that middle Miocene trench retreat drove westward motion of the Sierra Nevada and behind it, crustal extension across the Basin and Range and NW-dextral shear within the GVGR. During the Pliocene, the plate boundary to the west changed to NW-dextral shear between the Pacific and North American plates, which drove continued dextral slip along the same faults within the GVGR because they were fortuitously aligned subparallel to plate boundary motion.

Post-LGM faulting in Central Europe: LiDAR detection of the >50 km-long Sub-Tatra fault, Western Carpathians

The increasing availability of high-resolution LiDAR data reveals surprisingly strong topographic evidence of late Quaternary faulting in some intraplate regions characterized by weak recent tectonic activity. For the first time, we present evidence of a >50 km-long post-Last Glacial Maximum (LGM) tectonic rupture along the southern margin of the Tatra Mountains (Slovakia) in the highest part of the Carpathian Mountains. Using a high-resolution 1 × 1 m LiDAR DEM, we recognize that the normal, range-bounding Sub-Tatra fault (STF) and its eastward branch, the Ružbachy fault (RBF), offset late Quaternary glacial moraines, alluvial fans and colluvial slopes. The fault zone morphology is dominated by a nearly continuous, southward-facing, up to ~18 m-high main scarp locally accompanied by antithetic fault scarps and hanging wall grabens. Similar offsets of pre-LGM and LGM deposits and the absence of fault scarps within Holocene alluvium suggest that the STF and RBF were active for a relatively short time interval between ~18 ka and the Holocene. Using the vertical separation of dated LGM moraine crests, we estimate that the minimum average postglacial slip rates along the STF and RBF varied from ~0.1–0.3 mm/year, i.e., rates one order of magnitude higher than those reported for other faults in Central Europe. Additionally, deep-seated landslides and the presence of large rock avalanches along the fault suggest possible postglacial high-magnitude earthquake(s) producing coseismic rupture of the STF and RBF. We conclude that 1) southern range-bounding faults of the Tatra Mountains are the longest (>50 km) and only documented post-LGM ruptures revealing extensive offset of late Quaternary deposits in the Western Carpathians and 2) relatively small along-strike variations in postglacial offset mimic the long-term exhumation trends and imply “en block” uplift of the southern margin of the Tatra Mountains with the highest displacement along the east STF.

Regional relative tectonic activity of structures in the Pampean flat slab segment of Argentina from 30 to 32°S

The Pampean flat slab segment of the Central Andes in Argentina is a broad, active deformation zone subject to both plate boundary-related and intraplate deformation. To the west, plate boundary-related deformation is dominated by thin-skinned fold-and-thrusts of the Precordillera, while to the east, intraplate deformation involves thick-skinned reverse faults of the Sierras Pampeanas. GPS data and absolute dating of displaced geomorphic features suggest that most of the active permanent deformation occurs between these two regions at the Andean orogenic front and that there is a west-to-east trend of decreasing shortening rates. However, GPS stations and sites with geomorphic slip rate measurements are currently too sparse and cover too short a time period to make reliable inferences about the longer-term distribution and relative rates of deformation. Mountain range-scale geomorphic indices, on the other hand, reflect tectonic activity since at least the Pleistocene. In this study, we measured geomorphic indices (hypsometric integral and curves, basin elongation ratio, basin volume-to-area-ratio, valley floor width-to-height ratio, mountain front sinuosity, and normalized steepness indices of rivers) from 10 different N-S striking, range-bounding faults that span both the Precordillera and Sierras Pampeanas regions from west to east. We use these data to assess the regional relative tectonic activity of major Quaternary deformation features in the Pampean segment of the Central Andes. Even when variations in climate and geology are considered, mean values for each geomorphic index suggests uplift rates for the 10 mountain-range bounding faults display local trends that are closely associated with the structural style and tectonic evolution of these structures. Although vertical uplift rates vary little, horizontal shortening rates are higher for the Precordillera fold-and-thrusts in the west than the thick-skinned reverse faults of the Sierras Pampeanas in the east, due to shallower fault dips for Precordilleran structures. The similarity with decadal slip rate gradients from GPS studies could possibly indicate that the stress-field has been constant since at least the Pleistocene, but further slip rate studies over broader time intervals in the late Quaternary are necessary to confirm this. Variations observed in relative uplift rates along each fault point to sites that warrant further detailed study of slip rates and evaluation of associated seismic hazards.

NEOTECTONIC BASIN EVOLUTION IN CENTRAL-EASTERN MAINLAND GREECE: AN OVERVIEW

The neotectonic evolution of central-eastern mainland Greece (Sterea Hellas) is documented in the result of local extensional tectonics within a regional transtensional field, which is related to the westward propagation of the North Anatolian Fault. The observed tectonic structures within the neotectonic basins and their margins (range-bounding faults and fault zones, rotation of tectonic blocL·) suggest a close relation to the Parnassos Detachment Fault (PDF), which is a reused alpine thrust surface. Lokris basin (LB) occupied a central position in this neotectonic configuration, having received its first sediments in the Uppermost Miocene and subsequently been greatly affected by tectonic episodes, which continue until nowadays. LB is considered to have been separated from the present-day North Gulf of Evia not earlier than the Lower Pleistocene. Voiotihos Kifissos Basin, on the other hand, is tightly related to the activation of PDF, occupying the position of a frontal basin and having developed along the main detachment front.

Late Quaternary alluvial fan terraces: Langshan, Inner Mongolia, China

Alluvial fan terraces of the Langshan Range in the northwestern Hetao Basin of northern China records uplift on range-bounding faults and late Quaternary climate change. Alluvial fan profiles are segmented in the central part of the range front, which lack strath terraces that developed in the southwest (SW) and northeast (NE) parts. The stream channels in the central part have higher channel gradients and lower ratios of the valley floor width to valley height (V). Four episodes of alluvial fan terraces were upstream from the range-bounding fault, which formation ages were estimated as ~116, 64, 35, and 10ka BP according to optically stimulated luminescence (OSL) dating combined with previous 14C and OSL ages. The correlation between time of the terraces formation and climatic fluctuations recorded in a Guliya ice core reveals that alluvial fan aggradation was concentrated during the cold stages and that incision occurred during the succeeding cold-to-warm transitions. A majority of alluvial fan terraces developed in accelerated uplift since ~64ka, which indicates that tectonic uplift provided the driving force for continuous downcutting after climatic change triggered it. Geomorphic indices (channel gradient and V) distribution reconstructs the model of discrepant uplift in the Langshan Range with the fastest uplift rate in the central part, which is controlled by range-bounding faults.

Rates and style of Cenozoic deformation around the Gonghe Basin, northeastern Tibetan Plateau

The northeastern Tibetan Plateau constitutes a transitional region between the low-relief physiographic plateau to the south and the high-relief ranges of the Qilian Shan to the north. Cenozoic deformation across this margin of the plateau is associated with localized growth of fault-cored mountain ranges and associated basins. Herein, we combine detailed structural analysis of the geometry of range-bounding faults and deformation of foreland basin strata with geomorphic and exhumational records of erosion in hanging-wall ranges in order to investigate the magnitude, timing, and style of deformation along the two primary fault systems, the Qinghai Nan Shan and the Gonghe Nan Shan. Structural mapping shows that both ranges have developed above imbricate fans of listric thrust faults, which sole into decollements in the middle crust. Restoration of shortening along balanced cross sections suggests a minimum of 0.8–2.2 km and 5.1–6.9 km of shortening, respectively. Growth strata in the associated foreland basin record the onset of deformation on the two fault systems at ca. 6–10 Ma and ca. 7–10 Ma, respectively, and thus our analysis suggests late Cenozoic shortening rates of 0.2 +0.2/–0.1 km/m.y. and 0.7 +0.3/–0.2 km/m.y. along the north and south sides of Gonghe Basin. Along the Qinghai Nan Shan, these rates are similar to late Pleistocene slip rates of ∼0.10 ± 0.04 mm/yr, derived from restoration and dating of a deformed alluvial-fan surface. Collectively, our results imply that deformation along both flanks of the doubly vergent Qilian Shan–Nan Shan initiated by ca. 10 Ma and that subsequent shortening has been relatively steady since that time.

Active forearc shortening in Tohoku, Japan: Constraints on fault geometry from erosion rates and fluvial longitudinal profiles

Convexities in the longitudinal profiles of actively incising rivers are typically considered to represent the morphologic signal of a transient response to external perturbations in tectonic or climatic forcing. Distinguishing such knickzones from those that may be anchored to the channel network by spatial variations in rock uplift, however, can be challenging. Here, we combine stream profile analysis, 10Be watershed-averaged erosion rates, and numerical modeling of stream profile evolution to evaluate whether knickzones in the Abukuma massif of northeast Japan represent a temporal or spatial change in rock uplift rate in relation to forearc shortening. Knickzones in channels that drain the eastern flank of the Abukuma massif are characterized by breaks in slope–area scaling and separate low-gradient, alluvial upper-channel segments from high-gradient, deeply-incised lower channel segments. Average erosion rates inferred from 10Be concentrations in modern sediment below knickzones exceed erosion rates above knickzones by 20–50%. Although profile convexities could be interpreted as a transient response to an increase in rock uplift rate associated with slip on the range-bounding fault, geologic constraints on the initiation of fault slip and the magnitude of displacement cannot be reconciled with a recent, spatially uniform increase in slip rate. Rather, we find that knickzone position, stream profile gradients, and basin averaged erosion rates are best explained by a relatively abrupt spatial increase in uplift rate localized above a flat-ramp transition in the fault system. These analyses highlight the importance of considering spatially non-uniform uplift in the interpretation of stream profile evolution and demonstrate that the adjustment of river profiles to fault displacement can provide constraints on fault geometry in actively eroding landscapes.

Depositional history, tectonics, and detrital zircon geochronology of Ordovician and Devonian strata in southwestern Mongolia

Lower Paleozoic successions of the Gobi-Altai zone of southern Mongolia record an abrupt facies transition from deposition of predominantly fine-grained uppermost Ordovician through lowermost Devonian carbonate and marl facies to deposition of coarse clastic strata of the Lower Devonian Tsakhir Formation. The Tsakhir generally fines upward from alluvial-fan cobble and pebble conglomerate to interbedded coarse- and fine-grained marine siliciclastic and carbonate strata, which were deposited within a tectonically active basin. The marine strata, deposited in a storm-influenced proximal to distal fan delta, include unusual event beds that grade from pebble conglomerate to hummocky cross-stratified grainstone and sandstone. These beds represent sediment emplaced by gravity flows during flood events and reworked by large gravity waves associated with storm events. The interpreted link between flood deposition and storm wave reworking supports a hyperpycnal flow interpretation for these deposits. The sudden facies transition at the base of the formation represents the sedimentological and stratigraphic signature of Early Devonian tectonism in the Gobi-Altai zone. The general upward-fining pattern of the Tsakhir is interpreted as a response to the creation of accommodation space at a greater rate than progradation of the fan delta, in large part due to tectonic subsidence, although some component of eustasy may have been involved. The production of steep relief and deposition of associated volcanics suggest a transition from relatively passive deposition to active tectonics in this region during the Lochkovian to Pragian stages of the Early Devonian. We herein introduce the term “Tsakhir event” for this important tectonic transition. Range-bounding faults for this event are not preserved, but alluvial-fan deposition, the development of unconformities, renewed subsidence, and magmatism throughout the Gobi-Altai zone all suggest syndepositional tectonism. Detrital zircon spectra from both Ordovician and Devonian strata contain Archean to Paleozoic ages. Minor differences between Ordovician and Devonian samples suggest changes in source regions and/or transport paths prior to, and after, the Tsakhir event. The paleoenvironmental setting of the Tsakhir Formation requires short transport distances, and thus the age spectrum of a sample from this formation represents proximal basement rocks of the Shine Jinst region of the Gobi-Altai zone. Basement rocks are not exposed in the Shine Jinst region, but the wide variety of ages in all of the detrital spectra suggest a nearby continental source. Our detrital age spectra contain peaks that coincide with basement ages and magmatic events on the adjacent Mongolian microcontinent and also have strong similarities with recently published spectra of nearby landmasses in Neoproterozoic to Paleozoic paleogeographic reconstructions, namely, Siberia, North China, eastern Gondwana, and Tarim. These similarities extend to spectra of late Neoproterozoic to middle Paleozoic rocks throughout Gondwanaland and also Siberia, illustrating the somewhat limited utility of detrital spectra for determining the tectonic affinities of crustal blocks at this time in Earth history.

The Quaternary thrust system of the northern Alaska Range

The framework of Quaternary faults in Alaska remains poorly constrained. Recent studies in the Alaska Range north of the Denali fault add significantly to the recognition of Quaternary deformation in this active orogen. Faults and folds active during the Quaternary occur over a length of ∼500 km along the northern flank of the Alaska Range, extending from Mount McKinley (Denali) eastward to the Tok River valley. These faults exist as a continuous system of active structures, but we divide the system into four regions based on east-west changes in structural style. At the western end, the Kantishna Hills have only two known faults but the highest rate of shallow crustal seismicity. The western northern foothills fold-thrust belt consists of a 50-km-wide zone of subparallel thrust and reverse faults. This broad zone of deformation narrows to the east in a transition zone where the range-bounding fault of the western northern foothills fold-thrust belt terminates and displacement occurs on thrust and/or reverse faults closer to the Denali fault. The eastern northern foothills fold-thrust belt is characterized by ∼40-km-long thrust fault segments separated across left-steps by NNE-trending left-lateral faults. Altogether, these faults accommodate much of the topographic growth of the northern flank of the Alaska Range. Recognition of this thrust fault system represents a significant concern in addition to the Denali fault for infrastructure adjacent to and transecting the Alaska Range. Although additional work is required to characterize these faults sufficiently for seismic hazard analysis, the regional extent and structural character should require the consideration of the northern Alaska Range thrust system in regional tectonic models.

Seismicity within the actively deforming Eastern Sierras Pampeanas, Argentina

SUMMARY The Eastern Sierras Pampeanas of central Argentina have been interpreted by previous studies to represent the easternmost expression of continental deformation resulting from the flat-slab subduction of the Nazca Plate beneath the South American Plate. Yet, the seismic activity and deformation of this area, as well as its relationship to subduction more than 700 km to the west, are poorly understood. Using seismic data recorded by the Eastern Sierras Pampeanas broadband seismic array between 2008 August and 2009 August, we identified and located 445 local crustal earthquakes, most with magnitudes of 2.5 or less. We employed a double-difference location method to clarify the hypocentral distribution of local seismicity and to define zones of both clustered and diffuse seismicity. The relocated events define active structures in the region, including a horizontal zone of seismicity between 15 and 25 km depth, a prominent west-dipping structure beneath the central ranges and a vertical structure along the eastern margin of the study area. Overall, seismicity in this region during the study period is constrained to the upper ∼25 km of the crust and is mostly concentrated along minor structures and not along the major range-bounding faults. Focal mechanism solutions from the nine clearest events show mainly normal deformation with an oblique component and E–W oriented P-axes that likely result from shortening along structures with oblique orientations to the direction of plate convergence. This is the first seismic study to document that the Eastern Sierras Pampeanas are actively deforming in response to compression from the convergent margin.

Precise aftershock distribution and crustal structure in and around the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake

The 2008 Iwate-Miyagi Nairiku Earthquake (Mj 7.2) occurred on June 14, 2008, at the eastern flank of the Ou backbone range, in the central part of northern Honshu, Japan. In the northern part of the focal area, seismic reflection/refraction experiments conducted in 2006 and 2007 revealed a series of west-dipping faults. To investigate the relation between these faults and aftershocks, we conducted a high-density seismic array observation across the northern focal area. The arrival times of earthquakes were used in a joint inversion for earthquake locations and velocity structure. The V p structure shows tilted blocks and the block boundaries coincide with the revealed west-dipping faults. These faults were interpreted to have formed during the Miocene extension. The aftershock distribution shows a concentration on a plane dipping westward approximately 40° beneath the eastern margin of the Ou backbone range. The zone of aftershock concentration can be correlated to the known range-bounding fault inferred from seismic profiling and does not coincide with the known active reverse fault (Dedana fault). These results indicate the significance of the potential source fault located along the boundary of the Ou backbone range.

Range-Bounding Normal Fault of Smith Valley, Nevada: Limits on Age of Last Surface-Rupture Earthquake and Late Pleistocene Rate of Displacement

Abstract Smith Valley is bounded on its western edge by an active normal fault and the Pine Nut mountains. Displacement on the fault is responsible for development of one of the westernmost basins of the Basin and Range province of the western United States. Interpretation of an exposure afforded by excavation of a trench across the range-bounding fault places the most recent surface-rupture earthquake after 5176±130 cal yr B.P. and suggests it was close in time to 3530±82 cal yr B.P. Utilization of tephrochonologic and cosmogenic analyses and mapping of fault scarps across the Artesia Road fan are the basis for putting forth an initial estimate of the late Pleistocene vertical slip rate of the fault at between 0.125 mm/yr and 0.33 mm/yr.