Left Slip Research Articles

From Left Slip to Transpression: Cenozoic Tectonic Evolution of the North Altyn Fault, NW Margin of the Tibetan Plateau

AbstractThe Cenozoic North Altyn Fault (NAF) is a major splay of the Altyn Tagh Fault along the northwestern margin of the Tibetan Plateau, but its role in the development of this plateau margin in response to the India‐Eurasia collision is highly debated. Here, we investigate fault geometry, kinematics, and shortening magnitude along the westernmost 120 km of the NAF. Seismic surveys reveal minimal Cenozoic shortening in the subsurface of the Southeast Tarim Basin and support for large‐scale (>120 km) left slip on the NAF. Based on the satellite imagery, two new faults are identified to define the northern boundary of the NAF system, which together with the NAF to the south constitute a narrow transpressional shear zone comprised of three basement‐cored, fault‐bound slivers. Fission track data and thermal modeling indicate that the NAF zone experienced broad reburial during early Cenozoic that was locally interrupted by ∼40 to 35 Ma exhumation proximal to the NAF, followed by widespread but heterogeneous exhumation since ∼17 to 15 Ma associated with ∼9 to 11 km of total shortening across the NAF system via thick‐skinned faulting. We conclude that the NAF initiated as a left‐slip fault at ∼40 to 35 Ma and then became transpressional at ∼17 to 15 Ma during the middle‐Miocene reorganization of the Altyn Tagh Fault system. We find no evidence to support prior inferences of large‐scale (∼100 km) underthrusting of the Tarim Basin beneath the Tibetan Plateau along the NAF.

An Overview of the Geological Control on Groundwater Potential in Lacamutu River, Alaua Kraik Area, Baucau Municipality, Timor-Leste

Timor island has a tropical climate with relatively little rainfall and surface water is often not available throughout the year with groundwater relied on to fulfill daily domestic necessities. Geological reconnaissance mapping, hydrogeological investigation, and resistivity survey were undertaken in this study to systematically understand the hydrogeologic system (e.g., aquifer system, hydrostratigraphic units, groundwater flow direction) and its potentiality for water supply to human consumption in Alaua Kraik area, Baucau Municipality, Timor-Leste. Res2DInv, Dips 5.1, Surfer 16, Global Mapper 13, and ArcGIS 10.6 software was used to create geological reconnaissance maps, resistivity interpretation profile lines, and a hydrostratigraphic model. Rainfall precipitation, rainfall intensity, maximum rate of runoff and infiltration data are also used to interpret the groundwater potential in the study area. Two rock units occur in the study area; permeable alluvial deposits which unconformable overlie impermeable interbedded red marl-chert and calcareous shale. Structurally the area comprises the Lacamutu anticline, thrust fault, left slip fault, and normal right slip fault. Resistivity lines indicate three (3) types of lithologies: alluvial deposit, an intercalated layer of red marl-chert, calcareous shale and wet calcareous shale. The alluvial deposit and red marl-chert layer intercalated with calcareous shale units are classified as a hydrostratigraphic unit of intergranular and localised aquifer systems with low productivity. The groundwater flows through the existing fractures of the shear joint and tends to flow towards the left slip fault plane zone from the North to South direction. Much of the rainwater in the study area is most likely intercepted, evaporated, and or transpiration as opposed to running off and infiltrating into the ground. The permeable and heavily fractured impermeable rock units in the study area have good porosity but low permeability and represent poor aquifers. The springs and Lacamutu River have low discharge and are generally dry in the dry season as it does not have an adequate aquifer that can accumulate and pass groundwater with significant volumes even if the rainfall in the study area is classified as moderate rainfall.

Structural Control of Primary Tin Mineralization (Case Study: Parit Tiga, West Bangka Regency, Bangka Belitung)

Tins are important commodities of metal mineral deposits in Bangka and have been mined since the seventeenth century. The research aims to determine the characteristics of primary tin mineralization in Parit Tiga, West Bangka Regency, Bangka Belitung and the control of geological structures on mineralization. Rock samples were obtained from geological mapping and analyzed using X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), petrography, and mineragraphy method. From the analysis, the alteration can be divided into four zones: quartz + tourmaline alteration, quartz + halloysite + chlorite + pyrophyllite alteration, quartz + illite alteration, and quartz + kaolinite + chlorite + dickite alteration. The cassiterite, hematite, and pyrite are found in the mineralization zone associated with the greisen deposit. Based on the structural analysis data conducted on the normal left slip fault, the shear stress has an orientation of NE-SW and resulting in the forming of NW-SE gash fractures. These gash fractures have been filled by quartz and the tourmaline as disseminated minerals. The distribution of mineralization and alteration are found adjacent to the sheeted quartz-tourmaline veins. The dominated-tin concentration in ore deposits has the same orientation as the fault structure. These infer that the mineralization and alteration in the research area are controlled by geological structure.

Development and Interplay of Two Orthogonal Fault Systems in the NW Qaidam Basin, Northern Tibetan Plateau

How fold and thrust belts (FTBs) evolve over time and space in a transpressional regime remains poorly understood. Based on high-resolution 3D seismic reflection data and remote sensing images, we herein present a detailed structural analysis of the Cenozoic faults in the NW margin of the Qaidam Basin that is bounded to the north by the left-reverse Altyn Tagh fault system. Two sets of orthogonal, basement-involved faults with contrasting geometries, kinematics, and temporal development are identified. One set consists of generally E-W-striking, N-dipping, reverse faults with a component of sinistral shear. They are parallel or subparallel to the Altyn Tagh fault system, led to southward tilting of the basement, and formed a local unconformity between the middle Miocene Shangyoushashan formation and underlying strata. They developed in an out-of-sequence order, and were mostly active during 43.8–15.3 Ma but in relatively tectonic quiescence with limited weak reactivation since then. The second set is mainly composed of the NNW-striking reverse faults with dextral shear components. They are approximately perpendicular to the Altyn Tagh fault system, and intensively active since ∼15.3 Ma, much later than the initiation of the E-W-striking faults. Together with published results, we ascribe the development of these two sets of orthogonal faults as the transition from transpression to left lateral slip on the central segment of the Altyn Tagh fault system. The two fault sets interplayed with each other in two ways: 1) the older E-W-striking faults were offset by younger NNW-striking faults, and 2) the younger NNW-striking faults curved to link with the preexisting E-W-striking faults. Our findings reveal that transpressional-dominated FTBs evolve in a more complicated way than the contractional-dominated ones, and more site-based case studies are needed to reveal the underlying primary principles.

Geologi Daerah Dimito dan Sekitarnya Kabupaten Boalemo Provinsi Gorontalo

Administratively, the research area covers two districts, Wonosari Sub-district and Dulupi Sub-district, Boalemo District, Gorontalo Province. Based on the physiography of the Gorontalo region, the research area which is included in the southern mountain zone of Bone-Tilamuta-Modello generally consists of ancient volcano-sedimentary rock formations in Gorontalo, the Eocene-Oligocene. The purpose of this geological research is to discover the geological order that developed in the area of Dimito and surrounding are as covering several aspect such as geomorphology, stratigraphy, geological structure, and geological history. The result shows that the geomorphology of the research area divided into three geomorphic units, namely the units of the Alluvial Plain, Volcanic Hills, and Denudasional Hills. Stratigraphy of the research area divided into five types of units sorted from the early Miocene to the Holocene, i.e., Lava Basal Dimito (Early Eocene), Wackestone Dimito (Early Eocene), Andesit Kalidingin (Middle Miocene), Granodiorite Bualo (Middle Pliocene), and Alluvial Buaolo (Recen). Geological structures that work on the location of the research area relatively trend from northeast-southwest to northwest-southeast. Fault working on the location of the research area from the data processing consists of Normal Left Slip Fault Kalidingin and Lag Left Slip Fault Bualo.

Soil-structure interaction of existing buildings near the active fault zone (Burdur-Turkey)

Turkey is located in a tectonically active region. In the recent years, earthquakes occurred in settlement areas caused people dead and substantial damage to buildings. In order to reduce damages of possible earthquake effects on the buildings and people’s strengthening, the buildings and structuring in safer areas is increasingly gained importance. The Burdur city, situated at the tectonically active northeastern part of the Fethiye-Burdur Fault Zone (FBFZ), SW Turkey. This fault zone is approximately 300 km in length, and it contains several normal faults with left lateral oblique slip components. The Burdur settlement area is situated on the tectonically active fault segments of the FBFZ. This paper focuses on soil-structure interaction of existing buildings in Burdur settlement area built near the active FBFZ. In order to highlight the high risky, potentially risky, and low risky buildings for guiding decisions on retrofitting or renewal these buildings. In the study, to test and quantify the spatial distribution of the buildings built on the high or low liquefaction risk zones, the Getis-Ord Gi* statistics for hot and cold spot determination are applied to buildings which are weighted with the ground water level. According to results, the NE of the area buildings was built in high liquefaction prone areas. However, in the SW of the area buildings were built in low liquefaction prone areas. Other potentially risky areas located in the center of the area include those along some of the dried streams deposits, which produce the loose young soils that are particularly susceptible to liquefaction.

Late Quaternary Tectonic Deformation of the Eastern End of the Altyn Tagh Fault

AbstractThe Quaternary activity of the faults at the eastern end of the Altyn Tagh fault, including the Dengdengshan–Chijiaciwo, Kuantanshan and Heishan faults, was studied on the basis of interpretation of satellite images, trenching, geomorphologic offset measurements and dating. The Altyn Tagh fault has extended eastwards to Kuantanshan Mountain. The left–slip rates of the Altyn Tagh fault decreased through the Qilianshan fault and were transformed into thrust and folds deformation of many NW– trending faults within the Jiuxi basin. Meanwhile, under NE–directed compression of the Tibetan plateau, thrust dominated the Dengdengshan–Chijiaciwo fault northeast of the Kuantanshan uplift with a rate lower than that of every fault in the Jiuxi basin south of the uplift, implying that tectonic deformation is mainly confined to the plateau interior and the Hexi Corridor area. From continual northeastward enlargement of the Altyn Tagh fault, the Kuantanshan uplift became a triangular wedge intruding to the east, while the Kuantanshan area at the end of this wedge rose up strongly. In future, the Altyn Tagh fault will continue to spread eastward along the Heishan and Jintananshan faults. The results have implications for understanding the propagation of crustal deformation and the mechanism of the India–Eurasian collision.

Primary Surface Ruptures of the LudianMw 6.2 Earthquake, Southeastern Tibetan Plateau, China

Online Material: Google Earth images. A M w 6.2 earthquake struck Ludian County at 08:30:11 UTC (16:30:10.2 local time) on 3 August 2014 and is one of the most destructive earthquakes that has occurred on an unmapped active fault east of the Xianshuihe–Xiaojiang fault system between the Chuandian rhomb block (the Sichuan–Yunnan block), southeastern Tibetan plateau, and the South China block, a stable tectonic terrain (Fig. 1; Wen et al. , 2013; X. Xu, Jiang, et al. , 2014; Cheng et al. , 2015). Its hypocenter is located at 27.11° N, 103.35° E, at a depth of ∼12 km, adjacent to Longtoushan Township, ∼29 km southwest of Zhaotong City (Liu et al. , 2014; L. Xu et al. , 2014; Y. Zhang et al. , 2014). The relocation of the aftershocks and a little of the reported fieldwork results may demonstrate the seismogenic fault of the Ludian earthquake is the Baogunao–Xiaohe fault, a northwest-trending left-lateral strike-slip fault, which cut through the Zhaotong–Lianfeng fault zone (http://www.eq-igl.ac.cn/contents/41/867.html, last accessed August 2014; Fang et al. , 2014; Liu et al. , 2014; W. Wang, et al. , 2014; X. Xu, Cheng, et al. , 2014; X. Xu, Jiang, et al. , 2014). We know nothing at all about the former, whereas the latter is a highly developed northeast-trending reverse fault with a right-slip component (Wen et al. , 2013). The finite-source inversion using regional seismic broadband data suggests the hypocenter of the Ludian earthquake was initiated from ∼12 km in the upper crust, then propagated unilaterally to the southeast and upward to the shallow with a peak left slip of ∼0.7 m; it likely offset the ground to form a surface rupture zone 10 km in length (Liu et al. , 2014), but there is no any reliable evidence to support or oppose this view. The significant directivity of the rupture propagation during …

Wrench-Slip Reversals and Structural Inversions: Cenozoic Slide-Rule Tectonics in Sundaland

DOI: 10.17014/ijog.v1i1.174Most of continental Southeast Asia, that is, Sundaland and Indosinia, achieved a relative tectonic stability by the beginning of the Cenozoic. Since then a strong tectonic activity in Sundaland has been restricted to existing regional fault zones and to regional slow, vertical crustal movements elsewhere that produced small to very large sedimentary basins. On the other hand, regional deformation of Indosinia as a consequence of ductile shearing has continued into the Paleogene. Since the Oligocene, the northern part of Sundaland and Indosinia have been extruded differentially towards southeast along the Red River, Wang Chao (or Mae Ping, or Tonle Sap), and Three Pagodas - Axial Malay fault zones. The initial cause has been attributed to hard collision between Subplate India with Megaplate Eurasia. Plate dynamics in the region have changed substantially since Mid-Miocene as to force wrench-slip reversals along the major fault zones in Sundaland as well as in Indosinia. Concomitant structural inversions are demonstrated on seismic sections. In the core of Sundaland, earlier transtensional wrenching was succeeded by transpressive strike-slip faulting that on major faults of the Malay Basin manifested in reversals of sense. From the Hinge-line fault eastward, the transtensional left wrench slip was succeeded by transpressional dextral slip, while in the region to its west the wrench-slip kinematics was an earlier transtensional right slip followed by transpressional left slip. In the Strait of Malacca and eastern margin of Sumatra, right-lateral wrenching in the Neogene has been common. In certain places it could be established a wrench-slip of transtensional character in Oligocene-Early Miocene, and the transpressional wrench movement occurred mainly during the Middle to Late Miocene. The remarkable coincidence of termination of spreading of the South China Basin in Langhian, and that of the West Philippine and Caroline basins during Mid-Miocene invites further study.

Role of Cognition and Priming in Interlimb Generalization of Adaptive Control of Gait Stability

ABSTRACT The authors examined the impact of (a) awareness of impending slip and (b) priming (to accelerate learning) on interlimb transfer of acquired stability control. Among the participants, 12 of 24 young individuals were not informed of 2 impending slips on the nontrained left-side (UNWARN), whereas the other 12 knew the exact timing and location of the 1st left slip (WARN) following 24 right-side slips, given to all participants while walking. Both groups reduced the backward loss of balance (BLOB) incidence from 100% on the first slip to 0% on 24th slip. The authors found significantly better transfer of the improved stability control in WARN than in UNWARN, resulting in a greater reduction in BLOB on the 1st left slip (55% vs. 92%, respectively). The priming was evident with no BLOB on the 2nd left slip versus 50% on the 2nd right slip. Thus, the interlimb generalization can be improved by cognition and evidenced in priming.

Basin geometry and cumulative offsets in the Eastern Transverse Ranges, southern California: Implications for transrotational deformation along the San Andreas fault system

The Eastern Transverse Ranges, adjacent to and southeast of the big left bend of the San Andreas fault, southern California, form a crustal block that has rotated clockwise in response to dextral shear within the San Andreas system. Previous studies have indicated a discrepancy between the measured magnitudes of left slip on through-going east-striking fault zones of the Eastern Transverse Ranges and those predicted by simple geometric models using paleomagnetically determined clockwise rotations of basalts distributed along the faults. To assess the magnitude and source of this discrepancy, we apply new gravity and magnetic data in combination with geologic data to better constrain cumulative fault offsets and to define basin structure for the block between the Pinto Mountain and Chiriaco fault zones. Estimates of offset from using the length of pull-apart basins developed within left-stepping strands of the sinistral faults are consistent with those derived by matching offset magnetic anomalies and bedrock patterns, indicating a cumulative offset of at most ~40 km. The upper limit of displacements constrained by the geophysical and geologic data overlaps with the lower limit of those predicted at the 95% confidence level by models of conservative slip located on margins of rigid rotating blocks and the clockwise rotation of the paleomagnetic vectors. Any discrepancy is likely resolved by internal deformation within the blocks, such as intense deformation adjacent to the San Andreas fault (that can account for the absence of basins there as predicted by rigid-block models) and linkage via subsidiary faults between the main faults.

GPS-constrained inversion of present-day slip rates along major faults of the Sichuan-Yunnan region, China

A linked-fault-element model is employed to invert for contemporary slip rates along major active faults in the Sichuan-Yunnan region (96°–108°E, 21°–35°N) using the least squares method. The model is based on known fault geometry, and constrained by a GPS-derived horizontal velocity field. Our results support a model attributing the eastward extrusion of the Tibetan Plateau driven mainly by the north-northeastward indentation of the Indian plate into Tibet and the gravitational collapse of the plateau. Resisted by a relatively stable south China block, materials of the Sichuan-Yunnan region rotate clockwise around the eastern Himalayan tectonic syntaxis. During the process the Garze-Yushu, Xianshuihe, Anninghe, Zemuhe, Daliangshan, and Xiaojiang faults, the southwest extension of the Xiaojiang fault, and the Daluo-Jinghong and Mae Chan faults constitute the northeast and east boundaries of the eastward extrusion, with their left slip rates being 0.3–14.7, 8.9–17.1, 5.1 ± 2.5, 2.8 ± 2.3, 7.1 ± 2.1, 9.4 ± 1.2, 10.1 ± 2.0, 7.3 ± 2.6, and 4.9 ± 3.0 mm/a respectively. The southwestern boundary consists of a widely distributed dextral transpressional zone other than a single fault. Right slip rates of 4.2 ± 1.3, 4.3 ± 1.1, and 8.5 ± 1.7 mm/a are detected across the Nanhua-Chuxiong-Jianshui, Wuliangshan, and Longling-Lancang faults. Crustal deformation across the Longmenshan fault is weak, with shortening rates of 1.4 ± 1.0 and 1.6 ± 1.3 mm/a across the Baoxing-Beichuan and Beichuan-Qingchuan segments. Northwest of the Longmenshan fault lies an active deformation zone (the Longriba fault) with 5.1±1.2 mm/a right slip across. Relatively large slip rates are detected across a few faults within the Sichuan-Yunnan block: 4.4±1.3 mm/a left slip and 2.7±1.1 mm/a shortening across the Litang fault, and 2.7±2.3 mm/a right-lateral shearing and 6.7±2.3 mm/a shortening across the Yunongxi fault and its surrounding regions. In conclusion, we find that the Sichuan-Yunnan region is divided into more than a dozen active micro-blocks by a large number of faults with relatively slow slip rates. The eastward extrusion of the Tibetan Plateau is absorbed and adjusted in the region mainly by these faults, other than a small number of large strike-slip faults with fast slip rates.

Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau

We derive a detailed horizontal velocity field for the southeast borderland of the Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2004. Our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes, separated by strike‐slip and transtensional faults. Most notably, the regional deformation includes 10–11 mm/yr left slip across the Xianshuihe fault, ∼7 mm/yr left slip across the Anninghe‐Zemuhe‐Xiaojiang fault zone, ∼2 mm/yr right slip across a shear zone trending northwest near the southern segment of the Lancang River fault, and ∼3 mm/yr left slip across the Lijiang fault. Deformation along the southern segment of the Red River fault appears not significant at present time. The region south and west of the Xianshuihe‐Xiaojiang fault system, whose eastward motion is resisted by the stable south China block to the east, turns from eastward to southward motion with respect to south China, resulting in clockwise rotation of its internal subblocks. Active deformation is detected across two previously unknown deformation zones: one is located ∼150 km northwest of and in parallel with the Longmenshan fault with 4–6 mm/yr right‐slip and another is continued south‐southwestward from the Xiaojiang fault abutting the Red River fault with ∼7 mm/yr left slip. While both of these zones are seismically active, the exact locations of faults responsible for such deformation are yet to be mapped by field geology. Comparing our GPS results with predictions of various models proposed for Tibetan Plateau deformation, we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger, highly fragmented upper crust.

Active faulting in the Walker Lane

Deformation across the San Andreas and Walker Lane fault systems accounts for most relative Pacific–North American transform plate motion. The Walker Lane is composed of discontinuous sets of right‐slip faults that are located to the east and strike approximately parallel to the San Andreas fault system. Mapping of active faults in the central Walker Lane shows that right‐lateral shear is locally accommodated by rotation of crustal blocks bounded by steep‐dipping east striking left‐slip faults. The left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area, including Rattlesnake and Garfield flats; Teels, Columbus and Rhodes salt marshes; and Queen Valley. The Benton Springs and Petrified Springs faults are the major northwest striking structures currently accommodating transform motion in the central Walker Lane. Right‐lateral offsets of late Pleistocene surfaces along the two faults point to slip rates of at least 1 mm/yr. The northern limit of northwest trending strike‐slip faults in the central Walker Lane is abrupt and reflects transfer of strike‐slip to dip‐slip deformation in the western Basin and Range and transformation of right slip into rotation of crustal blocks to the north. The transfer of strike slip in the central Walker Lane to dip slip in the western Basin and Range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long‐lived feature of the deformation field. The complexity of faulting and apparent rotation of crustal blocks within the Walker Lane is consistent with the concept of a partially detached and elastic‐brittle crust that is being transported on a continuously deforming layer below. The regional pattern of faulting within the Walker Lane is more complex than observed along the San Andreas fault system to the west. The difference is attributed to the relatively less cumulative slip that has occurred across the Walker Lane and that oblique components of displacement are of opposite sense along the Walker Lane (extension) and San Andreas (contraction), respectively. Despite the gross differences in fault pattern, the Walker Lane and San Andreas also share similarities in deformation style, including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike‐slip and dip‐slip faults.

Cenozoic Left-Slip Motion along the Central Altyn Tagh Fault as Inferred from the Sedimentary Record

Several Cenozoic sedimentary basins are present along the central segment of the Cenozoic Altyn Tagh fault (ATF) that marks the northern boundary of the Tibetan Plateau. Field investigations reveal that basin sedimentation and subsequent deformation are controlled by left-slip motion along the Cenozoic ATF. In order to better understand the temporal and spatial interactions between the Altyn Tagh fault and the development of adjacent basins, we divided Cenozoic sedimentary sequences into three subunits based on lithologic variation and the presence of unconformities. From our own observations and regional correlations, we suggest that slip along the central ATF began in the Early Oligocene. Reconstruction of sedimentary relationships among the basin, slip along the fault, and offset topography suggests that the ATF experienced four stages of Cenozoic leftslip motion. The offset of a Late Miocene sequence and its possible correlation across the Altyn Tagh fault suggests 80-100 km left slip. This yields an average slip rate of 10-12.5 mm/yr assuming that the sequence was deposited at ~8 Ma.

Tectonics of the San Gabriel Basin and surroundings, southern California

The SanGabriel Basin is a Pliocene–Pleistocene Transverse Ranges structure bounded by the San Gabriel Mountains on the north, the San Jose and Puente Hills on the east, and the Repetto and Montebello Hills on the west. A basement boundary between gneissoid rocks of San Gabriel Mountains aspect and Peninsular Ranges rocks strikes east across the basin and connects with the San Jose fault. This boundary may represent the eastward continuation of the Canton fault, a middle Miocene precursor of the San Gabriel fault system with up to 30 km of right slip. Canton fault displacement is largely canceled by ∼22.5 km of left slip on a precursor to the Raymond fault such that an Oligocene dacite in the San Jose Hills and clasts derived from it show little evidence of large horizontal displacement between the San Gabriel Mountains and Puente Hills. Clockwise rotation in the middle Miocene probably influenced the distribution of lateral slip. The fill in the northeast-trending Pliocene–Pleistocene San Gabriel Basin is a basal shallow-marine sequence overlain by the nonmarine Duarte Conglomerate, in contrast to the deep-water Fernando Formation of the Los Angeles Basin. The paleoslope trends south in the direction of the Repetto and Puente Hills, evidence that uplift of these hills postdated deposition. The triangular San Gabriel Basin is currently a stable block bounded by the right-slip, northwest-striking East Montebello fault on the west and the left-slip(?), northeast-striking Walnut Creek fault on the east. Farther east, the San Jose and Puente Hills are underlain by an east-trending active fold belt, including the San Jose reverse fault and anticline, the Walnut anticline, and the Puente Hills anticline adjacent to the right-lateral Whittier fault. The East Montebello fault is bounded on the west by the Elysian Park anticlinorium, a fold belt in downtown Los Angeles. The left-slip Holocene Raymond fault joins the Sierra Madre reverse fault at its segment boundary marked by the Clamshell-Sawpit fault. The triangular stable block between east-trending fold belts is bounded on the south by the Puente Hills blind thrust and on the north by the Sierra Madre fault, documenting north-south Quaternary convergence. East-west extension is also indicated, although not confirmed by geodetic measurements.

Fault-Related Folding in California's Northern Channel Islands Documented by Rapid-Static GPS Positioning

Rapid-static Global Positioning System positioning was used to measure late Quaternary deformation of uplifted coastal terraces on Santa Rosa and San Miguel Islands, which have accumulated regional warping and local faulting over 100+ k.y. Late Quaternary deformation on Santa Rosa is sharply partitioned, with several hundred meters of surfacerupturing left slip, but 15+ m of vertical motion taken up by folding, resulting in anticlinal growth of at least 0.12 m/k.y. Deformation on San Miguel Island to the west is consistent with warping on the north limb of the regional anticlinal structure. Deformation on both islands matches activity to the east, suggesting that the Santa Rosa Island fault represents an en echelon segment of the larger Transverse Ranges Boundary fault system and the Northern Channel Islands antiform. The Northern Channel Islands have been the focus of a debate over differing geometrical models of fault-related folding, and the deformation measurements presented here suggest that uplift of the islands is occurring over a smoothly curved thrust ramp rather than a fault-bend fold. This reinterpretation is important, as current earthquake-hazard assessments on this and other buried reverse faults depend entirely on assumptions unique to the fault-bend fold geometry. We suggest that alternative models should be considered where geologically reasonable, and that surface uplift measurements across fault-related folds can provide a crucial test of subsurface geometry.

Quaternary faulting in the southern Mississippi embayment and implications for tectonics and seismicity in an intraplate setting

The recently recognized Saline River fault zone in the southwestern Mississippi embayment (strike = 135°) is characterized by moderate historic seismicity. We document paleoseismicity (possibly strong) on the Saline River fault zone that is outside the known region of neotectonism in the northern embayment. Six surface faults were excavated within the Saline River fault zone, and all faults displace marine Eocene units and fluvial Pliocene–Pleistocene units; five show post-Wisconsin loess movement (thermoluminescence [TL] age = 23.6 ± 3 ka), and three deform middle to late Holocene eolian silt (TL age = 5.1 ± 0.6 ka and 3.6 ± 0.5 ka) as fault-tip flexures. Along the principal excavated fault, fault-plane braiding and plunging drag folding suggest a strong component of left slip, and subsidiary faults show both normal and reverse displacement (<1m) of Pliocene−Pleistocene, Wisconsin, and Holocene units. We interpret these faults as components of a left-slip flower structure. A shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows Eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m, supporting a strike-slip interpretation. In addition, the only focal plane mechanism available in the area indicates left slip with a small reverse component. We conclude that the Saline River fault zone is a Paleozoic–Mesozoic basement fault zone reactivated as a left-slip system in a Quaternary east-west compressive stress field. Field evidence suggests that earthquakes with several meters of strike slip may be characteristic, and the width of a liquefaction field on the Saline River fault zone suggests an event of magnitude 5.5–6. Considering a similarity of strike and sense of Holocene movements of the Meers fault of southern Oklahoma and of the Saline River fault zone, there may be additional active fault zones that pose seismic hazards concealed by sediments in southern North America.

Late Cenozoic to Holocene deformation in southwestern Sichuan and adjacent Yunnan, China, and its role in formation of the southeastern part of the Tibetan Plateau

From at least 2–4 Ma to present, crust in the southeastern part of the Tibetan Plateau west of the convex-east Xianshuihe-Xiaojiang fault system has deformed internally and rotated clockwise around the eastern Himalayan syntaxis. The northwest-striking Ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78–100 km, of which ∼60 km is transferred to the Xianshuihe fault zone across a diffuse transfer zone, and ∼22–40 km is absorbed by bending of older structures and crustal shortening. Crustal shortening is expressed along and east of the eastern end of the Ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the Tibetan Plateau. A similar transfer of left slip into shortening occurs farther south across the Xianshuihe fault in the high mountains around and east of Gongga Shan (7556 m). The northwest-striking, convex-east, left-lateral Litang fault zone lies southwest of the Ganzi-Xianshuihe-Xiaojiang fault zone and appears to be less well developed but otherwise similar to the Ganzi fault zone. The Batang, Chenzhi, and other northeast-striking right-lateral faults of small displacement occur within the rotating crustal fragment. Together with the left-slip faults, they accommodate east-west shortening northeast of the eastern Himalayan syntaxis. South of this region of shortening, the crust is extending to form grabens within the Dali and southern Xiaojiang fault systems and in the Tengchong volcanic province. The progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern Himalayan syntaxis. The assemblage of structures in southwestern Sichuan geometrically resembles structures of Eocene to Miocene age in southern Yunnan that were positioned northeast of the eastern Himalayan syntaxis, similar to present-day southwestern Sichuan, at the time of their development. The similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past ∼50 m.y. Structures in Sichuan are less fully developed than older structures in southwestern Yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis. Deformation in these areas indicates that plateau formation has been complex, inhomogeneous, and diachronous at scales from 1000 km to less than 100 km.

Is the North Altyn fault part of a strike-slip duplex along the Altyn Tagh fault system?

Although the Altyn Tagh fault system has played an important role in the Indo-Asian collision, its geometry and tectonic evolution remain poorly known. Between 86° and 92°E, this system is at least 100 km wide and is bounded to the north and south by the North Altyn and Altyn Tagh faults, respectively. Mapping along the Jianglisai reach of the North Altyn fault indicates that Miocene(?) to Pliocene(?) motion was predominantly left to left-reverse slip, with transport vectors trending N45°–60°E. Map relationships suggest that total offset on the fault is >120 km. These results are inconsistent with previous models of the Altyn Tagh fault system in which oblique convergence along the northern margin of the Tibetan Plateau is partitioned into thrusting on the North Altyn fault and left slip on the Altyn Tagh fault. An alternative hypothesis is that the North Altyn fault is the northern boundary of a transpressional strike-slip duplex within which the structurally elevated Altyn Mountains were created. Our model suggests that transpressional deformation may be restricted to this strike-slip duplex and need not characterize the entire margin.