Late Quaternary active faulting on the inherited Baoertu basement fault within the eastern Tian Shan orogenic belt: Implications for regional tectonic deformation and slip partitioning, NW China

The deformation pattern and slip partitioning related to oblique underthrusting of the Tarim Basin in the eastern Tian Shan orogenic belt are not well understood because interior deformation images are lacking. The Baoertu fault is an E-W−striking, ∼350-km-long reactivated basement structure within the eastern Tian Shan. In this study, we quantify its late Quaternary activity based on interpretations of detailed high-resolution remote sensing images and field investigations. Three field observation sites along an ∼80-km-long fault segment indicate that the Baoertu fault is characterized by sinistral thrust faulting. Based on surveying of the displaced geomorphic surfaces with an unmanned drone and dating of the late Quaternary sediments using radiocarbon and optically stimulated luminescence (OSL) methods, we estimate a late Quaternary left-lateral, strike-slip rate of 1.87 ± 0.29 mm/yr and a N−S shortening rate of 0.26 ± 0.04 mm/yr for this fault. The lithospheric Baoertu fault acts as a decoupling zone and accommodates the left-lateral shearing caused by the oblique underthrusting of the Tarim Basin. In the eastern Tian Shan orogenic belt, the oblique convergence is partitioned into thrust faulting across the entire range and sinistral slip faulting on the high-dip basement structure within the orogen. This active faulting pattern in the eastern Tian Shan of sinistral shearing in the center and thrust faulting on both sides can be viewed as giant, crustal-scale positive flower structures.

Supplemental Material: Late Quaternary active faulting on the inherited Baoertu basement fault within the eastern Tian Shan orogenic belt: Implications for regional tectonic deformation and slip partitioning, NW China

Figure S1: Profile of velocity components normal to structure striking (E-W components) across the eastern Tian Shan (profile from (85.3°, 41.0°) to (85.3°, 45.0°) with a width of 240 km.

Supplemental Material: Late Quaternary active faulting on the inherited Baoertu basement fault within the eastern Tian Shan orogenic belt: Implications for regional tectonic deformation and slip partitioning, NW China

Figure S1: Profile of velocity components normal to structure striking (E-W components) across the eastern Tian Shan (profile from (85.3°, 41.0°) to (85.3°, 45.0°) with a width of 240 km.

Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

In Southern California, plate boundary motion between the North American and Pacific plates is distributed across several sub-parallel fault systems. The offshore faults of the California Continental Borderland (CCB) are thought to accommodate ∼10–15% of the total plate boundary motion, but the exact distribution of slip and the mechanics of slip partitioning remain uncertain. The Newport-Inglewood-Rose Canyon fault is the easternmost fault within the CCB whose southern segment splays out into a complex network of faults beneath San Diego Bay. A pull-apart basin model between the Rose Canyon and the offshore Descanso fault has been used to explain prominent fault orientations and subsidence beneath San Diego Bay; however, this model does not account for faults in the southern portion of the bay or faulting east of the bay. To investigate the characteristics of faulting and stratigraphic architecture beneath San Diego Bay, we combined a suite of reprocessed legacy airgun multi-channel seismic profiles and high-resolution Chirp data, with age and lithology controls from geotechnical boreholes and shallow sub-surface vibracores. This combined dataset is used to create gridded horizon surfaces, fault maps, and perform a kinematic fault analysis. The structure beneath San Diego Bay is dominated by down-to-the-east motion on normal faults that can be separated into two distinct groups. The strikes of these two fault groups can be explained with a double pull-apart basin model for San Diego Bay. In our conceptual model, the western portion of San Diego Bay is controlled by a right-step between the Rose Canyon and Descanso faults, which matches both observations and predictions from laboratory models. The eastern portion of San Diego Bay appears to be controlled by an inferred step-over between the Rose Canyon and San Miguel-Vallecitos faults and displays distinct fault strike orientations, which kinematic analysis indicates should have a significant component of strike-slip partitioning that is not detectable in the seismic data. The potential of a Rose Canyon-San Miguel-Vallecitos fault connection would effectively cut the stepover distance in half and have important implications for the seismic hazard of the San Diego-Tijuana metropolitan area (population ∼3 million people).

New constraints on Quaternary slip partitioning near the eastern termination of the Altyn Tagh Fault

<p>Slip partitioning along the northern boundary of the Tibetan Plateau is essential for understanding regional deformation and the seismic potential of the different faults that accommodate it. Within this framework the Altyn Tagh Fault (ATF) is commonly viewed as the primary structure that separates the Tibetan Plateau from the stable Gobi-Alashan block to the north. Late Quaternary sinistral slip rates of 8-12 mm/yr across the central ATF between 86° and 93°E decrease eastwards to zero as the fault approaches its mid-continental termination at ~97°E. To better understand how late Quaternary slip is partitioned along the ATF’s eastern termination we obtained new slip-rate measurements  for the ~200-km-long left-lateral ENE striking Sanweishan Fault (SSF) located ~60 km north of the ATF between 94°-96°E near the town of Dunhuang.</p><p>Multiple sinistral offsets ranging up to 600 m were identified by linking the clast assemblage of offset alluvial fan remnants with their provenance upstream of the fault.  Luminescence dating revealed depositional ages ranging from 100 - 200 ka for the offset features and time-invariant minimum sinistral slip of 2.5±1 mm/yr during the last ~200 ka, which is approximately an order of magnitude higher than previously reported slip-rates for the SSF. Our results indicate that the SSF and the eastern segment of the ATF accommodate comparable magnitudes of late Quaternary slip. Considering this substantial transfer of lateral slip as far as 60 km north of the eastern ATF we propose that the SSF may represent juvenile northeastward expansion of the Tibetan Plateau into previously stable parts of the Gobi-Alashan block.</p>

Field Response and Surface-Rupture Characteristics of the 2020 M 6.5 Monte Cristo Range Earthquake, Central Walker Lane, Nevada

The M 6.5 Monte Cristo Range earthquake that occurred in the central Walker Lane on 15 May 2020 was the largest earthquake in Nevada in 66 yr and resulted in a multidisciplinary scientific field response. The earthquake was the result of left-lateral slip along largely unmapped parts of the Candelaria fault, one of a series of east–northeast-striking faults that comprise the Mina deflection, a major right step in the north–northwest structural grain of the central Walker Lane. We describe the characteristics of the surface rupture and document distinct differences in the style and orientation of fractures produced along the 28 km long rupture zone. Along the western part of the rupture, left-lateral and extensional displacements occurred along northeasterly and north-striking planes that splay off the eastern termination of the mapped Candelaria fault. To the east, extensional and right-lateral displacements occurred along predominantly north-striking planes that project toward well-defined Quaternary and bedrock faults. Although, the largest left-lateral displacement observed was ∼20 cm, the majority of displacements were <5 cm and were distributed across broad zones up to 800 m wide, which are not likely to be preserved in the geologic record. The complex pattern of surface rupture is consistent with a network of faults defined in the shallow subsurface by aftershock seismicity and suggests that slip partitioning between east-striking left-lateral faults and north to northwest-striking right-lateral faults plays an important role in accommodating northwest-directed transtension in the central Walker Lane.