The seismogenic thickness of the crust, a proxy for brittle-crust thickness, is a geometric parameter related to crustal strength, seismic hazard, and the crust's thermo-mechanical nature. We use ...high-resolution earthquake-location data from California to construct a topographic map of the base of the seismogenic crust by calculating the depth above which 95% of seismicity (D95) is located for fixed width bins. Seismogenic thickness is highly variable, ranging from ~5 km to >30 km, with thicker D95 values in the Great Valley-Sierra Nevada and thinner values in the Walker Lane and northern coastal California. Seismogenic thickness is inversely correlated with surface heat flow in most locations, consistent with a steady-state conductive crust, and local deviations probably reflect non-steady-state conditions related to magmatism and/or hydrothermal circulation. Such correlation suggests that, at regional scale, brittle-ductile transition (BDT) depth is mostly controlled by geothermal gradients, and the base of the seismogenic crust essentially represents a BDT isotherm (~300–350 °C for quartz-dominated lithologies). Spatial variations of D95 depths across California can be used to evaluate or constrain the locations of future seismicity, propagation direction of earthquake ruptures, and maximum depth, rupture area, and magnitude of future strike-slip earthquake events. Thicker seismogenic crust has a greater integrated strength. Seismogenic depth asperities, which represent mechanically stronger crustal patches, may focus and nucleate future earthquake events and/or impede rupture propagation.
•Seismogenic thickness map (D95) made for California using relocated earthquakes•D95 varies between ~5 km and >30 km.•D95 correlates with heat flow, not strain rate, confirming temperature dependence.•Seismogenic thickness is a proxy for crustal strength and impacts seismic hazard.•Fewer earthquakes in D95 crust; ruptures may propagate toward thinner D95 values.
Collision-induced continental deformation commonly involves complex interactions between strike-slip faulting and off-fault deformation, yet this relationship has rarely been quantified. In northern ...Tibet, Cenozoic deformation is expressed by the development of the >1000-km-long east-striking left-slip Kunlun, Qinling, and Haiyuan faults. Each have a maximum slip in the central fault segment exceeding 10s to ~100km but a much smaller slip magnitude (~<10% of the maximum slip) at their terminations. The along-strike variation of fault offsets and pervasive off-fault deformation create a strain pattern that departs from the expectations of the classic plate-like rigid-body motion and flow-like distributed deformation end-member models for continental tectonics. Here we propose a non-rigid bookshelf-fault model for the Cenozoic tectonic development of northern Tibet. Our model, quantitatively relating discrete left-slip faulting to distributed off-fault deformation during regional clockwise rotation, explains several puzzling features, including the: (1) clockwise rotation of east-striking left-slip faults against the northeast-striking left-slip Altyn Tagh fault along the northwestern margin of the Tibetan Plateau, (2) alternating fault-parallel extension and shortening in the off-fault regions, and (3) eastward-tapering map-view geometries of the Qimen Tagh, Qaidam, and Qilian Shan thrust belts that link with the three major left-slip faults in northern Tibet. We refer to this specific non-rigid bookshelf-fault system as a passive bookshelf-fault system because the rotating bookshelf panels are detached from the rigid bounding domains. As a consequence, the wallrock of the strike-slip faults deforms to accommodate both the clockwise rotation of the left-slip faults and off-fault strain that arises at the fault ends. An important implication of our model is that the style and magnitude of Cenozoic deformation in northern Tibet vary considerably in the east–west direction. Thus, any single north–south cross section and its kinematic reconstruction through the region do not properly quantify the complex deformational processes of plateau formation.
•We propose a non-rigid passive bookshelf-fault model for deformation in north Tibet.•Right-lateral shear drives clockwise rotation and left-slip faulting on W–E faults.•Clockwise rotation of faults occurs against left-slip Altyn Tagh bounding fault.•Model explains bidirectional decrease in slip and four-quadrant strain pattern.•Protracted deformation in northern Tibet since Eocene
Uplift of the Tibetan Plateau and the distribution of deformation across it are the result of India-Asia collision, which bring an opportunity of understanding intracontinental tectonics in the ...context of continent-continent collision. The Tibetan Plateau is bound on the northern margin by the Qilian Shan thrust belt and the strike-slip Haiyuan fault. These Cenozoic fault systems play a critical role in accommodating continental convergence, yet the initiation age, deformation sequence and mechanisms of deformation are debated. In this study, integrated geologic mapping, field observations, and apatite fission track thermochronology were conducted to constrain the initiation ages of the localized thrust faults and the exhumation history of the central and northern Qilian Shan, northern Tibet. Our analyses reveal the central and northern Qilian Shan underwent rapid cooling during the Cretaceous as a result of a far-field tectonic event. In the Eocene-Oligocene, a period of thrust-related cooling occurred along the Shule Nan Shan, Tuolai Nan Shan and Tuolai Shan faults. Reactivation of the proximal thrust faults and initiation of the western segment of the Haiyuan fault occurred at ca. 16 Ma and drove final accelerated Miocene cooling and denudation to the surface. We argue that the Qilian Shan thrust belt has persisted as the stationary and internally deformed northern boundary of the Himalayan-Tibetan orogen since the early Cenozoic, involved overprinting out-of-sequence development starting by Eocene related to initiation of India-Asia collision, and the basins and ranges across the northern Tibetan Plateau have since experienced multi-phase of growth.
•The central and northern Qilian Shan experienced three-pulse cooling starting in the early Cretaceous.•Thrust systems and Haiyuan fault across the Qilian Shan initiated in Eocene-Oligocene, and Miocene, respectively.•The Qilian Shan thrust belt underwent out-of-sequence deformation starting in Eocene shortly after the Indo-Asia collision.
The Late Cretaceous to Paleogene Laramide orogen in the North American Cordillera involved deformation >1,000 km from the plate margin that has been attributed to either plate-boundary end loading or ...basal traction exerted on the upper plate from the subducted Farallon flat slab. Prevailing tectonic models fail to explain the relative absence of Laramide-aged (ca. 90-60 Ma) contractional deformation within the Cordillera hinterland. Based on Raman spectroscopy of carbonaceous material thermometry and literature data from the restored upper 15-20 km of the Cordilleran crust we reconstruct the Late Cretaceous thermal architecture of the hinterland. Interpolation of compiled temperature data (n = 200) through a vertical crustal column reveals that the hinterland experienced a continuous but regionally elevated, upper-crustal geothermal gradient of >40 °C/km during Laramide orogenesis, consistent with peak metamorphic conditions and synchronous peraluminous granitic plutonism. The hot and partially melted hinterland promoted lower crust mobility and crust-mantle decoupling during flat-slab traction.
Whether continental deformation is accommodated by microplate motion or continuum flow is a central issue regarding the nature of Cenozoic deformation surrounding the eastern Himalayan syntaxis. The ...microplate model predicts southeastward extrusion of rigid blocks along widely-spaced strike-slip faults, whereas the crustal-flow model requires clockwise crustal rotation along closely-spaced, semi-circular right-slip faults around the eastern Himalayan syntaxis. Although global positioning system (GPS) data support the crustal-flow model, the surface velocity field provides no information on the evolution of the India-Asia orogenic system at million-year scales. In this work, we present the results of systematic geologic mapping across the northernmost segment of the Indo-Burma Ranges, located directly southeast of the eastern Himalayan syntaxis. Early research inferred the area to have experienced either right-slip faulting accommodating northward indentation of India or thrusting due to the eastward continuation of the Himalayan orogen in the Cenozoic. Our mapping supports the presence of dip-slip thrust faults, rather than strike-slip faults. Specifically, the northern Indo-Burma Ranges exposes south- to west-directed ductile thrust shear zones in the hinterland and brittle fault zones in the foreland. The trends of ductile stretching lineations within thrust shear zones and thrust sheets rotate clockwise from the northeast direction in the northern part of the study area to the east direction in the southern part of the study area. This clockwise deflection pattern of lineations around the eastern Himalayan syntaxis mirrors the clockwise crustal-rotation pattern as suggested by the crustal-flow model and contemporary GPS velocity field. However, our finding is inconsistent with discrete strike-slip deformation in the area and the microplate model.
•Thrusting dominates the northern Indo-Burma Ranges.•Slip vectors rotate along the same thrusts from south to west.•Rotation of thrust vectors indicates non-rigid-body deformation.•Thrusting results from crustal flow around the eastern Himalayan syntaxis.
The Proterozoic‐Phanerozoic evolution of the Tarim and North China cratons is integral to the construction of the Eurasian continent. Throughout the Paleozoic, these continents were bound by the ...Paleo‐Asian and Tethyan Oceans to the north and south, respectively, and, thus, their paleogeography is critical to reconstructions of the oceanic domains. Specifically, it remains uncertain whether the Tarim and North China cratons were contiguous during the Paleozoic. Geologic observations from the Qilian Shan and Longshou Shan of western China provide valuable information regarding the paleotectonic relationships of these continents. Here we present detailed field, geochronological, and geochemical observations from key locations in the Qilian Shan and Longshou Shan to decipher complex relationships between the Kunlun‐Qaidam, North China, and Tarim continents. Paleoproterozoic deformation might have been associated with the northern North China orogen, whereas a Neoproterozoic collisional orogen occurred between the Kunlun‐Qaidam‐South Tarim and the North China‐North Tarim. Subsequent late Neoproterozoic rifting led to the opening of the Qilian ocean as an embayed marginal sea, and the Paleo‐Asian Ocean developed along the northern margin of the North China craton. South‐dipping subduction, arc magmatism, slab rollback, and convergence between Kunlun‐Qaidam and North China continued throughout the Early Silurian, which were recorded in the Qilian Shan and Longshou Shan regions. Our updated geotectonic framework requires reevaluation of previously published paleogeographic models, including ones that suggest the North China craton was affixed to Gondwana in the early Paleozoic. It is further considered a possible connection between the North China and southern Africa at ca. 2.05 Ga.
Key Points
The western North China craton records at least three orogenies from the Paleoproterozoic to the early Paleozoic
Mesoproterozoic strata in North China, Tarim, and the Qilian Shan are similar, suggesting continuity among these continents
Gondwana was not affixed to the western margin of North China in the Neoproterozoic‐early Paleozoic
The ~1,500‐km‐long, north trending Eastern Flanking Belt of the Himalayan‐Tibetan orogenic system is located along the eastern margin of the Indian subcontinent. Although the belt is a key element of ...the Cenozoic India‐Asia collisional zone, its tectonic evolution remains poorly understood. This lack of knowledge has impacted our ability to differentiate between competing hypotheses for the evolution of the India‐Asia collision. To address this problem, we integrate constraints on the structural framework and magnitude of Cenozoic shortening strain with thermochronology of the northernmost segment of the belt located directly southeast of the eastern Himalayan syntaxis (i.e., the northern Indo‐Burma Ranges). The study area exposes a southwest directed thrust belt that is bounded by the Indian craton in the west and the right‐slip Jiali fault zone in the east. New and existing (U‐Th)/He and 40Ar/39Ar thermochronologic data indicate that thrust‐related cooling occurred from ~36 Ma in the northeast to ~5.6 Ma in the southwest. Episodes of out‐of‐sequence thrusting occurred at ~30–20, ~14–12, and ~11–6 Ma within the thrust belt. Restoration of the thrust belt yields a minimum horizontal shortening of ~280 km (~86%). These results combined with (1) the recorded local absence of several major Himalayan‐Tibetan lithologic units (i.e., Tethyan Himalayan Sequence, Greater Himalayan Sequence, and southern Gangdese batholith) and (2) the southward decrease in the thrust‐belt width (33–5 km) suggest a complex history of thrusting in the northern Indo‐Burma Ranges and an spatial increase in Cenozoic crustal shortening and/or continental underthrusting from west to east across the eastern Himalayan syntaxis.
Key Points
The northern Indo‐Burma Ranges expose a Cenozoic southwest to west directed thrust belt that is bounded by two active right‐slip faults
Thrusting since ~36 Ma accommodated >280 km (86%) shortening during clockwise crustal flow around the eastern Himalayan syntaxis
Our findings support an increase in Cenozoic shortening and/or continental underthrusting along the easternmost India‐Asia plate boundary
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