Gray, K. D.Foster, D. A.Johnson, K.Isakson, V. H....
38页
查看更多>>摘要：Paleomagnetic data reviewed along an 800-km transect extending from northeastern Oregon into southwest Montana indicate adjacent crustal blocks experienced post-Jurassic vertical-axis clockwise rotation relative to North America. From west to east, rotated blocks include relict island-arc and related terranes of the Blue Mountains province (Oregon: >60 degrees rotation), calc-alkaline plutons along the arc-continent boundary (Idaho: similar to 30 degrees rotation), and displaced passive margin strata in the Rocky Mountain foreland (Montana: <= 60 degrees rotation). Assessment of polyphase contractional deformation shows oroclinal bending (similar to 35 degrees westward swing) of the Blue Mountains block concurrent with clockwise turning of the entire terrane province (our variant of conventional model; S.W. Carey, 1955-), with an approximated hinge along the Olympic-Wallowa lineament (similar to N30W). Restoration of clockwise rotation (similar to 66 degrees; Blue Mountains) reveals collisional mountain-building below a promontory buttress, coeval with torsional displacement on the continental interior.
查看更多>>摘要：The ongoing Cenozoic Indo-Asia continental collision not only formed the highly elevated Tibetan Plateau but also reactivated the crustal deformation in adjacent Paleozoic-Mesozoic orogens. This includes the renewed phase of deformation in the Qinling Orogen, northeast of the Tibetan Plateau, which was formed initially by Paleozoic closure of the Paleo-Tethys and subsequent early Triassic continental collision between the North and South China blocks. Debates continue as to when and how the reactivation initiated. To investigate this question, this study reports a vertical profile of apatite and zircon (U???Th)/He thermochronometric ages from the Guangtoushan Granite to estimate the exhumation history of the central Qinling Orogen, where previous models for the growth of the Tibetan Plateau inferred late Cenozoic northeastward growth of the Tibetan Plateau by outward flow of the lower crust from the plateau interior. Age-elevation relationship and thermal history modelling suggest two phases of relatively rapid exhumation during the early Cretaceous (-130???110 Ma) and the latest Cretaceous ??? early Paleocene (-70???50 Ma), respectively. The early Cretaceous exhumation coincided with the time of the Lhasa-Qiangtang collision, whose far field effects have been widely reported in regions of the central-northern Tibetan Plateau. In the southern and central Qinling, our finding of an early Paleocene (-70???50 Ma) increase in exhumation rate followed by minimal exhumation during the late Cenozoic suggests negligible impact of the lower crust flow from the expanding Tibetan Plateau on the rock exhumation of the Qinling. Instead, our study suggests instant strain migrations from the Lhasa-Qiangtang and Indo-Asia collision zones to the northeast margin of the Tibetan Plateau during the early Cretaceous and the early Paleocene, respectively, supporting the models highlighting the importance of rigid blocks in facilitating intracontinental strain migration.
查看更多>>摘要：Gravity changes related to large earthquake preparation are reported in broad seismogenic source regions, and crustal fluids may change the gravity potential and trigger earthquakes. However, what is the major factors affecting crustal fluids movement in the upper 10 km of the continental crust and altering the gravity potential is still poorly understood. In this study, we evaluated the gravity changes caused by invasion of crustal fluids into the depth of 6???10 km using finite element based numerical modeling. We conducted a series of numerical ex-periments to investigate the influences of permeability distribution, the fluid source, and the invasion depth on gravity changes. The sensitivity analysis and comparison results suggest that the invasion of high-pressure fluids increases fluid density and produces positive gravity changes within a decade. Fluid pressure is the key factor for the variations of fluid density, while temperature exerts a minor influence. Consistent with the dissipation of fluid pressure, the corresponding gravity changes are fast (>3 ??Gal/year) in the first 2???3 years of fluid invasion and become increasingly slow later. Therefore, the first few years are the more suitable time for monitoring the gravity changes caused by crustal fluids invasion. The fluid-related gravity changes are positively correlated to the porosity and permeability of rocks adjacent to the fluid source, the salinity and the scale of crustal fluids, and the initial temperatures, and are negatively correlated to the invasion depth. Transient invasion of crustal fluids can be detected by means of the high precision gravity observation under certain conditions subjected to the initial geological characteristics of the continental crust and the geochemical evolution of the fluid sources.
查看更多>>摘要：Early Cretaceous rift basins in Northeast Asia typically experienced varying degrees of post-rifting thermal subsidence and compressional deformation. However, previous workers believed that only a thin post-rifting sequence was developed in basins west of the Great Xing???an Range, making a proper understanding of the post-rift tectonic history in this part of Northeast Asia difficult. In this study, newly acquired shallow seismic reflection data were utilized to unravel the shallow deformation of the Erlian Basin, which to the west of the Great Xing???an Range contains a thin post-rifting sequence. Furthermore, we used apatite fission-track thermochronology to analyze the post-rifting deposition and uplift processes in the Erlian Basin, for which there are limited stratigraphic records. Our results show that the Erlian Basin experienced a typical post-rifting stage whose sequence from the Albian to the Campanian was not controlled by normal faults. The corresponding sedimentary thickness was up to 2 km, which contradicts the previously reported little or no post-rifting thermal subsidence. The current thin post-rifting sequence is the remaining stratum after 1.4???1.8 km of denudation at the end of the Erlian period. In addition, by further comparing the sedimentary thickness and time-lag in the onset of post-rifting to those of other basins in Northeast Asia, we found two driving mechanisms of rifting in Northeast Asia, which occurred simultaneously: the gravitational collapse and back-arc extension. Moreover, the basins in Northeast Asia experienced a gradual weakening in the compressional deformation from east to west during the Cenomanian to the Coniacian, and an intense compressional deformation in the east of Northeast Asia and largescale vertical uplift in the west during the Late Cretaceous Campanian to the Eocene.
查看更多>>摘要：Broadband and long-period magnetotelluric (MT) soundings were collected along a 600-km-long EW-trending profile across the southwestern segment of the Jiangnan Orogenic Belt (JOB) to explore the possible remnants of ancient tectonic processes beneath the central South China Block (SCB). Beneath the JOB, the resistivity model reveals a west-dipping, trans-lithosphere conductor ( 20 ?? ??? m) embedded within the relatively thick (80???120 km) and highly resistive ( 1000 ?? ??? m) lithosphere, possibly associated with relics of the Neoproterozoic subduction zone between the Yangtze and Cathaysia blocks. The crustal portion of this conductor is interpreted as solid conducting materials (e.g., graphite, sulfide) that were emplaced into the crust during subduction and orogenesis, whereas its lithospheric mantle portion is more likely to be volatile-bearing minerals formed by subduction-related metasomatic processes. We attribute the greatly reduced asthenospheric resistivities (1-10 ?? ??? m) below the JOB to low-degree melting of the early-formed metasomatized mantle. In contrast, the lithosphere of the Cathaysia Block is evidently thinned (60???100 km) and segmented by several subvertical, lithospheric-scale conductors (1-30 ?? ??? m) that are likely associated with fossil fluid pathways along reactivated shear/fault zones. Notably, the thinnest lithosphere sits above a prominent low-resistivity (10 - 30 ?? ??? m) anomaly in the asthenospheric mantle and is spatially coincident with the location of Late Jurassic basaltic outcrops. We interpret these features as signs of lithospheric extension and asthenospheric upwelling driven by roll-back of the Paleo-Pacific Plate during the Late Mesozoic. The results provide key constraints on subduction???accretion tectonics associated with the Neoproterozoic assembly of the SCB and the subsequent lithospheric evolution during the Phanerozoic.
查看更多>>摘要：The primary driver of magmatism, crustal deformation and metallogeny of the North China Craton in the Jurassic remains actively debated, either attributed to the subduction of the Paleo-Pacific (i.e., Izanagi) plate in the east or to the closure of the Mongol-Okhotsk Ocean in the north. This issue is addressed here by examining temporal and spatial variation of the contemporary volcanic rocks (i.e., Tiaojishan Formation) of the Yanshan belt in the northern part of the craton. These rocks are mainly calc-alkaline and intermediate-felsic in composition and show a highly coherent geochemical composition and unradiogenic zircon Hf isotope, indicating that they have similar origins and formed in a subduction-related setting. The eruption age of the Jurassic volcanic rocks decreases westward (i.e., inland-ward) from Western Liaoning (166???153 Ma), through Pingquan-Chengde-Luanping (162???153 Ma) and Jingxi-Xuanhua (158???149 Ma), to Yuxian (157???142 Ma). This temporal and spatial variation is consistent with the motion of the subducted Paleo-Pacific plate over that time. In addition to the westward migration, the earliest volcanic rocks in the Yanshan belt formed earlier than those in the adjacent southern Great Xing???an Range in the north. These observations suggest that the southward subduction and closure of the Mongol-Okhotsk plate played a limited role in generation of these volcanic rocks. Two episodes of Middle-Late Jurassic volcanism (166???153 Ma and 150???139 Ma) are observed in the northern Great Xing???an Range. They are interpreted as results of closure of the Mongol-Okhotsk Ocean and subduction of the Paleo-Pacific plate, respectively. Temporal-spatial variation of volcanic activity in the northern North China Craton and adjacent region demonstrates that the Paleo-Pacific subduction was the principal cause of the evolution of Eastern China during the late Mesozoic, whereas the Mongol-Okhotsk tectonic regime is largely confined to the northern Great Xing???an Range and the area north to it.
查看更多>>摘要：The West Helanshan fault is situated at the junction of the northeastern Tibetan Plateau, Alxa, and Ordos blocks, providing an opportunity to investigate the deformation in the transitional zone between the contractional northeastern Tibetan Plateau and extensional North China. Using field investigations and satellite imagery interpretations, we determined detailed geometry and kinematics of the West Helanshan fault. The West Helanshan fault is divided into three segments, including the northern, middle, and southern segments, based on varying fault strike, slip sense, and geomorphic expression. The northern and middle segments are dominated by rightlateral strike-slip motion as indicated by linear fault traces, horizontally offset streams and terrace risers, relatively insignificant vertical offset, and frequently changing facing direction of fault scarps. The late Quaternary right-slip rate is estimated to be 0.2-0.4 mm/yr, which is relatively low comparing with other strike-slip faults in the northeastern Tibetan Plateau, based on Be-10-dated offset fluvial/alluvial surfaces. The southern segment shows conspicuous fault scarps with significant systematic vertical offset, which has a reverse component as revealed in the trench. The timing of the most recent earthquake revealed in the trench is constrained to be between 6.2 +/- 0.7 ka and 5.9 +/- 0.4 ka. Notably, the West Helanshan fault cuts through the city center of Azuoqi town, which is the capital of Azuoqi county and has about 100,000 residents, carrying high potential earthquake hazard in the future. Finally, combining the regional fault geometry and kinematics with GPS observations, we propose a limited north-northeastward extrusion model for the southern Alxa Block.
NSTL
Elsevier