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 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：With an excess of 75 Moz of gold produced, the Timmins-Porcupine mining camp in the Abitibi green stone belt is the largest gold producing camp in Canada. Most of the endowment is associated with two major, east-west striking, crustal fault zones, the Porcupine Destor fault and the Pipestone fault. The seismic survey presented in this study crosses these prolific fault systems in the Matheson area, which represents the eastern extension of the Timmins-Porcupine mining camp. The survey includes two receiver lines, the east and west profiles, with unequal receiver spacing among them and seismic shots only acquired along the east profile. After careful considerations, we treated the survey as a parallel geometry and designated the east and the west processing profiles to potentially image subsurface architecture in crystalline rock environment. We considered three scenarios to process the data: the entire survey, the east, and the west profiles individually considered. A full pseudo pre-stack time migrated 3D cube of the Archean crust documents the usefulness in imaging the extent of the geological features. By integrating the 2D and 3D seismic images with the local magnetic susceptibility isosurface model, we interpreted the Porcupine Destor fault to dip shallowly to the south and the Pipestone fault as steeplydipping/subvertical. The techniques used here can be further applied to other areas with similar dense infrastructure requiring imperfect acquisition survey geometry, including crooked lines, to produce fully 3D or pseudo seismic cubes in such areas of great exploration interest.
查看更多>>摘要：We present results derived from a seismic refraction experiment and gravity measurements about the crustal structure of southern Sri Lanka and the adjacent Indian Ocean. A P-wave velocity model was derived using forward modelling of the observed travel times along a 509 km long, N-S trending profile at 81???E longitude. Our results show that the continental crust below southern Sri Lanka is up to 38 km thick. A - 65 km wide transition zone, which thins seawards to -7 km thickness, divides stretched continental from oceanic crust. The adjacent, 4.7 to 7 km thick normal oceanic crust is covered by up to 4 km thick sediments. The oceanic crust is characterized by intra-crustal reflections and displays P-wave velocity variations, especially in oceanic layer 2, along our profile. In the central part of the profile, the uppermost mantle layer is characterized by normal P-wave mantle velocities of 8.0???8.1 km/s. At the southern end of the profile, unusual low upper mantle seismic velocities, ranging from 7.5 to 7.6 km/s only, characterize the uppermost mantle layer. These low upper mantle velocities are probably caused by partially serpentinized upper mantle. At even greater depths the upper mantle layer is characterized by velocities of 8.3 km/s on average. The type of margin along our profile is difficult to identify, since it is characterized by features typical for different types of margins.
查看更多>>摘要：The Xiaojiang Fault (XJF) Zone locates in the southeastern of Tibetan Plateau and defines the boundary between the South China and Sichuan-Yunnan blocks. Historical large earthquakes were hosted on the XJF, though its seismic hazard in the near future is under debate. In this study, we utilize portable broad-band seismic network to unravel the microseismic activities along XJF, and to further investigate the fault structures and their properties. Adopting PALM, a newly developed earthquake detection algorithm, we obtained -13,000 relocated events. The micro-seismicity reveals widespread off-fault structures showing conjugate geometry, while the major faults present low seismicity. The fault branches conjugate to the main-fault present intensive microseismicity, which hosts repeating events and presents high b-value. Regional GPS stations reflect slips are mostly concentrated along the XJF, while the slip rate on off-fault branches correlates with seismic activities on these structures. Combining with other recent seismological and magnetotellurics evidences, we suggest a low strength on these off-fault structures, which may partially release tectonic stress loading and serve as a barrier for future big earthquakes. On the XJF, the microseismic events are clustered on the fault junctions with low b-value. A special set of clusters between 25 degrees N to 25.5 degrees N show an along-strike variation of depth from 10 to 25-km, imaging the boundary between creeping and locked fault portions. We revisit the seismic hazard problem of XJF, and conclude that XJF is at the late stage of inter-seismic period.
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：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：Zones of fluid-bearing anomalies occur in volcanic regions, on a plate interface, or in the mantle wedge of a subduction zone. The various causes include slab dehydration, serpentinization or metamorphic reactions in different thermal regimes. To reveal the impact factors that might have contributed to slab melting and arc magmatism behind the Japan Trench, we determined high-resolution three-dimensional (3-D) structures of seismic velocities (Vp, Vs), Vp/Vs ratio and temperatures. The temperatures (T) were calculated from the inverted seismic structures, and two petrological models of the upper mantle: a peridotite assemblage and a pyrolite assemblage, which differ from the results of the thermal simulation. The multi-parameter structures obtained for Vp, Vs, Vp/Vs and T show good consistency in the upper mantle behind the Japan Trench. The temperatures derived from the peridotite assemblage were found to match the mean geotherm and simulated temperature of the upper mantle more closely than those from the pyrolite model. A layer approximately 10 km thick with low-Vp and Vs, high-Vp/Vs, and slightly high-T anomalies was observed on the upper boundary of the subducting Pacific slab. This distinctive anomalous layer is interpreted as partial melting of the oceanic crust due to deep-seated metamorphic reactions whose characteristics are dependent on the source of fluids, mineral composition, and the thermal regime. Such a process enriches the peridotite content of the basalt underneath the island arc in the mantle wedge. Localized zones of significantly low-Vp and low-Vs, high-Vp/Vs and high-T perturbations were imaged in the mantle wedge under the active volcanoes, suggesting partial melting of peridotite-rich mantle material to produce tholeiitic magma. The present study demonstrates that fluids released from slab dehydration, mineral composition and the thermal regime play crucial roles in both arc magmatism, and slab melting in the subduction zone.
查看更多>>摘要：Coastal NW South China Sea is a critical region for evaluating the contrasting thermo-tectonic histories between the onshore and offshore regions, and the possible surface response to a deep-seated mantle anomaly. To further elucidate the regional post-late Mesozoic tectonic cooling history, low temperature thermochronological data are reported from 16 onshore granite samples. These new data, together with previously reported data, are used to constrain the thermo-tectonic evolution of the onshore southern margin of the South China Block (SCB). Results show that the area has mostly experienced a similar three-stage cooling pattern, characterized by a similar to 3-8 degrees C/Myr Early Cretaceous to early Late Cretaceous (similar to 130- similar to 80 Ma) cooling phase, followed by a period of relative thermal quiescence, and then a further cooling episode initiated in the early Eocene and for most samples continuing until the present day. The early cooling phase is recorded extensively along the southern SCB margin and is probably a combined response to several alternating compressional and extensional tectonic events during the Late Jurassic to early Late Cretaceous (similar to 150- similar to 80 Ma). The second phase of relative thermal quiescence suggests, in contrast to the offshore and the eastern coast, that the onshore southern SCB margin was not affected markedly by compressional tectonics during the latest Cretaceous. Post-Eocene enhanced cooling and exhumation are probably related to widespread Eocene to Early Oligocene extension, and subsequent uplift due to convergence between the Indo-Australian, Eurasian and Pacific Plates in the onshore southern SCB margin. Southeastward extrusion due to Indo-Asia collision resulted in a decrease in uplift amplitude from NW to SE, but is not recorded in the offshore region. Different tectonic effects result in spatial relief variation from the onshore to the offshore southern SCB margin in the latest Cretaceous obviously different from in the late Cenozoic. NE Hainan Island, which was most likely mountainous during the Late Cretaceous to Paleocene, but became an area of low elevation and relief with gradual exhumation, particularly following significant Eocene-Early Oligocene exhumation. Neogene thermal histories in coastal NW South China Sea do not record any marked cooling rate changes which could be related to significant recent surface uplift and erosion, indicating that the deep anomalous body beneath the LeMiong Depression might be just a branch of a much larger low-velocity anomalous structure. Our results suggest that the thermo-tectonic evolution of the onshore southern SCB margin (Guangdong) was mainly controlled by Cretaceous NE-trending subduction of the Palaeo-Pacific plate beneath the southeastern SCB, whilst during the Late Cenozoic it was mainly affected by the convergence of adjacent plates, particularly by the Indo-Asia collision.
查看更多>>摘要：The region of West Bohemia and Upper Palatinate belongs to the West Bohemian Massif. The study area is situated at the junction of three different Variscan tectonic units and hosts the ENE-WSW trending Ohre Rift as well as many different fault systems. The entire region is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of mantle derived CO2 in form of mineral springs and mofettes. Ongoing active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures. All these phenomena make the Ohre Rift a unique target area for European intra-continental geo-scientific research. With magnetotelluric (MT) measurements we image the subsurface distribution of the electrical resistivity and map possible fluid pathways. Two-dimensional (2D) inversion results by Munoz et al. (2018) reveal a conductive channel in the vicinity of the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of their model; however, their 2D inversions allow ambiguous interpretations of this feature. Therefore, we conducted a large 3D MT field experiment extending the study area towards the south. The 3D inversion result matches well with the known geology imaging different fluid/magma reservoirs at crust-mantle depth and mapping possible fluid pathways from the reservoirs to the surface feeding known mofettes and spas. A comparison of 3D and 2D inversion results suggests that the 2D inversion results are considerably characterized by 3D and off-profile structures. In this context, the new results advocate for the swarm earthquakes being located in the resistive host rock surrounding the conductive channels; a finding in line with observations e.g. at the San Andreas Fault, California.
NSTL
Elsevier