查看更多>>摘要：The Bohai Sea is proposed as the center of deformation associated with destruction of the North China Craton, and its tectonic history is essential for understanding the response of craton evolution to oceanic subduction. However, interior deformation of the craton remains unclear due to a lack of high-resolution imaging in this region. Here, we present high-resolution 3-D azimuthally-anisotropic shear-wave velocity structure of the crust and uppermost mantle using direct inversion of Rayleigh-wave phase-velocity dispersion from ambient noise. The anisotropic velocity structures reveal significant consistency between the regional strain field and seismic activity in the shallow crust. The N-S fast direction and relatively high velocity anomalies observed in the uppermost mantle beneath the Tanlu fault zone, as well as prominent Moho uplift, support the conclusion that the Tanlu fault zone has facilitated lithospheric modification of the eastern North China Craton since late Mesozoic. Anisotropy images also indicate that the northern and southern Bohai Bay Basin have undergone different deformation histories. The strong and consistent azimuthal anisotropy within the uppermost mantle beneath the southern Bohai Bay Basin may originate primarily from re-orientation arrangement of the olivine induced by new lithosphere formed from rapid cooling of upwelling asthenosphere after lithosphere delamination due to the subduction of the (Paleo-) Pacific Plate.
查看更多>>摘要：Ubiquitous wet cracks in fault damage zones alter the mechanical behavior of fault rocks. Quantifying the effects of crack damage and water on stress evolution is fundamental for earthquake rupture model establishment. We conducted increasing-amplitude cyclic loading experiments on dry and wet sandstones to investigate the effects of crack damage, confining pressure and water on the stress rotation and fault weakening. The energy dissipated in each loading-unloading cycle was quantified for inference of the evolution of crack damage within the fault zone. A multi-layer elastic model was used to investigate the effects of crack damage and water on stress rotation in the fault damage zone. The results show that the observed decrease of elastic modulus in both dry and wet rocks greatly increased the maximum principal stress in the fault core as deformation progressed, but reduced the differential stress and the angle between the maximum principal stress and the orientation of the fault damage zone. However, water within fault rocks results in a lower reduction in differential stress within the fault damage zone when compared to the dry condition. The observed decrease of differential stress implies that the stress state evolves away from failure, and we infer water affects the proximity to fault instability both by affecting the stress rotation and by reducing in local strength, where these two effects are in competition. These results and inferences indicate that the stress rotation amplitude of fault damage zone increased with the crack damage, demonstrating that the stress rotation is greater near the damage zone-core interface than that near the fault damage zone/host rock boundary. We also found that the changes in the elastic modulus of the deformed rocks remains almost the same under different confining pressures, indicating that stress rotation of fault damage zone is not affected by confining pressure.
查看更多>>摘要：Eclogites form in different geotectonic environments such as subduction zones or roots of overthickened continental crust. The specific environment can be identified by the construction of pressure-temperature (P-T) trajectories. We determined the P-T path of eclogite from the Les Essarts high-pressure unit, Southern Armorican zone in western France. The eclogite contains unusual atoll garnet characterized by a younger generation (Grt2) that replaced the core of an older generation (Grt1). Large-area (cm(2)) X-ray mapping with the electron microprobe was applied to determine the quantities of Grt1 and Grt2 in the studied eclogites. Detailed characterization of the mineral chemistry was undertaken followed by thermodynamic modelling. The assemblage of amphibole, quartz, rutile, and ilmenite enclosed in Grt1 formed in the amphibolite facies during a first metamorphic event. Grt1 grew at P-T conditions around 20 kbar and 540 degrees C probably after nearly isothermal burial during a second metamorphic cycle. An isobaric heating to ca. 650 degrees C followed at which Grt2 and, thus, atoll garnet formed. A time period of ca. 15 million years for exhumation in the temperature range 620-655 degrees C was estimated by diffusional modelling of the contact between Grtl and Grt2. We correlate the Southern Armorican zone with the Malpica-Tuy zone (NW Spain) that contains eclogite and related rocks with a similar metamorphic evolution. We outline a geodynamic scenario in which the eclogite-facies rocks of both zones were tectonically eroded from the upper continental plate by a subducting plate to reach maximum depths of 70-90 km in the Late Devonian. Exhumation to depths of ca. 45-50 km occurred in a subduction channel just before continent-continent collision started.
Kakihata, YukiMichibayashi, KatsuyoshiDick, Henry J. B.
11页
查看更多>>摘要：Microstructures and olivine crystal fabrics were studied in amphibole-bearing peridotite samples obtained from the Marion Fracture Zone of the Southwest Indian Ridge by dredge D19 of the 1984 PROTEA Expedition Leg 5 cruise of the RV Melville. The peridotites show various textures ranging from extremely fine-grained well-layered ultramylonites to heterogeneously strained tectonites. Electron back-scatter diffraction analyses revealed that olivine crystal-preferred orientations (CPOs), which are developed primarily in coarse granular peridotites in the mantle, become weaker with an increasing degree of grain-size reduction from coarser to finer grains, for both porphyroclastic and matrix olivine grains. However, two well-layered ultramylonites are characterized by bimodal CPOs of (010)[001] (B type) and (001)[100] (E type) or a strong maximum of [010] normal to the foliation and girdle patterns of both [100] and [001] on the foliation plane (i.e., an axial [010] pattern or AG type). Moreover, spinel grains within these well-layered ultramylonites have not only been broken down to form olivine and amphibole by hydrous reactions, but have also been fractured and their fragments pulled apart in the fine-grained matrix. These features indicate that shear deformation occurred as increasing stress under hydrous conditions during the final stage of deformation, which enabled the local occurrence of low-temperature plastic deformation, resulting in the development of a CPO and a foliation within the ultramylonites.
查看更多>>摘要：Situated in the far southeastern corner of the Eurasian plate, the South China block is traditionally considered to be created by the collision/amalgamation of the Yangtze craton and Cathaysia terrane. In this study, we use the continuous seismic ambient noise, recorded by dense permanent seismic stations deployed across South China, to image the isotropic and azimuthal anisotropic crustal structure of the South China block. In the short period (5-10s), which is most sensitive to the upper crustal structure, the lateral changes in the isotropic crustal structure show good agreement with surface geology, with lower velocities being associated with major sedimentary basins. More details about the tectonic evolution of the South China block are revealed by the lateral and vertical variations in azimuthal anisotropy. Spatial variations in isotropic model and model with azimuthal anisotropy have been observed across the Jiangshan-Shaoxing fault, related to the crustal suture zone between the Yangtze craton and Cathaysia terrane. The azimuthal anisotropy at short periods (5-10 s) shows a fast-propagation direction broadly parallel to the suture zone. This might reflect folding, thrusting, metamorphism structures, and basin-mountain foreland deformation system created by the intense crustal deformation due to the Yangtze craton colliding with Cathaysia terrane and following multiphase tectonic activities.
查看更多>>摘要：The thermochronological history of orogens along plate margins provides unique constraints on regional tectonic evolution. Given existing thermochronological data that identify late Mesozoic-Cenozoic multiphase cooling events, we perform new apatite fission-track and (U-Th)/He low-temperature thermochronology on Wula Shan and Dahong Shan rocks to reveal the first stages of cooling along the southern Yin Shan orogenic belt, northern margin of the North China Block (NCB). Our new thermal history modeling suggests that cooling of the southern Yin Shan began in the Late Triassic (ca. 230 Ma), with rapid cooling at ca. 210-178 Ma and ca. 10 similar to Ma in the Wula Shan and ca. 200-160 Ma in the southern Dahong Shan. Late Triassic-Early Jurassic cooling stages are related to uplift and exhumation of the northern NCB, which given the tectonic setting of the late Indosinian event, are mainly interpreted as signatures of Mongol-Okhotsk Ocean subduction. Subsequently, enhanced plate convergence of East Asia with closure of the Mongol-Okhotsk Ocean led to intense thrust-nappe and intracontinental orogeny during the early Yanshanian. Importantly, our new finding of ca. 10 Ma rapid cooling of the Wula Shan at a rate of similar to 3.5 degrees C/Ma corresponds to coeval uplift and synrift basin sedimentation in the Ordos Block periphery, western NCB, which could mark the response to craton-scale rotational deformation. The completion of this low-temperature thermochronological database allows us to reappraise the Mesozoic exhumation and ca.10 Ma reactivation history and its implications for the southern Yin Shan.
NSTL
Elsevier