Paleomagnetic Study of Mesozoic Continental Sediments Along the Northern Tien Shan (China) and Heterogeneous Strain in Central Asia

A paleomagnetic study of rocks from the northern foot of the Tien Shan and the southern border of the Dzungar Basin, east of Urumqi (44.2°N, 86.0°E), spanning ages from middle Jurassic to early Tertiary was carried out to constrain the tectonic evolution in central Asia since Mesozoic time. Five middle Jurassic sites reveal a remagnetized direction close to the present Earth field in geographic coordinates: D = 6.6°, I = 72.6° (α_(95) = 7.4°). Thirteen out of 17 upper Jurassic and lower Cretaceous sites yield a characteristic direction (stratigraphic coordinates) of D = 12.7°, I = 48.6° (α_(95) = 5.5°). Nine of 16 upper Cretaceous and lower Tertiary sites provide a characteristic direction of D = 12.5°, I = 51.3° (α_(95) = 6.9°). The latter two directions pass fold and reversal tests. The pole positions are close to each other and to the Besse and Courtillot [1989, 1990] Eurasian apparent polar wander path, for ages ranging from 130 to 70 Ma. However, the difference in paleolatitudes amounts to about 5.9° ± 3.7°, which could indicate significant continental shortening in the Altai Mountains and perhaps further north, subsequent to India-Asia collision. The pole positions from the Dzungar Basin are close to those found for the Tarim [Li et al., 1988a], leading to an insignificant paleolatitude difference (3.0° ± 6.9°), but showing a larger difference in declination (8.6° ± 8.7°). These paleomagnetic results are compatible with a model of heterogeneous deformation in the western part of the collision zone between India and Siberia. A significant shortening in the Altai, a slight counterclockwise rotation of the Dzungar block, the westward-increasing shortening in the Tien Shan with attendant clockwise rotation of the Tarim block are all consistent with this model, in which Tibet, the Tien Shan and the Altai undergo differential strain along strike in a relay fashion, with the total India-Siberia convergence remaining approximately constant

Updated Interpretation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea : Implications for the Tertiary Tectonics of Southeast Asia

International audienceWe present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ~50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ~16 Ma) and the average spreading rate was slower, ~35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo-Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large-scale "domino" models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left-lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India-Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion a long the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike-slip fault of SE Asia

Millennial Recurrence of Large Earthquakes on the Haiyuan Fault near Songshan, Gansu Province, China

International audienceThe Haiyuan fault is a major active left-lateral fault along the northeast edge of the Tibet-Qinghai Plateau. Studying this fault is important in understanding current deformation of the plateau and the mechanics of continental deformation in general. Previous studies have mostly focused on the slip rate of the fault. Paleo-seismic investigations on the fault are sparse, and have been targeted mostly at the stretch of the fault that ruptured in the 1920 M ϳ8.6 earthquake in Ningxia Province. To investigate the millennial seismic history of the western Haiyuan fault, we opened two trenches in a small pull-apart basin near Songshan, in Gansu Province. The excavation exposes sedimentary layers of alternating colors: dark brown silty to clayey deposit and light yellowish brown layers of coarser-grained sandy deposit. The main fault zone is readily recognizable by the disruption and tilting of the layers. Six paleoseismic events are identified and named SS1 through SS6, from youngest to oldest. Charcoal is abundant, yet generally tiny in the shallowest parts of the trench exposures. Thirteen samples were dated to constrain the ages of paleoseismic events. All six events have occurred during the past 3500–3900 years. The horizontal offsets associated with these events are poorly known. However, events SS3 to SS6 appear to be large ones, judging from comparison of vertical separations and widths of fault zones. The youngest event SS1 instead seems to be a minor one, probably the 1990 M w 5.8 earthquake. Thus, four large events in 3500–3900 years would imply a recurrence interval of about 1000 years. Three events SS2 to SS4 prior to 1990 occurred sometime during 1440–1640 A.D., shortly after 890–1000 A.D. and 0–410 A.D., respectively. We tentatively associate them with the 1514 A.D., 1092 A.D., and 143 or 374 A.D. historical earthquakes. Taking 10 ‫ע‬ 2 m of slip for large events (SS3 and SS4), comparable to the 1920 M Ͼ8 Haiyuan earthquake, their occurrence times would be consistent with the long-term 12 ‫ע‬ 4 mm/yr estimate of Lasserre et al. (1999). However, a more realistic evaluation of slip rate and its possible change with time requires a more rigorous determination of coseismic slip amounts of past earthquakes

Discrete Element Modeling of a Subduction Zone with a Seafloor Irregularity and its Impact on the Seismic Cycle

peer reviewedSeafloor irregularities influence rupture behavior along the subducting slab and in the overriding plate, thus affecting earthquake cycles. Whether seafloor irregularities increase the likelihood of large earthquakes in a subduction zone remains contested, partially due to focus put either on fault development or on rupture pattern. Here, we simulate a subducting slab with a seafloor irregularity and the resulting deformation pattern of the overriding plate using the discrete element method. Our simulations illustrate the rupture along three major fault systems: megathrust, splay and backthrust faults. Our results show different rupture dimensions of earthquake events varying from tens to ca. 140 km. Our results suggest that the recurrence interval of megathrust events with rupture length of ca. 100 km is ca. 140 years, which is overall comparable to the paleoseismic records at the Mentawai area of the Sumatran zone. We further propose the coseismic slip amounts decrease and interseismic slip amounts increase from the surface downwards gradually. We conclude that the presence of seafloor irregularities significantly affects rupture events along the slab as well as fault patterns in the overriding plate

Twenty million years of continuous deformation along theKarakorum fault, western Tibet: A thermochronological analysis.

The role of the Karakorum fault zone (KFZ) is debated. South of 33°N, ongoing dextral-oblique slip along the SW edge of the Gar basin exhumes metamorphic and magmatic rocks of the Ayilari range. Minerals have recorded a continuum of deformation from temperatures >600–400°C down to 20 Ma of deformation along the fault. Greenschist facies deformation superimposed upon the medium- to high-grade deformation marks a kinematic change from pure dextral to dextral-normal motion associated with the onset of rapid cooling. At the regional scale, the coexistence of transtension in the Gar basin with transpression documented along the Pangong range farther north suggests another example of the ‘‘zipper tectonics'' model developed along the Red River fault. The kinematic shift induced the rise of the Ayilari range starting at 16–12 Ma and the incision of major river courses. The Indus River might have become captive of the relief at this time. The river's 120 km of apparent offset implies dextral motion at a long-term rate of ca 8.5 ± 1.5 mm/yr

Effects of crystallographic anisotropy on fracture development and acoustic emission in quartz

Transgranular microcracking is fundamental for the initiation and propagation of all fractures in rocks. The geometry of these microcracks is primarily controlled by the interaction of the imposed stress field with the mineral elastic properties. However, the effects of anisotropic elastic properties of minerals on brittle fracture are not well understood. This study examines the effects of elastic anisotropy of quartz on the geometry of brittle fracture and related acoustic emissions (AE) developed during indentation experiments on single crystals at ambient pressure and temperature. A Hertzian cone crack developed during blunt indentation of a single crystal of flawless Brazilian quartz parallel to the c axis shows geometric deviation away from predictions based on the isotropic case, consistent with trigonal symmetry. The visible cone crack penetration depth varies from 3 to 5 mm and apical angle from 53 to 40. Electron backscatter diffraction (EBSD) mapping of the crack tip shows that fracturing initiates along a ~40 μm wide process zone, comprising damage along overlapping en echelon high-index crystallographic planes, shown by discrete bands of reduced electron backscatter pattern (EBSP) quality (band contrast).Coalescence of these surfaces results in a stepped fracture morphology. Monitoring of AE during indentation reveals that the elastic anisotropy of quartz has a significant effect on AE location and focal mechanisms. Ninety-four AE events were recorded during indentation and show an increasing frequency with increasing load. They correspond to the development of subsidiary concentric cracks peripheral to the main cone crack. The strong and complex anisotropy in seismic velocity (~28% Vp, ~43% Vs with trigonal symmetry) resulted in inaccurate and high uncertainty in AE locations using Geiger location routine with an isotropic velocity model. This problem was overcome by using a relative (master event) location algorithm that only requires a priori knowledge of the velocity structure within the source volume. The AE location results correlate reasonably well to the extent of the observed cone crack. Decomposition of AE source mechanisms of the Geiger relocated events shows dominantly end-member behavior between tensile and compressive vector dipole events, with some double-couple-dominated events and no purely tensile or compressive events. The same events located by the master event algorithm yield greater percentage of vector dipole components and no double-couple events, indicating that AE source mechanism solutions can depend on AE location accuracy, and therefore, relocation routine that is utilized. Calculations show that the crystallographic anisotropy of quartz causes apparent deviation of the moment tensors away from double-couple and pure tensile/compressive sources consistent with the observations. Preliminary modeling of calcite anisotropy shows a response distinct from quartz, indicating that the effects of anisotropy on interpreting AE are complex and require detailed further study

A922 Sequential measurement of 1 hour creatinine clearance (1-CRCL) in critically ill patients at risk of acute kidney injury (AKI)

Meeting abstrac