Two Phases of Cenozoic Deformation in Northeastern Tibet: Thrusting Followed by Strike-Slip Faulting

TWO PHASES OF CENOZOIC DEFORMATION IN NORTHEASTERN TIBET: THRUSTING FOLLOWED BY STRIKE-SLIP FAULTING     

http://www.sciencedirect.com/science/article/pii/S1674987110000319

The 3-D geometry of the seismicity in Hindu Kush–Pamir–western China region has been defined by seismic records for 1975–1999 from the National Earthquake Information Center, the U.S. Geological Survey, and over 16,000 relocated earthquakes since 1975 recorded by the Xinjiang seismic network of China. The results show that most Ms ≥ 5.0 hypocenters in the area are confined to a major intracontinental seismic shear zone (MSSZ). The MSSZ, which dips southwards in Pamir has a north-dipping counterpart in the Hindu Kush to the west; the two tectonic realms are separated by the sinistral Chaman transform fault of the India–Asia collisional zone. We demonstrate that the MSSZ constitutes the upper boundary of a south-dipping, actively subducting Pamir continental plate. Three seismic concentrations are recognized just above the Pamir MSSZ at depths between 45–65 km, 95–120 km, and 180–220 km, suggesting different structural relationships where each occurs. Results from focal mechanism solutions in all three seismological concentrations show orientations of the principal maximum stress to be nearly horizontal in an NNW–SSE direction. The south-dipping Pamir subduction slab is wedge-shaped with a wide upper top and a narrow deeper bottom; the slab has a gentle angle of dip in the upper part and steeper dips in the lower part below an elbow depth of ca. 80–120 km. Most of the deformation related to the earthquakes occurs within the hanging wall of the subducting Pamir slab. Published geologic data and repeated GPS measurements in the Pamir document a broad supra-subduction, upper crustal zone of evolving antithetic (i.e. north-dipping) back-thrusts that contribute to north-south crustal shortening and are responsible for exhumation of some ultrahigh-pressure rocks formed during earlier Tethyan plate convergence. An alternating occurrence in activity of Pamir and Chaman seismic zones indicates that there is interaction between strike-slip movement of the Chaman transform fault system and deep-subduction of the Pamir earthquake zone. Pamir subduction-related seismicity becomes shallower in depth with increasing distance east of the transform fault. Therefore, sinistral movement of the Chaman transform fault appears to be influencing continental deep-subduction in the Pamir region and may provide an explanation for the unusual south-dipping geometry of the intracontinental Pamir plate.     

南秦岭山柞旬-安康地区的两次逆冲推覆和Ⅱ型造山带基本特征

界于东经108°30'～110°,商丹带-勉略带(或大巴山带)之间的山柞旬-安康构造带是在古生代-三叠纪南北向裂陷-裂谷板内海相沉积盆地基础之上转化形成的Ⅱ型造山带,它与东、西两侧的武当-商南带、佛坪-汉南带的组成、结构均不相同。南秦岭的山柞旬-安康带开始从南向北以及后来再自北向南的两次逆冲推覆所形成的构造格局,与目前都认为该区是从北向南指向四川盆地的滑脱—逆冲式的薄皮构造模式是完全不同的。笔者还简要讨论了Ⅱ型造山带的基本特征。     

Invariant shape solutions of the spinning three craft Coulomb tether problem

In this paper we study shape-preserving formations of three spacecraft, where the formation keeping forces arise from the electric charges deposed on each craft. Inspired by Lagrange’s 3-body problem, the general conditions that guarantee preservation of the geometric shape of the electrically charged formation are derived. While the classical collinear configuration is a solution to the problem, the equilateral triangle configuration is found to only occur with unbounded relative motion. The three collinear spacecraft problem is analyzed and the possible solutions are categorized based on the spacecraft mass–charge ratio. Precise statements on the number of solutions associated with each category are provided. Finally, a methodology is proposed to study boundedness of the collinear solution that is inspired by past understanding and results for the 3-body problem. Given the initial position and the velocity vectors of each craft along with the charges, analytical solutions are provided describing the resulting relative motion.     

Contrasting pressure–temperature–deformation history across a vestigial craton–mobile belt boundary: the western margin of the Eastern Ghats Belt at Deobhog, India

At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn‐D_1M and D_2M) are reoriented along N–S‐trending and E‐dipping shear planes (S_3M), with (S_1M–S_3M) intersection lineations having steep to moderate plunges. The near‐peak P–T condition was syn‐D_3M (≥900 °C, 9.5 kbar), as inferred from syn‐D_3M Grt+Opx‐bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high‐Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies. The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D_1B & D_2B), with the L_2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn‐D_2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D_3B shear deformation post‐dates granite and dolerite intrusions and is characterized by top‐to‐the‐west movement along N–S‐trending, E‐dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn‐D_3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S_3B shears to the west, and post‐D_2B neosome segregation along D_3B folds and shears to the east. The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre‐D₃P–T –deformation histories. The shared D₃ deformation in the two units, the syn‐D₃ inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para‐autochthonous/allochthonous unit emplaced on and amalgamated to the craton.     

Spores from a carboniferous section in the hunter valley,New South Wales

In this initial systematic study of Carboniferous spores from New South Wales, Australia, fifteen species (all but one of them new) are formally described and are distributed among eight established genera and two new genera (Rattiganispora, a distally annulate trilete form, and Psomospora, an inaperturate or proximally hilate form). The species were selected as being the most characteristic and distinctive forms found in the Italia Road Formation at its well‐exposed type section in the Hunter Valley, east‐central New South Wales. The formation is a cyclical non‐marine unit, over 300 metres (1,000 ft) thick, consisting of lithic arenites together with carbonaceous shales, claystones, and siltstones; its age is regarded as West‐phalian‐Stephanian. The microfiora is compared with those known from sediments of similar age elsewhere and its place in the Australian Palaeozoic palynostratigraphic record is discussed.

New specific institutions are as follows: Punctatisporites lucidulus, P. sub‐tritus, Verrucosisporites aspratilis, V. italiaensis, Raistrickia accincta, R. radiosa, Reticulatisporites asperidictyus, R. magnidictyus, Foveosporites pellucidus, Rattiganispora apiculata (type species), Kraeuselisporites kuttungensis, Grandispora maculosa, Psomospora detecta (type species), and Wilsonites australiensis.     

Garzón Massif basement tectonics: Structural control on evolution of petroleum systems in upper Magdalena and Putumayo basins,Colombia

The Garzón Massif, is an active Laramide style basement uplift flanked by the Upper Magdalena Valley (UMV) and the Putumayo Basin. In this paper we use new gravity, magnetic, well and seismic data for the first geophysical interpretation of the Garzón Massif. The Garzón/Algeciras fault has been previously interpreted as a right-lateral strike-slip fault. The new seismic, well, and gravity data demonstrates that the Garzón fault is also a low-angle (12–17°) Andean age fault thrusting PreCambrian basement 10–17 km northwestward over Miocene sediments of the UMV in a prospective footwall anticline.The new geophysical data as well as previous field mapping were used to produce the first gravity and magnetic maps and retrodeformable structural cross section of the northern Garzón Massif. The new model distinguishes for the first time distinct episodes of “thin-skinned” and “thick-skinned” deformation in the Garzón Massif. The model indicates approximately 43 km of Early to Middle Miocene shortening by “thin-skinned” imbricate thrusting contemporaneous with the uplift of the nearby southern Central Cordillera (∼9–16 Ma) and the main hydrocarbon expulsion event for the UMV and Putumayo Basin. This was followed by at least 22 km of Late Miocene (3–6 Ma) “thick-skinned” Andean shortening and 7 km of uplift on the symmetrical Garzón thrust and a SE-verging basement thrust fault zone. The Andean uplift interrupted and exposed the hydrocarbon migration pathways to the Putumayo Basin.3-D volume fracture analysis was used for the first time in this paper together with the first seismic and well data published for the Topoyaco and Miraflor structures to test closure models for the Topoyaco foothills. Intense fracturing is observed in the Topoyaco basement monocline from the near-surface to depths of over 3.5 km. The high level of fracturing permitted freshwater flushing and oil biodegradation and hydrocarbon escape. In contrast, the Miraflor-1 well, located just southwest of the Topoyaco block, tested light gravity oil and is sealed from groundwater flushing and biodegradation by a backthrust.     

台阶式逆冲推覆－安徽沿江煤田的基本构造型式

运用现代构造几何最新理论，指出安徽沿江煤田的基本构造型式为台阶式逆冲推覆构造，并对其构造类型进行详细的分类。     

云南金顶超大型铅锌矿床的成矿地质背景

采用构造－沉积综合分析的方法，研究了金顶超大型铅锌矿床成矿的盆地、构造和深部地质背景。研究表明，控矿的古新世－中始新世盆地为走滑拉张盆地，研究区先后经历了古新世－中始新世早期的走滑和拉伸，中始新世－渐新世的挤压推覆和中新世的隆升和走滑，分析了盆地演化、沉积体系、同生断裂活动和逆冲推覆等对金顶超大型铅锌矿床的控制作用，探讨了可能的成矿过程。     

Une carte gravimétrique haute résolution du massif du Mont-Blanc : implications structuralesA high-resolution gravity map of the Mont-Blanc massif: structural consequences

We performed a gravity survey in the Mont-Blanc and Aiguilles Rouges ranges in order to improve the gravity data coverage of the Alpine crystalline external ranges, and to constrain the deep geometry of a crustal scale thrust. Preliminary results allow us to propose a geometry for the deep structure of the ranges, taking into account a major reverse fault that bounds the Mont-Blanc range: the Mont-Blanc shear zone. To cite this article: F. Masson et al., C. R. Geoscience 334 (2002) 1011–1019.