Li Z.H.,Chinese Academy of Geological Sciences
Science China Earth Sciences | Year: 2014
Continental subduction and collision normally follows oceanic subduction, with the remarkable event of formation and exhumation of high-to ultra-high-pressure (HP-UHP) metamorphic rocks. Based on the summary of numerical geodynamic models, six modes of continental convergence have been identified: pure shear thickening, folding and buckling, one-sided steep subduction, flat subduction, two-sided subduction, and subducting slab break-off. In addition, the exhumation of HP-UHP rocks can be formulated into eight modes: thrust fault exhumation, buckling exhumation, material circulation, overpressure model, exhumation of a coherent crustal slice, episodic ductile extrusion, slab break-off induced eduction, and exhumation through fractured overriding lithosphere. During the transition from subduction to exhumation, the weakening and detachment of subducted continental crust are prerequisites. However, the dominant weakening mechanisms and their roles in the subduction channel are poorly constrained. To a first degree approximation, the mechanism of continental subduction and exhumation can be treated as a subduction channel flow model, which incorporates the competing effects of downward Couette (subduction) flow and upward Poiseuille (exhumation) flow in the subduction channel. However, the (de-)hydration effect plays significant roles in the deformation of subduction channel and overriding lithosphere, which thereby result in very different modes from the simple subduction channel flow. Three-dimensionality is another important issue with highlighting the along-strike differential modes of continental subduction, collision and exhumation in the same continental convergence belt. © 2013, Science China Press and Springer-Verlag Berlin Heidelberg.
An M.,Chinese Academy of Geological Sciences
Geophysical Journal International | Year: 2012
The resolution matrix of an inverse problem defines a linear relationship in which each solution parameter is derived from the weighted averages of nearby true-model parameters, and the resolution matrix elements are the weights. Resolution matrices are not only widely used to measure the solution obtainability or the inversion perfectness from the data based on the degree to which the matrix approximates the identity matrix, but also to extract spatial-resolution or resolution-length information. Resolution matrices presented in previous spatial-resolution analysis studies can be divided into three classes: direct resolution matrix, regularized/stabilized resolution matrix and hybrid resolution matrix. The direct resolution matrix can yield resolution-length information only for ill-posed inverse problems. The regularized resolution matrix cannot give any spatial-resolution information. The hybrid resolution matrix can provide resolution-length information; however, this depends on the regularization contribution to the inversion. The computation of the matrices needs matrix operation, however, this is often a difficult problem for very large inverse problems. Here, a new class of resolution matrices, generated using a Gaussian approximation (called the statistical resolution matrices), is proposed whereby the direct determination of the matrix is accomplished via a simple one-parameter non-linear inversion performed based on limited pairs of random synthetic models and their inverse solutions. Tests showed that a statistical resolution matrix could not only measure the resolution obtainable from the data, but also provided reasonable spatial/temporal resolution or resolution-length information. The estimates were restricted to forward/inversion processes and were independent of the degree of inverse skill used in the solution inversion; therefore, the original inversion codes did not need to be modified. The absence of a requirement for matrix operations during the estimation process indicated that this approach is particularly suitable for very large linear/linearized inverse problems. The estimation of statistical resolution matrices is useful for both direction-dependent and direction-independent resolution estimations. Interestingly, even a random synthetic input model without specific checkers provided an inverse output solution that yielded a checkerboard pattern that gave not only indicative resolution-length information but also information on the direction dependence of the resolution. © 2012 The Author Geophysical Journal International © 2012 RAS.
Junchang L.,Chinese Academy of Geological Sciences
Acta Geologica Sinica | Year: 2011
A new species of Darwinopterus, D. robastodens sp. nov. is described and named. Based on the new specimen, the diagnostic characters of Dorwinopterns are amended and include: rostra! dentition composed of well-spaced, spike-like teeth; the longest teeth are confined to the anterior half of the tooth row; tooth alveoli have raised margins; nasoantorbital fenestra continent; inclined quadrate; elongate cervical vertebrae with low neural spine and reduced or absent ribs; long tail of more than 20 caudals partially enclosed by filiform extensions of the pre- and postzygapophyses; short metacarpus less than 60 per cent length of humerus, fifth toe with two elongate phalanges and curved second pedal phalanx of the fifth toe with the angle between the proximal and distal segments about 130 degrees. The complete specimen of the new pterosaur D. robustodens sp. nov. provides much more osteological information. The differences in tooth morphologies between Dorwinopterus inoduloris and D. robustodens sp. nov. suggest that they filled different ecological niches. The hard integument- bearing Coleoptera may have been the main food source of Darwinopterus robustodens. Copyright © 1999-2011 John Wiley & Sons, Inc. All Rights Reserved.
Gao L.-E.,Chinese Academy of Geological Sciences |
Zeng L.,Chinese Academy of Geological Sciences
Geochimica et Cosmochimica Acta | Year: 2014
Identifying the timing of formation and geochemical nature of the Cenozoic granites along the Himalayan orogen is essential to test or formulate models that link crustal anatexis with tectonic transition during the evolution of large-scale collisional orogenic belts. The Malashan gneiss dome, one of the prominent domes within the Tethyan Himalaya, experienced Barrovian-type metamorphism and partial melting of pelitic rocks at relatively deep levels during the collision between India and Eurasia. New LA-MC-ICP-MS zircon U-Pb analyses yielded that the Malashan two-mica granites formed at a time span of 17.6±0.1 to 16.9±0.1Ma. The Malashan two-mica granites are characterized by: (1) high SiO2 (>71.3wt.%), Al2O3 (>14.8wt.%), and relatively high CaO (>1.3wt.%); (2) relatively high Sr (>146ppm), but low Rb/Sr ratios (<1.3) which are nearly constant relative to large variations in Ba concentrations; (3) enrichment in LREE, depletion in HREE, and no or weak negative Eu anomalies (Eu/Eu*=0.7-0.9); (4) as compared to granites in the other Northern Himalayan Gneiss Domes and High Himalayan Belt, relatively lower initial 87Sr/86Sr ratios (0.7391-0.7484) and similar unradiogenic Nd isotope compositions (εNd(t)=-13.7 to -14.4). These characteristics imply that the two-mica granites were derived from fluid-fluxing melting of metapelite, possibly triggered by the E-W extension. Our new data in combination with literature data indicate that there are three types of granites with diverse geochemical characteristics and distinct formation mechanisms along the Himalayan orogen since the Cenozoic India-Eurasia continental collision. Conceivably, our new results will provide new insights on how the partial melting behavior of relatively deeper crustal rocks evolved as the tectonic evolution of large orogenic belts. © 2014.
Condie K.C.,New Mexico Institute of Mining and Technology |
Kroner A.,University Mainz |
Kroner A.,Chinese Academy of Geological Sciences
Gondwana Research | Year: 2013
Oceanic arcs are commonly cited as primary building blocks of continents, yet modern oceanic arcs are mostly subducted. Also, lithosphere buoyancy considerations show that oceanic arcs (even those with a felsic component) should readily subduct. With the exception of the Arabian-Nubian orogen, terranes in post-Archean accretionary orogens comprise < 10% of accreted oceanic arcs, whereas continental arcs compose 40-80% of these orogens. Nd and Hf isotopic data suggest that accretionary orogens include 40-65% juvenile crustal components, with most of these (> 50%) produced in continental arcs. Felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs. They have lower Th/Yb, Nb/Yb, Sr/Y and La/Yb ratios, reflecting shallow mantle sources in which garnet did not exist in the restite during melting. The bottom line of these geochemical differences is that post-Archean continental crust does not begin life in oceanic arcs. On the other hand, the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs. During the Archean, however, oceanic arcs may have been thicker due to higher degrees of melting in the mantle, and oceanic lithosphere would be more buoyant. These arcs may have accreted to each other and to oceanic plateaus, a process that eventually led to the production of Archean continental crust. After the Archean, oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges, hence, oceanic lithosphere is more subductable. Widespread propagation of plate tectonics in the late Archean may have led not only to rapid production of continental crust, but to a change in the primary site of production of continental crust, from accreted oceanic arcs and oceanic plateaus in the Archean to primarily continental arcs thereafter. © 2011 International Association for Gondwana Research.