Alexandra Guy

Anatexis of accretionary wedge, Pacific‐type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt

Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the “Khantaishir” arc (520–495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

Geophysical constraints for terrane boundaries in southern Mongolia

The Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes. In theory, these terranes should be characterized by contrasting magnetic and gravity signatures owing to their dissimilar petrophysical properties. To test this hypothesis, the extent of tectonostratigraphic terranes in southern Mongolia was compared with the potential field data. The analysis reveals that the terrane boundaries are not systematically defined by strong gravity and magnetic gradients. The correlation of the magnetic signal with the geology reveals that the magnetic highs coincide with Late Carboniferous to Early Permian volcanic-plutonic belts. The matched filtering shows a good continuity of signal along the boundaries of these high magnetic anomalies toward the deeper crustal levels which may indicate the presence of deeply rooted tectonomagmatic zones. The axes of high-density bodies in the western and central parts of the study area are characterized by periodic alternations of NW-SE trending gravity anomalies corresponding to up to 20 km wide cleavage fronts of Permo-Triassic age. The matched filtering analysis shows good continuity of signal to the depth of these gravity highs which may indicate presence of deeply rooted high-strain zones. The magnetic signal is interpreted to be as the result of a giant Permo-Triassic magmatic event associated with lithosphere-scale deformation, whereas the gravity pattern is related to the postaccretionary shortening of the CAOB between the North China and Siberia cratons.

MELTING OF ACCRETIONARY WEDGE AND BUILDING MATURE CONTINENTAL CRUST: INSIGHTS FROM THE MAGMATIC EVOLUTION OF THE CHINESE ALTAI OROGEN, CENTRAL ASIA

Tectonic-magmatic reworking of accretionary wedges is a key process responsible for differentiation and stabilization of continental crustal in accretionary orogens. This generic problem can be exemplified by magmatic evolution of the Chinese Altai which represents a high-grade core of the worlds largest accretionary system, namely the Central Asian Orogenic Belt (CAOB). In the Chinese Altai, voluminous SilurianDevonian granitoids intruding a greywacke-dominated Ordovician flysch sequence. These intrusions are classically interpreted to originate from predominant (70‒90 %) juvenile (depleted mantle-derived) magma. However, their close temporal and spatial relationship with the regional anatexis of flysch rocks, allows us to examine the possibility that they were mainly derived from flysch rocks.

Moho depth model for the Central Asian Orogenic Belt from satellite gravity gradients

The main purpose of this study is to construct a new 3-D model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim Cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. First, we estimate an initial crustal thickness by inversion of the vertical gravity component of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and DTU10 models. Second, 3-D forward modeling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry, and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection, and receiver function studies and geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3-D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

Hybrid accretionary/collisional mechanism of Paleozoic Asian continental growth: new plate tectonic perspective

Continental crust is formed above subduction zones by well-known process of “juvenile crust growth”. This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Laurussian or later Eurasian continent along the “Alpine Himalayan collisional/interior orogens” in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along “circum-Pacific accretionary/peripheral orogens” at the border of the PaleoPacific and modern Pacific oceanic plate.