SEISMIC AND CRUSTAL IMPLICATIONS FOR EVOLVING ACCRETIONARY WEDGE-BASIN: LAKE VAN, EASTERN ANATOLIA ACCRETIONARY COMPLEX (E-TURKEY)

Eastern Anatolia Accretionary Complex (EAAC) provides a unique natural laboratory to constrain geodynamics associated with continent-continent collision, subsequent lithospheric delamination, crustal thinning and formation. EAAC is responsible for major growth of the continental lithosphere, through the addition of juvenile magmatic products, but are also major site of consumption and reworking of continental crust through time. It can be broken into retreating and advancing types, based on its kinematic framework and resulting geological character. This research deals with some critical aspects of post-orogenic processes and discusses a prominent example of accretionary orogens in the world and its crustal growth through neotectonic history. An examination and evaluation of multi-channel seismic reflection and high-resolution chirp data cover the geological consequences of the largely Neogene delamination that characterizes the segments of interest, in response to late stage reorganization of the subduction zone, the delamination and break-off of the subducting slab. The aim of this research is to provide a better understanding and overview of post-collisional processes in accretionary orogens and their role in the formation and evolution of the continental crust.

It becomes clearer that questions on evolution of EAAC are answered in tectonic patterns and deformation styles of the accretionary wedge-basins in the region. The peculiar one of these wedge-basins is orogen-parallel Lake Van trough (with 2 km elevation), emplaced at N-end of Bitlis-Pötürge Massive (BP-M) along Muº suture, separating EAAC from BP-M. Lake Van region is a place where no mantle lid exists. Dome-shaped topography of Lake Van is interpreted as indicating asthenospheric upwelling. Lake Van, together with its W-continuation, Muº basin, is generally considered as a prototypical sediment-rich accretionary wedge-basin complex, its tectonic and magmatic evolution is important for understanding accretionary plate margins, the surficial tectonic effects of delamination and decompressional melting magmatism. In multi-channel seismic reflection research of Lake Van, the largely seismic data-based contributions investigate the following special subjects of the preliminary results to exhibit and interpret tectonic and magmatic record of orogeny in Lake Van accretionary wedge-basin. Seismic structural interpretation of overall deformations observed in and along Lake Van leads to clear evidences of basement reactivation, basin inversion, strike-slip deformation, orogen-parallel extension as well as extensional magma propagation, post-magmatic hydrothermal alterations, and magma-hydrothermal sediment deformations. Seismic sections show a late stage of extension-transtension of W- and S-margins of Lake Van, implying that there is a growing appreciation of the role of extensional/strike-slip tectonics in accretionary orogen. This gives an important view that local or large-scale tectonic contacts such as suture complex in orogenic belt have changed. Reflection data also illustrate seismic pattern, external geometry and basinal emplacement of shallow level magmatic intrusions and their cryptoexplosions and reveal clear evidences of dynamics of crustal magma transfer, storage and differentiation. An interpretation of these findings can be extended into an overview of the evolution of the convergent crust beneath the lake. This suggests that extensional magmatism is subject to a series of processes that lead to its differentiation during transfer through and storage within the convergent crust. This provides new insights to the subject of magmatic processes operating within the crust, and remarks important links between subsurface processes and magmatism.

The vast amount of the magmatic cover in the lake suggests a corresponding abundance of alkaline melting materials. These inject into the accretionary complex and there become solidified. The generated partial melts by adiabatic decompression pool under the accretionary structure, penetrate it and begin melting it. This is the first-order mechanism of upper crustal emplacement of rising magmas, confirming extensive marginal distribution of extensional magmatism in the lake. This mechanism builds the crust and drives hydrothermal circulation and consequent melting of the wet accretionary complex rocks, implying basalt-hosted hydrothermal system in Lake Van. This evident the transport of extensional alkaline magmas from the uppermost mantle through the basin-bounding fault zones/suture trends and its cooling and crystallization. Decompressional melting magmatism is correlated with alkaline magmatic surges around Lake Van and clearly appears associated to the present-day area of the crustal thinning defined by the previous studies. It is obvious that the timing of these magmatic surges is related to upwelling pulses from the uppermost mantle, dependent of the extensional and strike-slip tectonic setting of Lake Van. All these considerations deduced from seismic structural observations give a clue to how continental crust forms in E-Turkey and how it begins its journey towards complete consolidation in the form of a continent.