STRUCTURE AND GEODYNAMICS OF FORMATION OF ACTIVE CONTINENTAL MARGINS

Abstract

The structure of active continental margins is characterized by development of volcanic and terrane fold-and-nappe orogenic belts, which collectively ensure the growth of continental margins both in area and crustal thickness. Accretionary prisms (wedges) developing in the frontal part of volcanic belts differ in principle from terrane fold-and-nappe belts by their structural peculiarities and genesis. Unlike these belts, accretionary prisms are largely made up of turbidites (with olistostrome Horizons), i. e., deposits characteristic of the inner slope of a deep-sea trench, and to a lesser degree, deep-sea oozes of the subducted oceanic plate. Accretionary prisms exhibit an imbricate-thrust structure but with insignificant amplitudes of horizontal displacements of rocks. Blocks of allochthonous rocks (terranes) account for a small portion of the volume. In contrast, the main features of terrane orogenic belts are as follows: Large (a few hundreds of km and more) amplitudes of horizontal movements of rocks; tectonic contact of allochthonous assemblages (terranes) strongly differing in genesis, age, and depositional environments (including climatic conditions); sheets of ophiolite including ultrabasite; tectonic splitting of terranes; development low-high pressure metamorphic zones (attended by anatectic melts) in the soles of tectonic nappes; obduction of tectonic nappes onto continental margins, and complex fold-thrust and nappe-thrust structure. The evolution of active continental margins displays a pulsation character determined by alternation in time of volcanic and orogenic process in these margins. This is conditioned by alternation in time of subduction and accretion-collision processes at convergent boundaries of oceanic and continental plates. Terrane orogenic belts begin to form when subduction (and accordingly, volcanism) stops, i. e., in the course of the change of plate convergence by accretion-collision processes along continental margins. In this connection, each orogenic belt in the continental margin acts as an indicator of the stage of reconstruction of geodynamic regimes at convergent boundaries of plates. The peculiarities of formation of active continental margins reflect geological events in the oceans and eventually, endogenic activity of the Earth manifested as mantle upwelling.