Seismic Tomogram of the Earth's Mantle: Geodynamic Implications

Orogens in the evolving Earth: from surface continents to ‘lost continents’ at the core–mantle boundary

Orogens and their posthumous traces are the basic elements that can be used to understand the material circulation within the Earth. Although information preserved in the rocks on the surface ranging in age from 4.4 Ga to the present has been used to characterize orogens, it is important to understand orogens on a whole-Earth scale to evaluate global material circulation through time. In this paper, we synthesize the general concepts and characteristics of orogens and orogenic belts. The collision type and accretionary type constitute the two end-member types of orogens, both sharing similar structural features of subhorizontal disposition, bounded above and below by paired faults. Their exhumation generally occurs in two steps: first by wedge extrusion to form a sandwich structure with subhorizontal boundaries, which is followed by domal uplift of all the units. In the accretionary type, oceanic lithosphere subducts under the continental margin, and in the collision type, buoyant continents collide with each other. Of the various types of subduction and collision processes, arc–arc collision orogeny may have been widespread in the Archaean, although most of the intra-oceanic arc crust must have been destroyed and dragged down to the Archaean core–mantle boundary (CMB). Here we propose a broad two-fold classification of orogens and their subducted remnants, based on (1) their thermal history and (2) temporal constraints. Based on their thermal history, orogens are grouped into three types: cold orogens, hot orogens and ultra-hot orogens. Two extreme situations, which are anomalous and unlikely to occur on Earth, termed here super-cold and super-hot orogens, are also proposed. We discuss the characteristics of each of these subtypes. Based on temporal constraints, we group orogens into Modern and Ancient, where in both cases regional metamorphic belts occupy the orogenic core. In both groups, the overlying and underlying units of the regional metamorphic belts are weakly metamorphosed or unmetamorphosed, and are either accretionary complex in origin (Pacific type) or continental basement and cover (collision type). Major structures are subhorizontal with oceanward vergence of deformation, for both types. Orogens in the Modern Earth are grouped into four sub-categories: (1) deeply subducted orogens that are taken down to mantle depths and never return to the surface, termed here ‘ghost orogens’; (2) those that are subducted to deep crustal levels, undergo melting and are recycled back to the surface, forming resurrected and temporarily ‘arrested orogens’; (3) ‘extant orogens’, which are partly returned to the surface after deep subduction; (4) ‘concealed orogens’, which have been deeply subducted and only the traces of which are represented on the surface by mantle xenoliths carried by younger magmas. The preservation of orogens on the surface of the Earth occurred through an unusual return process from their natural course of total destruction, a phenomenon that operated more efficiently in the Phanerozoic through exhumation from ultra-deep domains against the slab-pull force of the plate, aided by fluids derived by dehydration of subducted lithosphere. Orogens at present represented on the surface of the Earth constitute only a fraction of the total volume formed in Earth history. Traces of the deeply subducted ‘lost orogens’ are sometimes returned to the surface in the form of melt or mantle xenoliths through combined processes of plume and plate tectonics. From a synthesis of the processes associated with the various categories of orogens proposed in this study, we trace the time-dependent transformations of orogens in relation to the history of the evolving Earth.