COLLISION TECTONICS-CORRELATION BETWEEN NATURE AND DURATION OF MAGMATISM AND PRE-COLLISION GEOMETRY OF CONTINENTAL MARGINS OF THE APPALACHIAN OROGEN

The inter-relationship between magmatism and tectonic processes, especially in collision zones, provides a window into the thermo-tectonic evolution of orogenic belts. In the Appalachian orogen (Maine to Alabama) the spatial and temporal distribution of igneous rocks has provided compelling evidence for both collisional and extensional kinematics of plate reorganization. Although many distinct magmatic cycles have been recognized (late Precambrian to Carboniferous), the most enigmatic episode belongs to the Siluro-Devonian (Acadian orogeny). The magmatic record of this event in the northern Appalachians has been attributed to both collisional and plume related tectonic processes. In contrast to the voluminous Acadian age igneous activity present in the northern Appalachians, the more modest record of magmatism in the southern region provides a continuum of ages and complex geochemical signatures ranging from the middle Ordovician (Taconic orogeny) through the Devonian. Although traditional two stage collisional processes (similar to the northern segment) have been proposed, we propose a model that more uniquely explains the enigmatic record of the igneous activity. We suggest that the spatial, temporal and compositional record of magmatism is uniquely related to the geometry of the continental margin prior to collision and crustal thickening. Along the late Precambrian- Cambrian margin of southern Laurentia, the distribution and thickness of synrift sedimentary and volcanic rocks correspond systematically to the outline of promontories and embayments of the continental margin. Embayments host thick successions of synrift sedimentary and volcanic rocks, as well as thicker passive margin shelf successions, whereas promontories are essentially devoid of synrift deposits and have a thin blanket of passive margin sediments. We suggest that collisional tectonics superimposed on these contrasting sedimentary environments has resulted in a magmatic record that images the availability of fertile lithologies required for generating melts in overthrust regions. Accordingly, the " pseudo- Acadian" plutons of the southern Appalachian reflect a time-depth variation in melting conditions within the embayment environments following middle Ordovician collisional processes.