Accretionary Tectonics of the North American Cordillera

Abstract

Continental geology stands on the threshold of a change that is likely to be as fundamental as plate-tectonic theory was for marine geology. Ongoing seismic-reflection investigations into the deep crustal structure of North America are verifying that orogenic zones are underlain by low-angle faults of regional extent (Brown et al 1981). The growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments. Paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture-bounded fragments, particularly within the North American Cordillera. Such interdisciplinary studies are leading to a consensus that the Cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in Mesozoic and early Cenozoic time. Such crustal growth is referred to as accretionary tectonics. In this paper, we review some of the important concepts in accretionary tectonics, discuss the nature of the materials accreted between central Alaska and southern California in Jurassic and Cretaceous time, and consider the general relations between Cordilleran accretion and the movement of lithospheric plates. The concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest. With the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen, along with mechanisms for crustal attrition. Accretion mechanisms include the growth of imbricated sedimentary prisms along inner-trench walls, slicing off of submarine topographic irregularities within subducting plates, and collision of continents and volcanic arcs by ocean-basin closure. Tectonic attrition mechanisms include rifting, transform faulting, and strike-slip or underthrust removal of inner-trench wall materials coincident with or in place of accretionary prism growth. Growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments. An important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins. Since attrition by nature leaves little material evidence of having operated, one of the major problems confronting Cordilleran geologists lies in the recognition of such attrition within the ancient record, particularly when interspersed with accretionary events. The spectrum of accretion and attrition mechanisms viewed at cm yr^-1 plate-transport rates over time scales of 100 m.y. leads one to suspect a highly mobile history for continental-margin orogens. The serial arrangement of subducting, transform, and rifting links along the modern Cordillera plate-juncture system and both serial and parallel arrangements in the western Pacific systems show the complex interplay of such mechanisms through space. Similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the Cordillera, which presently resemble a collage of crustal fragments (Davis et al 1978). Recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust, and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and Earth history. The first problem to be considered is the recognition of native North American crust from exotic fragments that have been accreted to its edge.

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