Abstract: Aulacogens and Megashears: Natural Habitat for Oil and Mineral Deposits

John M. Browning

Sedimentary basins associated with linear structural features such as megashears, deflections, and geofractures striking at high angles to cratonic interiors have been prospected for oil since early in the history of petroleum exploration. Although long conceived as regional features, these linear structures only recently have been recognized to have a common origin as aulacogens or "failed arms" of plume-generated triple junctions formed during the process of continental breakup.

These features, trending at high angles from the rifted continental margin, mark ancient plate margins and establish the time of crustal rifting. Some, formed at an early stage in the evolution of the earth's crust, have remained active or become reactivated as major sites of crustal dislocation during subsequent episodes of continental breakup and collision. Others, governing the course of major rivers, become the site of deltas that have augmented the sedimentary fill. These long structural and varied depositional histories and the critical timing of the ongoing tectonic processes, often of a transcurrent nature, together with the accompanying sedimentation have given rise to major hydrocarbon accumulations. In addition to the formation of favorable source beds, and structural and s ratigraphic traps, the high heat-flow characteristic of these features often enhances the generation of oil and gas.

Some aulacogens are marked only by a line of alkaline intrusions, others have complex igneous histories and commonly become the site of base-metal mineralization of a syngenetic type.

Three of these features--the Athapuscow aulacogen of northern Canada, the Wichita aulacogen of southern Oklahoma, and the Huancabamba deflection of central South America--illustrate the problems related to time of formation and the sedimentary and tectonic evolution and illustrate the formation of local structures and depositional basins and their relation to the evolution of the major tectonic feature.

Several other regional features having oil and gas potential, e.g., the Delaware basin of West Texas, the Bathurst Inlet of northern Canada, and the Mississippi embayment of the Gulf Coast, that now are termed aulacogens offer comparisons.

AAPG Search and Discovery Article #90973©1976-1977 AAPG Distinguished Lectures