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Shear margins

Continent-ocean transform and fracture zone boundaries

Bird Geophysical, Houston, Texas, U.S.

Corresponding author: D. Bird, dale@birdgeo.com

Editor's Note: The Geologic Column, which appears monthly in TLE, is (1) produced cooperatively by the SEG Interpretation Committee and the AAPG Geophysical Integration Committee and (2) coordinated by R. Randy Ray and Lee Lawyer.

The first 20% of the full text of this article appears below.

As exploration extends into deeper water, it has become more important to understand the nature of the earth's crust beneath offshore sediments. That is, heat flow and related source rock potential usually require that sediments be deposited over continental crust. Hence, delineating the boundary between oceanic and continental crust becomes important. The material in this article was originally presented at the workshop "The Crust and its Structure" at SEG's recent Annual Meeting in Calgary. These results, part of a larger dissertation research project, also have been presented to several oil and gas exploration companies in Houston.

Although continental-oceanic crustal boundaries for shear margins are among the easiest to delineate, they are important to understand because their formation usually produces distinct geologic features. The remainder of this article reviews global geophysical data over several shear margins and focuses on their evolution and related structural, sedimentary, and thermal effects. These margins typically form after: (1) shearing of continental crust and complex rifting; (2) development of an active transform boundary separating oceanic and continental crust; and (3) passive margin formation along an inactive fracture zone that also separates oceanic and continental crust (Figure 1).

View larger version (25K): [in this window] [in a new window]   Figure 1. Generic three-stage model for shear margin formation (after Lorenzo, 1997): (1) rift: continent-continent shearing; (2) drift: continent-ocean transform boundary (active margin); and (3) passive margin: continent-ocean fracture zone boundary.

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