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Advance of allochthonous salt sheets in passive margins and orogens

¹ Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924; michael.hudec{at}beg.utexas.edu
² Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924

Mike Hudec received his Ph.D. from the University of Wyoming in 1990. He has worked for Exxon Production Research and taught at Baylor University. He joined the Bureau of Economic Geology in 2000, where he is codirector of the Applied Geodynamics Laboratory. His current research interests include palinspastic restoration of salt structures, salt-sheet emplacement mechanisms, and minibasin initiation.

Martin Jackson established and coleads the Applied Geodynamics Laboratory, a prominent research group on salt tectonics. His honors include AAPG's Sproule Award, Matson Award, and Dott Award. His current research blends three-dimensional seismic interpretation, modeling, and fieldwork focusing on allochthonous salt sheet advance and folded evaporite canopies in the Canadian High Arctic.

Allochthonous salt sheets advance in four ways: (1) extrusive advance, (2) open-toed advance, (3) thrust advance, and (4) salt-wing intrusion. These mechanisms are determined primarily by the geometry and thickness of the roof that overlies the advancing sheet. An extrusive sheet spreads without a roof or with a roof of negligible mechanical strength. An open-toed sheet is partially covered by a mechanically significant roof but has an extrusive toe. An overthrusting sheet advances along a thrust fault at its leading edge, carrying its roof with it. A salt wing intrudes from the flank of a diapir into a shallower salt layer.

Extrusive, open-toed, and overthrusting salt sheets are found in both passive margins and orogens. A sheet typically evolves through two or more of these mechanisms. Three lineages, or evolutionary paths, are common. Plug-fed extrusions emanate from the top of a salt dome or salt wall. Plug-fed thrusts form the base of the hanging wall of thrust faults that are rooted in the tops of salt domes or salt walls. Finally, source-fed thrusts initiate as thrust faults rooted in the autochthonous salt layer. Source-fed thrusts form the largest individual salt sheets, some covering thousands of square kilometers. Salt-wing intrusions form only under special circumstances, so they are not part of the three major lineages. These intrusions are restricted to compressionally inverted basins containing multiple salt layers and are known only in the Zechstein salt basin.

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