Martin Jackson

Lecture MJ1: Continental-Scale Salt Tectonics on Mars and the Origin of Outburst Channels

The nearly 3000-km-wide Thaumasia Plateau is the most striking plateau on Mars. Its morphology of is typical of a thin-skinned “mega-slide,” in which extensional deformation connects via lateral zones of transtension and strike-slip to a broad zone of compressional uplift and shortening. The low regional slope (≈1°) of the 4-km-high plateau results in gravitational body forces that are too small to deform the basaltic lava flows conventionally thought to compose this area. Instead, we propose that geothermal heating and topographic loading of buried deposits of salts (or mixtures of salts, ice, and basaltic debris) would allow for weak detachments and large-scale gravity spreading. We propose that the generally linear chasmata of Valles Marineris reflect collapse and excavation along fractures radial to the adjoining Tharsis volcanoes then reactivated as a lateral margin of the Thaumasia gravity-spreading system. The other lateral margin is a massive dextral splay of extensional faults forming the Claritas Fossae, which resembles a trailing extensional imbricate fan. The compressional mountain belt of the Coprates Rise and Thaumasia Highlands forms the toe of the "mega-slide." A failed volcanic plume below Syria Planum could have provided both thermal energy and topography to initiate regional deformation. Higher heat flow during Noachian time, or heating due to burial by Tharsis-derived volcanic rocks, would have promoted flow of salt deposits, as well as yielded groundwater from melting ice and dewatering of hydrous salts. Connection of overpressured groundwater from aquifers near the base of the detachment through the cryosphere to the martian surface created the outflow channels of Echus, Coprates and Juventae chasmata at relatively uniform source elevations along the northern margin of the "mega-slide" where regional groundwater flow would have been directed toward the surface. Our hypothesis explains the perplexing relationships between the Tharsis volcanism, deformation of the Thaumasia Plateau, and the formation of Valles Marineris and associated outburst floods. .

Lecture MJ2: How and where is the Sigsbee Escarpment Advancing?

The Sigsbee Escarpment is the largest deformation structure in the Gulf of Mexico, where it separates the lower Texas-Louisiana slope from the continental rise. Because of its vast size and unrivaled coverage by reflection seismic data, the Sigsbee Escarpment is the type example for the leading edge of an advancing salt canopy. However, no systematic study has ever been published of the structural styles along the escarpment. We surveyed the 750 km-length of the Sigsbee Escarpment using ~1300 seismic profiles, mostly from 3D seismic datasets. We infer that ~40% of the Sigsbee Escarpment is static over escarpment distances as long as 130 km. These static lobes of the salt canopy are not advancing but are commonly inflating, overlain by draped but continuous roof strata. In contrast, the other ~60% of the Sigsbee Escarpment is actively advancing. Advance by salt extrusion may have dominated in the past, but <1% of the advancing scarp length now exposes salt. In contrast, more than 99% of the active salt front is buried. It advances by thrusting, most commonly as a thrust rooted into the tip of the salt sheet. The salt sheet and its protective roof advance together, minimizing salt dissolution. Additionally, imbricate thrust wedges sole into either the salt tip or the salt floor. These accretionary wedges comprise thrust slices of thin, weak strata. They form rapidly during surges of canopy advance. Stacked imbricate wedges record cycles of shortening then burial then detachment climb during renewed shortening. Salt-roof thrusts are rare and form where the sedimentary roof advances faster than the underlying salt.

Jackson, M. P. A., Hudec, M. R., Jennette, D. C., and Kilby, R. E., 2008, Evolution of the Cretaceous Astrid thrust belt in the ultradeep-water lower Congo Basin, Gabon: AAPG Bulletin, v. 92, p. 487–511 (also cover photograph).

Jackson, M. P. A., and Harrison, J. C., 2006, An allochthonous salt canopy on Axel Heiberg Island, Sverdrup Basin, Arctic Canada: Geology, v. 34, no. 12, p. 1045–1048.

Jackson, M. P. A., and Hudec, M. R., 2005, Stratigraphic record of translation down ramps in a passive-margin salt detachment: Journal of Structural Geology, v. 27, p. 889–911.