Salt Tectonics The following notes are abbreviated versions of Mark Rowan’s three-day industry short course on salt tectonics. Introduction Diapirs and diapirism Diapir initiation during differential Distribution and origin of salt basins loading Diapir initiation during extension or Mechanics of salt deformation contraction Active and passive diapirism Salt withdrawal structures and welds Reactivation of diapirs during Turtle structures extension or contraction Expulsion rollovers Diapir interiors and margins Welds Near-diapir deformation - folding and faulting Tectonic styles of salt deformation Thick-skinned extension Collisional mountain belts 1 INTRODUCTION There has been an enormous revolution in our understanding of salt tectonics in the past decade or so Structural restoration. The technique of cross- (see Jackson, 1995, for an excellent history of salt section restoration was first applied to salt tectonics research). The beginnings of the revolution date structures in the late 1980s (e.g., Worrall and back a little farther within some of the exploration Snelson, 1989). In the past decade, numerous companies, but their ideas only became public starting in people have used restoration to reconstruct the about 1989. Our increased understanding of the history of salt movement and associated geometry and evolution of salt bodies and associated deformation of surrounding strata. strata is due in large part to the fortuitous convergence of advances in four areas: Field studies. Armed with new ideas, various researchers have reexamined exposed salt basins Seismic imaging. There has been a steady improvement throughout the world, leading to improved in seismic data acquisition and processing over the years. But with the advent of such techniques as pre- understanding of the processes of salt-related stack depth migration, images of salt bodies became deformation. much clearer, with improved pictures of the bases of salt In this course, we will concentrate on the new sheets and overhangs and the steep flanks of many ideas about salt tectonics. Many of the illustrations diapirs (e.g., Ratcliff, 1993). used here are examples of the four areas of Experimental and numerical modeling. Attempts to advance listed above. Much of the work has been model salt deformation have been made for many concentrated in the northern Gulf of Mexico, but decades (e.g., Nettleton, 1934), but until fairly recently, the impact of the advances has spread to salt both salt and its overburden were modeled as viscous basins worldwide. Thus, we will also examine the fluids. Starting in the late 1980s, however, B. Vendeville and coworkers started modeling the overburden as a geometries and structural styles of salt from such brittle material, more in keeping with the known places as the North Sea, the Red Sea, offshore mechanical behavior. The results demonstrated salt’s West Africa, offshore Brazil, the Precaspian more passive role of reacting to, rather than causing, Basin, and onshore Mexico. deformation (e.g., Vendeville and Jackson, 1992a, b) and fundamentally changed the ways most people look at salt deformation. 2 DISTRIBUTION AND ORIGIN OF SALT BASINS SE oriented transfer faults. The rift geometry, coupled with the rate of sedimentation and the relative time of Salt basins are found throughout the world (Fig. 1), but a evaporite formation, determines the areal extent and quick look will show that they occur primarily in rift thickness distribution of evaporites. Salt may be restricted basins and along passive margins, as well as in their to individual half-grabens (Fig. 5 it may be regionally deformed counterparts, such as the Alpine/Himalayan tabular (Fig. 6), or it may have an intermediate geometry, system. According to a review by Jackson and Vendeville with a regional distribution but significant thickness (1994), many salt deposits were formed during the early variations (Fig. 7). postrift phase, including the basins of offshore Brazil, offshore West Africa, the U.S. Gulf Coast, and the Red Salt commonly occurs in paired basins on either side of Sea (Fig. 2). Others were formed either during rifting or oceanic crust, such as across the Gulf of Mexico, the South during lulls between distinct rift episodes, for example Atlantic, and the North Atlantic. Thus, it has generally many of the basins on either side of the northern Atlantic been thought that evaporite deposition occurs only on Ocean (Fig. 2). Finally, a few salt basins appear to be older continental crust, with subsequent oceanic spreading than rifting, namely those in the North Sea and Persian separating a once contiguous basin into two parts (Fig. 8). Gulf (Fig. 2). Many passive margins have seaward-dipping reflectors, or SDRs (Fig. 9) – wells show that these consist of subaerial Many rift basins have a similar history, one that is basalts that are considered the initial expression of oceanic conducive to evaporite deposition. They form during spreading. Autochthonous salt in parts of offshore Brazil extension of the continental crust, and grabens are initially occurs at a stratigraphic level above the SDRs, leading to a filled with nonmarine clastics because of the high heat model in which salt deposition postdates the onset of flow and associated regional uplift during rifting. oceanic spreading (Fig. 10). Salt in the South Atlantic Subsidence of the grabens, either during rifting or, more (offshore West Africa or Brazil) is interpreted to occur typically, during postrift thermal and loading subsidence, above the breakup unconformity and thus is underlain by a leads to marine incursion. If the climatic conditions are combination of landward continental crust and basinward appropriate, it is during the transition from nonmarine to oceanic crust (Fig. 11). There is then a transition to marine environments that evaporites are formed (often shallow-water carbonates and then deeper-water facies. episodically). In the typical (but not universal) scenario, The interbedded nature of the salt-carbonate transition and continued subsidence leads to true marine conditions. the similar seismic velocities means that there is typically Rift basins have a distinct basement architecture made a low acoustic impedance contrast and thus no good top- up of grabens and half-grabens segmented by transverse salt reflector for the autochthonous salt layer. In contrast, structures such as accommodation zones or transfer faults the salt is usually in contact with underlying clastic (Fig. 3). An example from the Brazilian margin is shown redbeds or basement so that there is a good base-salt in Figure 4, where the rift system includes a series of NW- reflector. 3 The evolution for the northern Gulf of Mexico is shown in The Precaspian Basin example nicely shows the kinds of Figure 12. Initial rifting during the Early Jurassic resulted vertical and lateral facies variations commonly found in in the SSW movement of Yucatan away from N. America, evaporite basins. Although salt dominates the basin center, whereas oceanic spreading resulted in southern movement there is also interbedded anhydrite that becomes more and rotation about a nearby pole. This means that dominant towards the northern margin. There can also be basement structures will have different orientations in the significant components of both carbonates and thinned continental crust and the oceanic crust. The siliciclastics that are concentrated along basin margins. boundary between the two is interpreted to be in the Another example is provided by the central North Sea, approximate position of the present-day shelf edge, so that where non-salt facies in the proximal part of the basin most of the deepwater province is underlain by oceanic disappear towards the center (Fig. 16). Of course, the crust (Fig. 13). So instead of one salt basin on continental presence of other lithologies means that there can be crust that was subsequently split by spreading (Fig. 12), significant, even coherent, reflectivity within the evaporite there were most likely two salt basins with the downdip layer (Fig. 17). edges defined by the incipient spreading center (Fig. 13). Although most salt basins are closely associated with rifting, salt deposits can form anytime conditions are appropriate for evaporite formation. Thus, any restricted basin with an arid climate is a potential salt basin. The modern examples are the sabkha deposits of the Arabian peninsula, but these are rare in the geologic record and, similarly, the types of salt basins common in the past are not observed today (Warren, 1999). One ancient setting for evaporite deposition is a basin with an open marine connection that gets closed off. This will lead to the development of various evaporite facies as the basin essentially dries up (Fig. 14). Examples include basins with narrow entrances that become emergent during major sea-level drops, such as the Mediterranean (Messinian salinity crisis) and the Red Sea. Another example is when plate tectonic motions close off a basin, as in the case of the Precaspian Basin during the Permian (Fig. 15). 4 Figure 1. Locations of salt basins around the world (Jackson and Talbot, 1991). 5 Figure 3. 3-D block diagram of basement rift architecture with offset grabens separated by an accommodation zone (Stonely, 1981). Note that more asymmetric rift geometries are also possible. 6 Figure 5. Example from east Africa where the salt is isolated within a single half-graben (Malek-Aslani, 1985). 7 Figure 6. Example from west Africa, where salt is high enough in the section to form a regionally continuous, tabular salt body (Perrodon, 1981). 8 Figure 7. Model for the onshore northern Gulf of Mexico, where the salt is continuous but of highly variable thickness because it partially fills in the rift-basin architecture (Adams, 1989). 9 Figure 11. Regional cross section across Angolan margin showing salt deposition above both continental and oceanic crust (Jackson et al., 2000). 10 Figure 12. Present-day geometry of the Gulf of Mexico and three reconstructions to the Early Jurassic (beginning of rifting), Late Jurassic (onset of seafloor spreading), and Early Cretaceous (end of 11 seafloor spreading) (Pindell et al., 2000).