article, Nettleton (1934) used physical experiments to investigate the progressive ... similarities between Trusheim's observations and Nettleton's experimental.
A New Interpretation of Trusheim’s Classic Model of Salt-Diapir Growth
Vendeville, Bruno C. Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924
Abstract In 1960, Trusheim described three evolutionary stages of salt movement (pillow, diapir, and postdiapir) recorded in adjacent sediments by three types of peripheral sinks (primary, secondary, and tertiary). Assuming that overburden rocks were viscous, Trusheim interpreted such evolutionary stages as typical of buoyant Rayleigh-Taylor instabilities. We use mechanical reasoning and physical models to reinterpret Trusheim’s observations, assuming a brittle overburden. Results suggest that nonpiercing salt pillows do not represent immature diapirs but form as contractional buckle folds or as salt swells triggered by differential sediment loading. Extension also forms salt pillows, but there, the roof of the pillowlike structure is breached early by normal faults. Subsidence of the overburden below the regional datum forms peripheral sinks in between the diapirs. The diapir stage, during which strata thicken against the diapir, corresponds to subsidence of diapir flanks driven by late extension. During the postdiapir stage, the diapir crest rises above the regional datum while the adjacent depocenters neither subside nor rise. This late stage corresponds to tectonics- or gravitydriven contraction that rejuvenates dormant diapirs.
Introduction In a benchmark article published in AAPG Bulletin in 1960, F. Trusheim set the foundation for interpreting kinematic evolution of salt structures. The originality of Trusheim’s work was that it demonstrated clearly and simply how salt diapir evolution is recorded by specific patterns of lateral thickening or thinning of strata in the sediment depocenters adjacent to the diapirs. Trusheim further developed his evolutionary model by interpreting his observations in light of the general theory for salt-diapir growth that prevailed at the time (buoyant rise by a mechanism of Rayleigh-Taylor instabilities). Trusheim’s approach was so successful that it has since become the norm in analyzing the evolution of salt diapirs. Rock mechanics data indicate, however, that nonevaporitic sedimentary rocks in the upper continental crust are typically brittle rather than viscous. Because the development of Rayleigh-Taylor instabilities requires that the overburden be weak and viscous, this mechanism is not a viable process for the rise of salt diapirs (e.g., Vendeville and Jackson, 1992a; Weijermars et al., 1993). Accordingly, despite Trusheim’s remarkable observations, some aspects of his evolutionary model need to be updated in light of the new concepts in salt tectonics. This article presents an alternative interpretation to his observations.
Trusheim’s Original Model After compiling data from more than 200 salt structures in northern Germany, Trusheim summarized their main characteristics using a synthetic cross section of diapirs and associated depocenters typical of the region (Fig. 1). The depocenters in this cross section comprise three main groups of synkinematic overburden strata (I, II and III in Fig. 1), each having its own characteristic pattern of lateral thickness changes. Strata in the lowest group (I in Fig. 1), deposited during the early stages of salt movement, are thickest in or near the middle of the depocenter and gradually thin toward the depocenter’s flanks, a geometry called the Primary Peripheral Sink by Trusheim (and often referred to as the Primary Rim Syncline by many authors). This pattern indicates that the central part of the depocenter subsided fastest while the flanks subsided less or even rose, owing to the rise of the adjacent salt body. Once restored to its original shape (Fig. 1C), a primary peripheral sink has the geometry of a bowl-shaped depocenter whose base is convex downward. 943
Gulf Coast Association of Geological Societies Transactions, Volume 52, 2002
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Figure 1. Restoration of a schematic cross section of a diapir and adjacent depocenters (Trusheim, 1960). A. Present-day (postdiapir stage) showing strata in the tertiary peripheral sink (III) thinning above the diapir crest. B. Diapir stage showing strata in the secondary peripheral sink (II) thickening toward the diapir. C. Pillow stage showing strata in the primary peripheral sink (I) thinning toward the diapir. D. Initial stage before salt movement.
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^ Strata in the middle group (II in Fig. 1), deposited during the middle stages of salt movement, are thinnest in the center of the depocenter and gradually thicken toward the depocenter’s flanks, a geometry named the Secondary Peripheral Sink or Secondary Rim Syncline. This reversal in the sense of stratigraphic thickness change indicates that the center of the depocenter was no longer subsiding. By the time the secondary peripheral sink started to form, there was little or no salt left beneath the center of the depocenter. The base of the depocenter grounded onto the top of the subsalt basement while the depocenter’s flanks started to subside (Fig. 1B). Subsidence of the depocenter’s flanks caused its convex-downward base to progressively deform until it became horizontal, thereby bending the overlying strata into a convex-upward geometry, a pattern typical of turtle-structure anticline. Strata in the upper group (III in Fig. 1), deposited during the latest stages of salt movement, thin laterally only above the crest of the salt diapir. Elsewhere, within the depocenter and on its flanks, the strata have laterally constant thickness. This geometry is that of a Tertiary Peripheral Sink or Tertiary Rim Syncline. The lack of thickness changes within the depocenter and its flanks indicates that neither the depocenter nor its flanks were subsiding during formation of the tertiary peripheral sink, whereas only the crest of the salt diapir rose. Trusheim further developed his conceptual model using the above results to draw a genetic, rather than kinematic only, interpretation of the growth of salt structures. When Trusheim wrote his article, the conventional wisdom was that the genesis and growth of salt diapirs were the result of Rayleigh-Taylor instabilities. Prior to Trusheim’s article, Nettleton (1934) used physical experiments to investigate the progressive evolution of spontaneous and autonomous, buoyant rise of a low-density fluid (the salt) through a denser fluid (the overburden). Results (illustrated in Fig. 2 by more recent numerical models by Podladchikov et al., 1993) showed that instabilities started as low-amplitude, domelike swells (Fig. 2C). The overburden located away from the rising instability subsided, whereas the overburden above and near the rising, less-dense material was vertically thinned and horizontally stretched, a pattern very similar to Trusheim’s primary peripheral sink. The second stage of Nettleton’s experiments showed a rapid rise of the buoyant material associated with subsidence and thickening of the denser overburden near the rising structures (Fig. 2B). Once again this behavior mimicked that observed by Trusheim. Finally, during the last stage of Nettleton’s experiments, the diapir kept rising while its stem was pinched off, and the overburden located above the diapir crest was thinned (Fig. 2A). 944
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A New Interpretation of Trusheim’s Classic Model of Salt-Diapir Growth
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Figure 2. Cross sections from a numerical model by Podladchikov et al. (1993) illustrating successive stages of development of Rayleigh-Taylor instabilities, including the pillow stage (C), diapir stage (B), and postdiapir stage (A).
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^ Because of the striking geometric similarities between Trusheim’s observations and Nettleton’s experimental results, Trusheim concluded that the three types of peripheral sinks he had observed corresponded to the three evolutionary growth stages of Rayleigh-Taylor instabilities (illustrated in Figure 1). First, during the Pillow Stage (Fig. 1C), a wide, dome-shaped salt body rises spontaneously while adjacent primary peripheral sinks form. Second, during the Diapir Stage (Fig. 1B), the wide base of the diapir narrows, and the flanks of the adjacent depocenters subside, forming secondary peripheral sinks. Third, during the Post-Diapir Stage (Fig. 1A), the diapir pinches off and continues to rise, deforming only the overburden located above the diapir crest (tertiary peripheral sinks). It is important to emphasize that this evolutionary model assumes that (1) salt rise is a spontaneous process resulting solely from buoyancy forces and (2) there is no change in the cross-section length (i.e., neither regional extension nor shortening: Fig. 1). Since Trusheim’s work, it has been demonstrated that overburden rocks are too strong to allow for salt-diapir growth by Rayleigh-Taylor instabilities (Weijermars et al., 1993). This statement, however, does not entirely invalidate Trusheim’s kinematic model. In the following sections, we illustrate how each evolutionary stage can be reinterpreted in light of the recent understanding of salt tectonics.
Pillow Stage Conceptual Inconsistencies of the Evolutionary Model The North German salt basin comprises full-grown diapirs having pierced their overburden, as well as pillowshaped salt bodies that have not subsequently grown into diapirs (Fig. 3). The overburden above the crest of these pillowlike structures is affected by normal faults (Fig. 3A) or has remained intact (Fig. 3B). Trusheim logically inferred that such intact pillow structures represent the earliest evolutionary stage of present-day full-grown diapirs. However, some inconsistencies shed doubt on (1) whether a salt pillow can spontaneously rise or stop rising and (2) whether some of the present-day pillowlike structures formed by buoyant rise alone. Rock mechanics data indicate that the overburden roof is typically too strong to be breached by the sole effect of buoyancy forces. Moreover, although differential loading could trigger salt movement during the early stages of deformation (i.e., when the sediment overburden is not compacted enough to be denser than the underlying salt), such loading would favor formation of bathymetric lows above the salt highs and thereby oppose further rise. Finally, there is no explanation why, even if the overburden were initially denser than salt (so that salt could rise buoyantly early on), a pillow would spontaneously stop rising. The presence of salt structures that have been preserved in their pillow stage is incompatible with results from Nettleton’s experiments indicating that, once Rayleigh-Taylor instabilities are initiated, they continue to rise inescapably until the entire source layer has been depleted. This last inconsistency suggests that some of the present-day, pillowlike structures may not have formed by simple buoyant rise, but by other, more episodic processes. We list below what such processes may be. 945
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Figure 3. Two cross sections showing dormant salt pillows in the German salt basin (Kockel, 1990).
^ Alternative Interpretations of Pillowlike Structures One process that can form pillowlike structures is regional, thin-skinned shortening. Shortening creates buckle-fold anticlines that rise above the regional datum even in the absence of density contrast between salt and overburden while the adjacent synclines, like primary peripheral sinks, accumulate thicker sediments in their center. The southern North Sea provides examples of pillowlike structures having intact overburden roofs that were initially interpreted as salt pillows (Fig. 4; Hughes and Davison, 1993) but were later proven to be mere buckle folds that formed in response to gravity gliding during regional doming (Coward and Stewart, 1995). A shortening origin for such structures explains why they stopped rising. When regional shortening stopped, the pillow had not grown tall enough for the differential load to be high enough to allow for continued, buoyancy-driven growth. The buckle folds simply stopped rising. Buckle folding does not lead to much thinning of the roof of anticlines, which, therefore, seldom evolve into diapirs, except for a few examples described by Coward and Stewart (1995), in which the overburden located above the fold crest was removed by erosion. Pillowlike structures can also form as residual salt highs located between rising diapirs (Fig. 5). As salt flows into each diapir, the source layer thins in areas adjacent to the diapirs while less or no salt is withdrawn from the central area located farther away from the diapir. The resulting convex-upward relic salt body retains an intact roof and typically remains dormant thereafter. Note that the salt relic never actually rose. A third process leading to pillowlike structures is differential loading caused by thin-skinned extension (Figs. 6, 7). Vendeville and Jackson (1992a) described how normal faulting of the overburden allows the underlying salt to rise reactively even in the absence of density inversion (e.g., during the early stages when sediments are thin and little compacted). The grabens and their underlying reactive diapirs are raised above the regional datum while the adjacent overburden blocks subside and are flexed synformally (Fig. 6). The resulting geometry differs from that of Trusheim’s salt pillows because the roof of such an extensional pillow is breached by normal faulting early on. In Trusheim’s restoration of the early stage of diapir rise, the present-day cross section of full-fledged diapirs does not allow determination of whether the roof of the salt body was intact or breached during the pillow stage. Although present-day pillowlike structures having intact roofs cannot be attributed to extension, the pillow stage of many present-day diapirs having pierced their roofs can be. Typically, reactive diapirs subsequently grow into passive diapirs, once the overburden is thick enough to become denser than salt. 946
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Figure 4. Cross section from the southern North Sea illustrating pillowlike structures interpreted as buoyant salt pillows by Hughes and Davison (1993).
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B Figure 5. Cartoon illustrating how salt withdrawal between two diapirs can leave a pillowlike relic salt high.
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Figure 6. Cross section in an experimental model illustrating the formation of a primary peripheral sink in response to graben faulting and reactive diapirism. The overburden comprised one prekinematic layer (L1) and two synkinematic layers (L2 and L3). Each layer was made of several stratified sublayers. The lowermost layer (L1) was flexed upward as the shoulders of the graben and the underlying reactive diapir rose. The primary peripheral sink (I) is indicated by thinning of the basal sublayer of Layer 1 toward the diapir. Note that no flexure formed during subsequent extension because deposition of Layers 2 and 3 thickened the overburden, making it too rigid to bend.
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Figure 7. Pillowlike structures that formed during thin-skinned extension and reactive diapirism, Eastern Mediterranean (Garfunkel and Almagor, 1987).
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Finally, this alternative origin for pillowlike structures can explain why, as Trusheim himself noted, some diapirs show no signs of having passed through a pillow stage during their early evolution. For example, such diapirs may have been triggered by a phase of thin-skinned extension that occurred after sediments were already too thick— hence too rigid—to allow for significant flexure of the overburden blocks.
Diapir Stage Conceptual Inconsistencies of the Evolutionary Model During the diapir stage (Figs. 1B, 8, 9), the base of the depocenter stops subsiding while its flanks subside, squeezing the underlying wedge of salt to feed the diapir. As observed by Trusheim, a significant number of diapirs have adjacent depocenter flanks that never subsided (Figs. 9A, 10), which suggests that some depocenters may have been too strong to invert spontaneously to form turtle structures. However, there is no clear correlation between whether a depocenter has deformed and the mechanical properties of its sediment strata. By contrast, there appears to be a correlation between the depocenter’s ability to deform and (1) the shape of the depocenter and (2) the tectonic setting. First, depocenters having gently dipping flanks commonly deform (e.g., the pseudoclinoforms associated with progradation; Ge et al., 1997), whereas depocenters that have not deformed have steeper flanks. Second, along salt-bearing, passive margins, the flanks of most depocenters located in the upper slope have subsided, whereas those located in the lower slope typically have not. Present
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Figure 8. Cross-section restoration showing the development of secondary peripheral sinks (II), Siegelsum Dome, Germany.
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Figure 9. Cross sections in two separate models in which diapirs grew passively as the adjacent depocenters subsided. A. In the model not subjected to late extension, the depocenters did not invert and no secondary peripheral sink formed. B. In the model subjected to late extension, the depocenters inverted and a secondary peripheral sink formed (II). 948
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A New Interpretation of Trusheim’s Classic Model of Salt-Diapir Growth Luneburg
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Figure 10. Example of a diapir that did not develop secondary peripheral sinks, Luneburg Dome, Germany (Kockel, 1990).
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^ Alternative Interpretation of the Diapir Stage The above observations can be explained in simple mechanical terms. On the one hand, the stresses driving subsidence of the depocenter’s flanks are related to loading by the overburden and to the density difference between salt and overburden (typically 10% or less). On the other hand, the strength of the overburden block, which is partly related to the dip of its flanks, resists deformation. Depocenters having gentle flanks can readily subside and deform, whereas thicker depocenters having steeper flanks are stronger and can rarely deform spontaneously. In addition, Vendeville and Jackson (1992b) demonstrated that thin-skinned extension occurring after grounding of the depocenters causes subsidence of diapirs and of the flanks of adjacent depocenters. Even steeply flanked depocenters can subside because extension greatly lowers stresses within the salt body, which therefore can no longer support the overlying depocenter’s flank. The cross sections of two models in Figure 9 illustrate the influence of extension onto the evolution of depocenters. The first model (Fig. 9A) was not subjected to late extension and therefore did not invert to become a turtle structure. By contrast, secondary peripheral sinks formed in the second model (Fig. 9B), which was subjected to late extension. Because the upper slope of passive margins is typically subjected to an extensional regime, subsidence of depocenters’ flanks and formation of secondary peripheral sinks are common there, whereas in the lower slope (commonly subjected to a compressional regime), the depocenters cannot deform. In conclusion, depocenters that have inverted into turtle structures to form secondary depositional sinks may have initially had gentle flanks or may have been subjected to late extension, whereas other depocenters never deformed because they had steep flanks and were not subjected to late extension.
Postdiapir Stage Conceptual Inconsistencies of the Evolutionary Model During the postdiapir stage (Figs. 1A, 11), the diapir rises, even though the adjacent depocenters and their flanks do not subside. This stage is also characterized by diapirs having steep flanks. Trusheim attributed this late deformation stage to buoyant rise of a diapir detached from the source layer after the diapir stem had pinched off. Such mechanism is, however, not mechanically viable because it would require the steep diapir flanks to spontaneously collapse and the adjacent overburden to deform. Furthermore, the depleted source layer is no longer capable of supplying additional salt to feed the rising diapirs. As for the other two stages, Trusheim noticed that, although some diapirs exhibit tertiary peripheral sinks (e.g., Fig. 11), many diapirs simply stopped rising and subsequently remained dormant (e.g., Fig. 8).
Alternative Interpretation of the Postdiapir Stage There are two alternative processes that can lead to the formation of tertiary peripheral sinks despite depletion of the source layer. First, modest amounts of late diapiric rise can be caused by differential compaction, owing to continued sediment aggradation during the postdiapir stage. As younger sediments accumulate, the entire overburden sequence, including the strata adjacent to the diapir, further compacts. Because rock salt is uncompressible and does not compact, overburden compaction causes relative vertical movement between the diapir and the adjacent country rocks. Note that compaction causes subsidence of the overburden, rather than true rise of the diapir. 949
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Figure 11. Two seismic-reflection profiles showing tertiary peripheral sinks (III) above diapirs rejuvenated by late shortening.
^ Another, more powerful process, thin-skinned shortening, can lead to large, genuine late diapiric rise after source-layer depletion. Because rock salt is uncompressible and much weaker than the adjacent overburden rocks, thin-skinned shortening is preferentially accommodated by squeezing of the diapirs, which therefore continue to rise and deform their roofs (Fig. 11). Examples include diapirs located in the lower slope of passive margins (Vendeville and Rowan, 2002) and diapirs rejuvenated by tectonic inversion (Vendeville and Nilsen, 1995; Guérin and Vendeville, 1998). Diapir rise driven by late contraction does not require additional salt supply because narrowing of the diapir compensates for its vertical rise. Eventually, late shortening can entirely pinch off the diapir stem, resulting in the classic inverted teardrop geometry common to many salt diapirs. Finally, attributing the formation of tertiary peripheral sinks to late contraction can explain why some diapirs have kept rising (sometimes episodically), whereas others have not. Diapirs located in tectonically quiet areas simply stopped rising after source layer depletion and remained dormant. Diapirs subjected to one or more pulses of late tectonic shortening were episodically rejuvenated.
Conclusions The above discussion shows that despite the kinematic similarities between Trusheim’s evolutionary model and that of Rayleigh-Taylor instabilities, the two models are mechanically and genetically different. Whereas Trusheim proposed that diapiric rise is an autonomous and spontaneous process independent of regional tectonic events, our analysis demonstrates the importance of early or late regional extension and shortening in driving salt-diapir evolution. Our study also suggests that not all present-day dormant salt pillows can be considered analogs of the early stages of diapir development. Dormant salt pillows may be buckle folds or residual salt highs that never rose buoyantly and were unlikely to ever evolve into full-grown diapirs. In contrast, the primary peripheral sinks observed around the base of present-day diapirs may have formed in response to early thin-skinned extension, normal faulting, and reactive diapiric rise. The formation of secondary peripheral sinks may also depend on regional tectonics: steeply flanked depocenters can invert and form turtle structures only if they are subjected to late thick-skinned extension. Late diapiric rise and formation of tertiary peripheral sinks appear to be intimately linked with tectonic shortening, which (1) allows the diapirs to keep rising despite the lack of additional salt supply from the depleted source layer and (2) provides the large stresses necessary to deform thick diapir roofs. Overall, a schematic restoration of full-grown diapirs based on Trusheim’s cross section (Fig. 12) shows changes in the section length that are caused by regional tectonics and that correspond to the three successive stages of diapir and depocenter evolution: early extension promotes reactive diapirism and formation of primary peripheral sinks (Fig. 12 C). Late extension forces the depocenters to invert and form secondary peripheral sinks (Fig. 12 B). Late contraction rejuvenates the diapir, forming tertiary peripheral sinks. 950
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Figure 12. Schematic cross-section restoration illustrating how successive tectonic events can contribute to the development of peripheral sinks. Early extension triggers reactive diapirism and formation of primary peripheral sinks. B. Late extension forces the flanks of the depocenters to subside, forming a secondary peripheral sinks. A. Late contraction rejuvenates the diapir, forming tertiary peripheral sinks above its crest.
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^ The theory of Rayleigh-Taylor instabilities, on which Trusheim based his genetic evolutionary model, predicts that, once initiated, the salt body rises inescapably and continuously until the source layer is entirely depleted. On the other hand, Trusheim’s own observations indicate that most diapirs in northern Germany have undergone distinct episodes of growth and quiescence. He also noted that some salt basins or subbasins comprised salt structures having different histories: some had all three evolutionary stages, although others did not. This behavior cannot be explained by the continuous rise of buoyancy-driven Rayleigh-Taylor instabilities. Our analysis emphasizes the influence of regional tectonics, a process that often occurs by pulses and is not regionally widespread. Assuming that regional extension or shortening may have influenced or controlled salt-diapir evolution may help provide an explanation for this episodic history and differences between salt structures within a salt basin. Some salt structures may have undergone extension or shortening that allowed then to continue to rise, whereas others remained dormant because they were not subjected to regional tectonics.
References Coward, M., and S. Stewart, 1995, Salt-influenced structures in the Mesozoic-Tertiary cover of the southern North Sea, U.K., in M.P.A Jackson, D.G. Roberts, and S. Snelson, eds., Salt Tectonics: a Global Perspective: American Association of Petroleum Geologists Memoir, v. 65, p. 229-250. Garfunkel, Z., and G. Almagor, 1987, Active salt dome development in the Levant Basin, southeast Mediterranean, in I. Lerche and J.J. O’Brien, eds., Dynamical Geology of Salt and Related Structures: Academic Press, New York, p. 263-300. Ge, H., M.P.A. Jackson, and Vendeville, B.C., 1997, Kinematics and dynamics of salt tectonics driven by progradation: American Association of Petroleum Geologists Bulletin, v. 81, p. 398-423. Guérin, G., and B.C. Vendeville, 1998, Structural decoupling by the Zechstein salt during multiphase tectonics in the southern Norwegian North Sea (abs.): American Association of Petroleum Geologists International Conference and Exhibition abstracts: American Association of Petroleum Geologists Bulletin, v. 82, p. 1920. Hughes, M., and I. Davison, 1993, Geometry and growth kinematics of salt pillows in the southern North Sea: Tectonophysics, v. 228, p. 239-254.
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Vendeville Kockel, F., 1990, Morphology and genesis of Northwest German salt structures, in Proceedings, Symposium on Diapirism with Special Reference to Iran, v. 2: Bandarabbas, Iran, Ministry of Mines and Metals, Tehran University, and Governery of Hormozgan, p. 225-245 Nettleton, L.L., 1934, Fluid mechanics of salt domes: American Association of Petroleum Geologists Bulletin, v. 27, p. 51-63. Podladchikov, Y., C. Talbot, and A.N.B. Poliakov, 1993, Numerical models of complex diapirs: Tectonophysics, v. 228, p. 189-198. Trusheim, F., 1960, Mechanism of salt migration in northern Germany: Association of Petroleum Geologists Bulletin, v. 44, p. 1519-1540. Vendeville, B.C., and M.P.A. Jackson, 1992a, The rise of diapirs during thin-skinned extension: Marine and Petroleum Geology, v. 9, p. 331-353. Vendeville, B.C., and M.P.A. Jackson, 1992b, The fall of diapirs during thin-skinned extension: Marine and Petroleum Geology, v. 9, p. 354-371. Vendeville, B.C., and K.T. Nilsen, 1995, Episodic growth of salt diapirs driven by horizontal shortening, in C.J. Travis, B.C. Vendeville, Holly Harrison, F.J. Peel, M.R. Hudec, and B.F. Perkins, eds., Salt, Sediment, and Hydrocarbons: Society of Economic Paleontologists and Mineralogists, Gulf Coast Section, 16th Annual Research Conference Program and Extended Abstracts, p. 285-295. Vendeville, B.C. and M.G. Rowan, 2002, 3-D kinematics of minibasins and salt ridges remobilized by late contraction: physical models and seismic examples (southeast Mississippi Canyon, Gulf of Mexico) (abs.): American Association of Petroleum Geologists Annual Meeting Official Program, v. 11, p. 182. Weijermars, R., M.P.A. Jackson, and B.C. Vendeville, 1993, Rheological and tectonic modelling of salt provinces: Tectonophysics, v. 217, p. 143-174.
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