Structural Evolution of Salt-Influenced Fold-and-Thrust belts: A Synthesis

3 Oliver B. Duffy, Tim P. Dooley, Michael R. Hudec, Martin P.A. Jackson, Naiara Fernandez, 4 Christopher A-L. Jackson, Juan I. Soto 5 6 Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University 7 Station, Box X, Austin, Texas, 78713-8924, USA 8 Basins Research Group (BRG), Department of Earth Science & Engineering, Imperial College, Prince Consort 9 Road, London, United Kingdom, SW7 2BP 10 Departamento de Geodinamica and Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), 11 Universidad de Granada, Campus Fuentenueva, 18071-Granada, Spain 12 13 * Corresponding Author: oliver.duffy@beg.utexas.edu 14 15


Abstract
Lateral shortening is expressed in unique ways in salt basins, especially if pre-shortening diapirs 24 are present. We present an overview and new 3-D conceptual models capturing the evolution of 25 shortening structures formed in salt provinces dominated by precursor isolated diapirs (termed 26 isolated-diapir provinces). In such provinces, isolated diapirs form only a minor volumetric 27 component of a sedimentary basin, however, due to the relative weakness of rock salt and their 28 ability to localize strain, during shortening they have a disproportionately large influence on 29 structural development. 30 We find three key mechanical principles govern the processes and structural styles 31 developed during shortening of isolated-diapir provinces. First, salt diapirs shorten before 32 surrounding sedimentary rocks due to their relative weakness, and so form salients in the thrust 33 front during early shortening. Second, diapirs tend to nucleate folds and faults, which radiate out 34 from the diapirs. Third, as diapir walls converge, the roof must shorten. Extrusive salt sheets are 35 expelled through thin roofs, but thicker roofs resist piercement and so tend to undergo complex 36 folding and faulting. 37 As a result of these principles, the first-order controls on the structural styles expressed 38 across a shortened isolated-diapir province are the pre-shortening configuration of diapirs, the 39 connectivity of the diapirs prior to shortening, total strain magnitude, and diapir roof thickness. 40 Second-order controls include the initial cross-sectional and map-view geometry of diapirs, 41 diapir size, and diapir orientation with respect to the shortening direction.

47
The geometry and kinematics of fold-and-thrust belts are generally well understood as a result of 48 their spectacular exposure in mountain ranges around the world (e.g., Bally et al., 1966; 49 Dahlstrom, 1969; Boyer and Elliott, 1982). A relatively poorly-understood aspect of these 50 systems involves fold-and-thrust belts that detach on, and are influenced by mobile salt. In these 51 settings 3-D shortening styles can be particularly complex and diverse due to: i) salt being much 52 weaker than local sedimentary rocks, creating a strength anisotropy during shortening; and ii) the 53 ability of salt to flow and thus be heterogeneously distributed prior to the onset of shortening 54 (e.g. Davis and Engelder, 1985;Letouzey et al., 1995). Thus, the configuration of salt prior to 55 shortening exerts a major control on salt-detached structural styles, with three styles standing 56 out. 57 In the first and simplest case, bedded salt is undeformed, forming a continuous gently-58 dipping layer prior to shortening. In this situation, where shortening occurs with no precursor salt 59 structures present, the salt simply acts as a décollement, 'lubricating' predominantly linear fold-60 and-thrust belts above that show extremely low taper angles (e.g. Davis and Engelder, 1985;  Jackson et al., 2008;Dooley et al 2009;. Third, when the walls of the 127 diapir converge, the roof also shortens Dooley et al., 2009;. Variations in the map-view, profile, size, and orientation with respect to the shortening direction 136 of diapirs means that each salt diapir will respond differently to shortening (e.g. Vendeville and 137 Callot et al., 2007;Jackson and Hudec, 2017 shortened. For this we assume that the diapir is initially elliptical in map-view here (i.e. a stock), 148 but the generic features described are typical of most common diapir geometries.

149
The sequence of structures that develop during the shortening of an isolated diapir is strongly 150 conditioned by salt's weakness in combination with strain magnitude. At low strains, far-field 151 hinterland shortening thickens the sediment pile driving pressurised source-layer salt towards the 152 foreland and into the diapir (Dooley et al., 2009;. The mechanical weakness of the salt 153 diapir relative to the stronger surrounding sedimentary rocks means that the diapir flanks begin 154 to converge. As this begins the diapir, in this case a circular stock, narrows and rises, and the 155 roof shortens (Fig. 3a)  . This squeezing of the diapir and shortening of the 156 roof occurs well ahead of the advancing thrust-front such that the diapir forms a thrust-front 157 salient ( Fig. 3a) (Dooley et al., 2009(Dooley et al., , 2015. At these low strains, even though the diapir and its    where the diapir roof is thick and strong, and thus does not break-up, there is no conduit for 246 pressurized salt to extrude (Fig. 7). Consequently, for the diapir flanks to converge, salt in the 247 diapir is pumped downward as an outward plume that is expelled back into the source layer,

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Having described shortening styles associated with single isolated diapirs, we now broaden the 253 scope of our study to describe shortening styles in arrays of isolated diapirs so as to explore the 254 consequences for the evolution of the fold-and-thrust belts. Studies examining shortening styles 255 in diapir arrays and the implications for fold-and-thrust belts are relatively rare, the work of and  Settings where a cluster of precursor diapirs are located adjacent to a broad region of flat-280 lying salt that has no precursor diapirs are also of note. As these settings shorten, each diapir 281 forms a local primary indenter and salient in the deformation front as described previously (e.g.  Overall, these concepts provide a general framework as to how diapirs interact. However, 288 we now consider how the precursor configuration of diapirs controls the geometry, kinematics

How does the precursor diapir configuration influence the structural style of fold-and-
292 thrust belts? 293 Prior to shortening, an array of isolated diapirs may be configured in a variety of ways, for 294 example they may be: i) aligned perpendicular to the shortening direction; or ii) tangentially-or 295 obliquely offset with respect to one another both parallel to, and perpendicular to, the shortening 296 direction (Fig. 9). These different configurations control the overall structural styles that develop 297 during shortening (Fig. 10)  Arrays of stocks and walls that are not aligned with one another perpendicular to the 312 shortening direction may have varying degrees of offset with one another both parallel-and 313 perpendicular-to the shortening direction (Fig. 9). In these scenarios, the precise arrangement of 314 the diapirs with respect to one another, along with the roof thickness, will control the type and 315 orientation of structures that develop to connect the diapirs. Where thin-roofed diapirs are 316 tangentially offset from one another, faults nucleate at, and propagate away along-strike from, 317 each of the diapirs (Fig. 10a). The striking feature in this particular configuration is that the 318 diapirs link to one another via a single tear fault, or series of small tear faults within a narrow 319 strike-slip fault zone, oriented parallel to the shortening direction. In contrast, if the diapir roof is 320 thick, the diapirs may link by the formation of a pop-up structure that is oriented oblique to the 321 shortening direction. Where thin-roofed diapirs are obliquely-offset from one another, the diapirs 322 link to one another via a transpressional pop-up structure oriented oblique to the shortening 323 direction (Fig. 10b). On the other hand, if the roof is thick, faults that propagate away from the 324 diapirs may curve at their tips to link the obliquely-offset diapirs.

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A further consideration is the degree of connectivity of the diapirs prior to shortening. In 326 some cases, such as in the sub-canopy system of the northern Gulf of Mexico, the isolated diapirs 327 are connected at depth by a polygonal network of buried salt anticlines or ridges (Fig. 11). These 328 anticlines or ridges radiate and plunge away from each of the diapirs to form an egg-carton-like 329 precursor geometry (Fig. 11) (Rowan and Vendeville, 2006). During shortening, both the diapirs 330 and the deep anticlines or ridges localize the shortening strain. As such, faults and folds nucleate 331 at the diapirs, and propagate away following the axes of the deep anticlines or ridges. The result 332 is a polygonal network of faults and folds in the supra-salt that reflects the map-view pattern of 333 the underlying anticlines or ridges (e.g. Rowan and Vendeville, 2006). It is likely that the more 334 deeply-buried the anticlines or ridges are relative to the isolated diapirs, the lesser the effect they   340 There are two fundamental reasons as to why structural styles vary across a diapir array. First, 341 shortening strains must propagate across the array in the dip direction, so some diapirs within the 342 array may be more strained than others. In systems driven by plate tectonics (e.g. Fars Province 343 of the Zagros Mountains in Iran, Betic foreland, Pyrenean foreland), shortening strains propagate 344 from the hinterland toward the foreland, thus diapirs located closer to the hinterland will be 345 shortened earlier and to a higher degree than those located towards the foreland. In contrast, in  Perpendicular walls are the easiest to weld shut because they maximize the volume of expelled 374 salt for an increment of shortening (Fig. 12d). However, the strength of sedimentary rocks 375 around the ends of the wall means that the centre of the wall may be squeezed more than the 376 ends. The tips may thus resist welding leaving two remnant diapirs connected by a vertical weld 377 (e.g. La Popa weld in Rowan and Vendeville, 2006) (Fig. 12c). Parallel walls extrude much less 378 than perpendicular walls given the same amount of shortening and require therefore more 379 shortening to close (Fig. 12d). Structural styles become more complex with oblique walls and 380 can vary according to the degree obliquity to the shortening direction (Fig. 13). Oblique walls 381 experience transpressional stresses and hence form uplifts bounded by oblique-slip reverse faults.

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Based on the preceding observations from natural and physically-modelled systems, we present a 389 series of conceptual models that synthesise how roof thickness, diapir configuration, and 390 shortening magnitude influence structural styles developed during shortening of isolated-diapir 391 arrays (Figs. 14 and 15). The pre-shortening configuration of the models is shown in Figure 2. 392 The models assume that shortening is thin-skinned and propagates from the hinterland on the 393 right, to the foreland on the left. Comparing structural styles in the foreland and hinterland we 394 thus capture temporal evolution at increasing strain. We also assume no syn-shortening 395 sedimentation, erosion, or variation in shortening rates. We vary roof thickness and strain 396 magnitude.

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At high strains, all diapirs in the array, even those located toward the foreland, experience 409 strong deformation (Fig. 14b). Diapirs tend to be welded shut, with some secondary welds 410 reactivated as thrusts, or, in the case of very high strains, offset by short-cut thrusts. The   The major differences between thick-roofed and thin-roofed diapir arrays at low strains are that         secondary weld and the salt has extruded through the thin roof leaving an overturned flap; and c) at high 550 strains the vertical secondary weld has been offset by a new shortcut thrust (note that in some cases, the 551 secondary weld can be reactivated as a thrust weld, particularly if the initial weld was dipping). 552 stretched roof; b) as shortening increases, the pressurised salt continues to rise, breaking through the weak 555 radial and peripheral grabens in the roof; c) salt lobes extrude through the weak points in the roof before 556 coalescing. R1 to R5 refer to individual portions of the roof or rafts. 557 propagated laterally such that they now link the diapirs perpendicular to the shortening direction. 580   during shortening (Callot et al., 2007); c) conceptual model of a salt wall that is oriented perpendicular to 599 the shortening direction. The strength of rocks around the wall tips mean it is harder to squeeze the tips of 600 the wall than the ends. When the wall preferentially welds in the centre, leaving two remnant diapirs at 601 the ends ('Q-tips') that are connected by a secondary weld (Rowan and Vendeville, 2006); d) differences 602 in how a perpendicular wall (wall with long axis oriented perpendicular to the shortening direction) and 603 parallel wall (wall with long axis oriented parallel to the shortening direction) respond to incremental 604 shortening (stage 1 is the lowest strain and stage 3 is the highest strain). A-A' and B-B' are schematic 605 cross-sections through the perpendicular wall before and after shortening, respectively. C-C' and D-D' 606 show schematic cross-sections through the parallel wall before and after shortening, respectively. Walls in 607 d) are all assumed to have the same area and volume prior to the shortening. 608   High pop-up