As a primary driving force, margin tilting is crucial for gravity-driven thin-skinned salt tectonics. We investigated how instant versus progressive margin tilting mechanisms influence salt tectonics using an analogue modeling setup where tilting rate could be controlled. Instant tilting resulted in initially high deformation rates, triggering widely distributed upslope extension and downslope contraction. Later, both the extensional and contractional domains migrated upslope as early extensional structures were successively deactivated, while deformation rates decreased exponentially. In contrast, progressive tilting led to downslope migration of the extensional domain by sequentially formed, long-lived normal faults. Contraction localized on a few, long-lived thrusts before migrating upslope. We attribute the distinct structural evolution of thin-skinned deformation, especially in the extensional domain, in the two tilting scenarios mainly to mechanical coupling between the brittle overburden and underlying viscous material. The coupling effect in turn is largely controlled by the deformation rate. By demonstrating the spatiotemporal variations of structural style and kinematic evolution associated with instant versus progressive tilting, we suggest that such variation is identifiable in nature and therefore can provide a new way to analyze margin tilting histories.
Gravity-driven thin-skinned salt tectonic activity is typically characterized by linked upslope extension and downslope contraction (e.g., Brun and Fort, 2011; Rowan et al., 2004), which significantly affect the tectono-stratigraphic evolution of salt-bearing passive margins (e.g., the South Atlantic margins; Marton et al., 2000; Mohriak et al., 2008) and intracratonic rift basins (e.g., the Central Graben, North Sea; Karlo et al., 2014). Sediment loading and margin tilting associated with thermal subsidence and tectonic activity are two major controls of gravity-driven salt tectonics (e.g., Brun and Fort, 2011; Rowan et al., 2004). In nature, margin tilting associated with subsidence is a continuous and protracted process lasting for tens of million years (Fig. 1). Up to now, analogue and numerical models commonly applied an instantaneous, static tilting as the boundary condition, despite its lack of appropriate geological meaning (e.g., Dooley et al., 2007, 2018; Fort et al., 2004; Gaullier et al., 1993; Ings et al., 2004; Mauduit et al., 1997). Although some studies have highlighted the impact of the amount of tilting (Adam et al., 2012; Brun and Fort, 2011; Fort et al., 2004; Ings et al., 2004; Mauduit et al., 1997) and a few have investigated the effect of stepwise (incremental) tilting (Adam et al., 2012; Brun and Fort, 2018; Quirk et al., 2012) upon the structural evolution, there are no modeling studies of thin-skinned salt tectonics where the margin tilting is simulated progressively. As a viscous material, the strength of salt scales proportionally to strain rate, which in turn is mainly controlled by gravitational force during margin tilting, although some other factors may also come into play (e.g., salt thickness; e.g., Dooley et al., 2018; Fort et al., 2004; Weijermars et al., 1993; Zwaan et al., 2016). In simplified terms, salt is expected to be relatively weak during progressive tilting with slowly growing gravitational force, and relatively strong during instant tilting with initially strong gravitational force. Such different tilting scenarios thus may lead to weak versus strong coupling, respectively, between cover sediments and underlying salt. Brittle-viscous coupling plays a key role in distributing strain in lithospheric-scale deformation (e.g., Schueller et al., 2005), as strong and weak coupling effects have been attributed to control wide and narrow rifts, respectively (e.g., Brun, 1999). However, such a coupling effect associated with margin tilting is poorly understood for thin-skinned salt tectonic deformation. Here, we studied the effect of instant versus progressive margin tilting on salt-tectonic structural style and evolution in scaled analogue experiments and compared them to natural salt-influenced margins. In order to mimic natural salt basins, we used a generic basin geometry with a double-wedge shape (Brun and Fort, 2011), such that the salt thickness varied downslope, and included a hinge zone between the two wedges, which typically controls the downslope change from extension to compression (Fig. 2; e.g., Dooley et al., 2018).
The analogue model setup was similar to previous studies where granular materials and polydimethylsiloxane (PDMS) silicone oil were used to simulate brittle sedimentary cover and viscous salt, respectively (e.g., Adam et al., 2012; Fort et al., 2004; Withjack and Callaway, 2000). We used a mixture of quartz sand and foam glass spheres as the cover material to achieve a reasonable density ratio of 1.16 between brittle and viscous layers. The silicone used in this study (KORASILON G30 M) behaves like a Newtonian fluid up to a strain rate of ∼10−2 s−1, which is well beyond our experimental range (Rudolf et al., 2016). The brittle behavior of the granular mixture used here (Warsitzka et al., 2019) is similar to natural rocks (e.g., Byerlee, 1978). The geometric scaling factor (1 cm in model = 1 km in nature) and time scaling factor (4 h in the model ≈ 1 m.y. in nature) were derived from common scaling procedures (e.g., Adam and Krezsek, 2012; see Table DR1 in the GSA Data Repository1 for details). The base of the experiment was a rigid plate. A basal sand body served as a mold for two identical silicone basins per experiment (Fig. 2). The resultant silicone wedge was thickest at 2 cm in the hinge zone and pinched out gradually toward the basin margins (Fig. 2). The tilting of the basal plate was driven by a computer-controlled motor that started after sieving of a pre-kinematic layer over the silicone basins.
We present two experiments, representing quasi-instant and progressive tilting scenarios, in which a final slope of 3.5° was established in 3.5 min (experiment 1) and at 1°/d (experiment 2), respectively (Fig. 3A). For both experiments, we investigated deformation of two suprasalt, pre-kinematic cover layers with a thickness of 1 mm and 5 mm, respectively. The experiments were run for 5 d with 1 mm (on average) of cover material sieved every 12 h to simulate synkinematic sedimentation (Fig. 2). The models were later sliced to provide cross-sectional views of the final structural styles. During the experiment, the surface of the model was monitored by two charge-coupled device (CCD) cameras, allowing digital image correlation (DIC) for three-dimensional surface analysis at high precision (<0.1 mm) and resolution (e.g., Adam et al., 2005). In the following section, we use downslope surface velocities (Vx), longitudinal surface strain (εxx), and surface strain rates (dεxx/dt, 1 h averages) to present experiment results for a pre-kinematic cover thickness of 1 mm (Figs. 3 and 4). The experiments for a thicker pre-kinematic cover (5 mm) gave a similar pattern of structural evolution and so are not described in detail (see Figs. DR1 and DR2). Experimental data were published in Ge et al. (2019).
INSTANT MARGIN TILTING
Instant tilting of 3.5° (experiment 1; Fig. 3A) triggered early basinwide thin-skinned deformation consisting of upslope extension and downslope contraction at high strain rates (3–4 mm/h), which decayed exponentially as the system approached gravitational stability (Fig. 3A). In the initial stage of the experiment, the width of the extensional domain covered ∼65% of the basin, bounding the hinge zone (50–60 cm wide; Figs. 3B and 4A). After 37 h, extension retreated to the upper 40% of the basin (Fig. 3B) and further reduced to ∼20% toward the end of the experiment (Fig. 3B). Narrowing of the extensional domain occurred by successive upslope deactivation of normal faults (Fig. 4A). After 72 h, the extensional faults near the upslope edge also became inactive due to the depletion of silicone and subsequent welding (Fig. 4A). Early stage contraction was also widely distributed, occupying ∼35% of the basin, mainly to the downslope side of the hinge zone (Fig. 3B). The contraction gradually localized onto three main folds and thrusts and migrated upslope in the first 24 h (Fig. 4A), and eventually occupied over 50% of the basin after 72 h (Figs. 3B and 4A). The cross section shows the small extensional structures, which were initially active and widely distributed and then became sequentially inactive and buried at a later stage, when continued extension only occurred near the upslope edge (Fig. 4A). Models with increased pre-kinematic cover thickness (to 5 mm) had a similar structural evolution but smaller extensional and contractional domains, due to the stronger cover (Figs. DR1B and DR2A).
PROGRESSIVE MARGIN TILTING
Progressive tilting (experiment 2; Figs. 3A and 3C) caused basinwide deformation rates to increase slowly, up to 1 mm/h, before decaying exponentially as tilting stopped (Fig. 3A), resulting in different structural evolution compared to instant tilting. Early extension occurred in a narrow zone in the upslope affecting ∼10% of the basin area (Figs. 3C and 4B). Instead of retreating upslope, as in the instant margin tilting case, the extensional domain gradually expanded downslope to cover ∼30% of the basin area by the end of the experiment (Figs. 3C and 4B). Also in contrast to the instant tilting, the normal faults in the extensional domain mostly stayed active until the end of the experiment with progressive tilting, lacking the deactivation and welding processes observed in the experiment with instant tilting (Fig. 4B). Furthermore, new normal faults (e.g., F1 in Fig. 4B) initiated and grew on the downslope side of the extensional domain, almost 12 h after the formation of the first normal fault (Fig. 3B). In the downslope area, contraction was initially distributed over a wide area, covering over 60% of the basin and encompassing the hinge zone (Fig. 3C). However, after 12 h, when the strain rates increased, the contraction localized on two folds and thrusts, to the downslope side of the hinge zone (Figs. 3A, 3C, and 4B). Minor upslope migration of contraction occurred after 48 h (Fig. 4B). With a thicker pre-kinematic cover (5 mm), the onset of basinwide deformation was significantly delayed (72 h later), and the deformation zones were narrower due to stronger cover, but the overall structural evolution remained similar (see Fig. DR2B).
Our experiments suggest that the structural and kinematic evolution of gravity-driven thin-skinned salt tectonics is to a first order controlled by the rate and timing of margin tilting, which dominate other factors like basin geometry, cover, and salt thickness. Instant tilting causes initially high deformation rates and basinwide deformation on multiple, short-lived structures (Fig. 4A). In contrast, progressive tilting triggers lower rates of deformation that localize onto fewer, but longer-lived structures (Fig. 4B). This difference is similar to the brittle-viscous coupling effect that has been recognized in two-layer models of continental rifting experiments (Brun, 1999). For example, Brun (1999) showed that brittle-viscous models subjected to fast extension generate more widely distributed normal faults (“wide rift models”) than those subjected to slow extension, where deformation localizes in a single extensional graben (“narrow rift models”). Such variation has been interpreted as due to the strength contrast between the brittle and viscous layers, which is controlled by the strain rate. Specifically, high strain rates lead to a strong viscous layer and strong mechanical coupling between the brittle and viscous layers, while low strain rates result in a weak viscous base flowing laterally and acting as a detachment decoupling the two layers (Brun, 1999).
During the thin-skinned deformation with instant tilting, our salt analogue deformed faster and therefore appeared stronger than the case of progressive tilting. Consequently, strong brittle-viscous coupling caused an initially wide area of deformation that became narrower as strain rates decreased (Figs. 3A and 4A). In contrast, during progressive tilting, the salt analogue acted as a weak detachment, allowing the deformation to focus onto a few dominant, long-lived structures as the strain rates gradually increased (Figs. 3A and 4B).
An important observation of progressive tilting is the sequence of faulting. As the extensional domain expands, additional extensional structures form sequentially downslope. For example, in experiment 2, significant extension on fault F1 occurred almost 12 h (3 m.y. in nature prototype) after the formation of the first normal fault (Fig. 4B). Such a sequential deformation style contrasts with the simultaneous occurrence and subsequent deactivation of normal faults observed in instant tilting, which is further complicated by the welding processes that occurred near the upslope edge (Figs. 4A and 4B). Tilting variations have only a limited influence on the contractional domain due to the hinge zone effect (e.g., Dooley et al., 2018). For example, during the initial stage, the hinge zone more effectively delineates the areas of extension and contraction during instant tilting compared to the progressive tilting scenario (Figs. 4A and 4B). Later, in both experiments, the strain is localized onto a few large structures to the downslope side of the hinge zone, before migrating upslope (e.g., Figs. 4A and 4B). The pre-kinematic layer thickness also has a limited impact upon the structural evolution, as a thicker and stronger cover only alters the timing and location of individual structures without changing the deformation style (Fig. DR2).
The variation in extensional faulting sequence can be observed in nature on a semiregional scale of tens of kilometers. As an example, based on a 20-km-long seismic section from the Lower Congo Basin (Valle et al., 2001), we identified an extensional fault in the downslope on which the earliest growth strata are younger than those of the faults in the upslope area (Fig. 4C). Similar examples can also be found in the Campos Basin (Quirk et al., 2012, their figure 4) and the Gulf of Mexico (Curry et al., 2018, their figure 3). We interpret such diachronous growth to indicate the sequential development of extensional structures under progressive margin tilting.
Using an original analogue modeling approach, we provide the first assessment of the influences of instant versus progressive margin tilting on the structural and kinematic evolution of gravity-driven thin-skinned deformation. Our experimental results suggest that instant margin tilting causes early widespread extension with high strain rates, which then retreat upslope by successive deactivation of normal faults as strain rates decrease. In contrast, progressive margin tilting causes sequential extensional faulting toward the downslope area as strain rates gradually increase. Such variation of extensional styles occurs because the brittle and viscous coupling effect, controlled by strain rates, is initially strong but continuously weakens during instant tilting, whereas it gradually strengthens during progressive tilting. The evolution of the contractional domain is less diagnostic for the mode of tilting and may be more affected by other factors (e.g., basin geometry). The spatiotemporal variation of deformation, especially the sequence of normal faulting, seen in our experiments has implications for subsurface data interpretation and thus may provide a new way to analyze margin tilting. Importantly, this may also help us to identify whether the structures are controlled by gradual thermal subsidence and long-term tectonic activity (progressive tilting) or short-term, more rapid tectonic events (quasi-instant tilting). Finally, we suggest that inclusion of margin tilting scenarios, among many other factors (e.g., salt thickness, basin geometry, etc.), is important for studies of thin-skinned salt tectonics.
We thank E.ON Stipendienfonds (Germany) and the 2018 Trans-national Access (TNA) program of the European Plate Observing System (EPOS) Thematic Core Service Multi-scale Laboratories for funding. Ge thanks Equinor (Norway) for supporting his postdoctoral fellowship at the University of Bergen. Frank Neumann and Thomas Ziegenhagen are thanked for device construction and technical assistance. Thilo Wrona and Leo Zijerveld are thanked for commenting on an early version of the paper. We thank Tim Dooley, Jürgen Adam, and an anonymous reviewer for their reviews, which improved the quality and clarity of the manuscript. We also thank the editor, Dennis Brown, for editing suggestions.