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. INTRODUCTION 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 tectonostratigraphic 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. Brittleviscous 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). MODEL SETUP 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 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4836977/g46485.pdf by guest on 31 October 2019 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America 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 1GSA Data Repository item 2019389, results of Basin 1b and 2b, Figures DR1 and DR2, and Table DR1 (scaling relationships), is available online at http://www.geosociety.org/datarepository/2019/, or on request from editing@geosociety.org. Data underlying this study are published open access via the Helmholtz Centre Potsdam GFZ Data Services in Ge et al. (2019), http://doi .org/10.5880/GFZ.4.1.2019.006. Figure 1. Typical subsidence curves (thermal subsidence–dominated) of modern passivemargin basins and tilting curves applied in this study. Note progressive subsidence curves in all natural cases. 1—South China Sea margin (Lin et al., 2003); 2—Moroccan margin (Le Roy et al., 1998); 3—Côte d’Ivoire–Ghana margin (Clift et al., 1997); 4—Western Australia margin (Falvey and Mutter, 1981); 5—United States Atlantic margin (Swift et al., 1987); 6—Campos Basin, Brazilian margin (Beglinger et al., 2012). Dashed lines are instant and progressive tilting curves (trans

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含盐大陆边缘渐进倾斜控制薄皮变形

作为主要驱动力，边缘倾斜对于重力驱动的薄皮盐构造至关重要。我们使用可以控制倾斜速率的模拟建模设置研究了即时与渐进边缘倾斜机制如何影响盐构造。瞬时倾斜导致最初的高变形率，触发广泛分布的上坡伸展和下坡收缩。后来，随着早期的伸展结构相继失活，伸展和收缩域都向上坡迁移，而变形率呈指数下降。相比之下，渐进倾斜通过顺序形成的、长期存在的正断层导致伸展域的下坡迁移。在迁移上坡之前，收缩局限于一些长期存在的推力。我们将薄皮变形的独特结构演变，特别是在拉伸域中，在两种倾斜情况下主要归因于脆性覆盖层和下方粘性材料之间的机械耦合。耦合效应又在很大程度上受变形率的控制。通过展示与即时倾斜和渐进倾斜相关的结构风格和运动学演化的时空变化，我们认为这种变化本质上是可识别的，因此可以提供一种新的方法来分析边缘倾斜历史。引言重力驱动的薄皮盐构造活动的典型特征是上坡伸展和下坡收缩相关联（例如，Brun 和 Fort，2011 年；Rowan 等人，2004 年），显着影响含盐被动边缘（例如，南大西洋边缘；Marton 等，2000；Mohriak 等，2008）和克拉通内裂谷盆地（例如，中央地堑、北海；Karlo 等）的构造地层演化等，2014）。与热沉降和构造活动相关的沉积物加载和边缘倾斜是重力驱动的盐构造的两个主要控制因素（例如，Brun 和 Fort，2011；Rowan 等，2004）。本质上，与沉降相关的边缘倾斜是一个持续且持续数千万年的持续过程（图 1）。迄今为止，模拟和数值模型普遍采用瞬时静态倾斜作为边界条件，尽管其缺乏适当的地质意义（例如，Dooley 等，2007，2018；Fort 等，2004；Gaullier 等，2018）。 , 1993; Ings et al., 2004; Mauduit 等人，1997 年）。尽管一些研究强调了倾斜量的影响（Adam 等人，2012 年；Brun 和 Fort，2011 年；Fort 等人，2004 年；Ings 等人，2004 年；Mauduit 等人，1997 年）和一些已经研究了逐步（增量）倾斜（Adam 等人，2012 年；Brun 和 Fort，2018 年；Quirk 等人，2012 年）对构造演化的影响，没有对边缘的薄皮盐构造进行建模研究倾斜是逐步模拟的。作为一种粘性材料，盐的强度与应变率成比例，而应变率主要受边缘倾斜期间的重力控制，尽管其他一些因素也可能起作用（例如，盐厚度；例如 Dooley 等人， 2018；Fort 等人，2004 年；Weijermars 等人，1993 年；Zwaan 等人，2016 年）。简单来说，预计盐分在重力缓慢增长的渐进倾斜过程中相对较弱，而在初始强重力的瞬时倾斜过程中相对较强。因此，这种不同的倾斜情况可能会分别导致覆盖沉积物和下伏盐分之间的弱耦合和强耦合。脆性粘性耦合在岩石圈尺度变形中的应变分布中起着关键作用（例如，Schueller 等，2005），因为强和弱耦合效应已分别归因于控制宽裂缝和窄裂缝（例如，Brun，1999）。然而，对于薄皮盐构造变形，人们对这种与边缘倾斜相关的耦合效应知之甚少。这里，我们在比例模拟实验中研究了瞬时和渐进边缘倾斜对盐构造结构样式和演化的影响，并将它们与天然盐影响的边缘进行了比较。为了模拟天然盐盆地，我们使用了双楔形的通用盆地几何形状 (Brun and Fort, 2011)，使得盐厚度在斜坡下变化，并在两个楔子之间包括一个铰链区，这通常控制着从延伸到压缩的下坡变化（图 2；例如 Dooley 等人，2018 年）。模型设置 模拟模型设置类似于以前的研究，其中颗粒材料和聚二甲基硅氧烷 (PDMS) 硅油分别用于模拟脆性沉积覆盖层和粘性盐（例如，Adam 等，2012；Fort 等，2004；威杰克和卡拉威，2000 年）。我们使用石英砂和泡沫玻璃球的混合物作为覆盖材料，以实现脆性层和粘性层之间的合理密度比为 1.16。本研究中使用的有机硅 (KORASILON G30 M) 在应变率高达 ~10-2 s-1 时表现得像牛顿流体，这远远超出了我们的实验范围（Rudolf 等人，2016 年）。此处使用的颗粒混合物的脆性行为（Warsitzka 等人，2019 年）类似于天然岩石（例如，Byerlee，1978 年）。几何比例因子（模型中的 1 厘米 = 自然界中的 1 公里）和时间比例因子（模型中的 4 小时 ≈ 自然界中的 1 米）源自常见的比例程序（例如，Adam 和 Krezsek，2012；参见表 DR1 GSA 数据由访客于 2019 年 10 月 31 日从 https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4836977/g46485.pdf 下载 2 www.gsapubs。组织| 卷XX | 编号 XX | 地质 | 美国地质学会资料库 1 了解详情）。实验的基础是一块刚性板。每个实验中，一个基底砂体用作两个相同的硅树脂盆的模具（图 2）。由此产生的硅胶楔在铰链区 2 厘米处最厚，并逐渐向盆地边缘挤压（图 2）。基板的倾斜由计算机控制的电机驱动，该电机在硅树脂盆上的预运动层筛分后启动。我们提出了两个实验，分别代表准瞬时和渐进倾斜场景，其中最终斜率 3.5° 分别在 3.5 分钟（实验 1）和 1°/d（实验 2）内建立（图 3A）。在这两个实验中，我们研究了两个厚度分别为 1 毫米和 5 毫米的盐上预运动覆盖层的变形，分别。实验运行 5 天，每 12 小时筛分 1 毫米（平均）的覆盖材料以模拟同步沉降（图 2）。这些模型后来被切片以提供最终结构样式的横截面视图。在实验过程中，模型的表面由两个电荷耦合器件（CCD）相机监控，允许数字图像相关（DIC）以高精度（<0.1 mm）和分辨率（例如，Adam et等，2005）。在下一节中，我们使用下坡表面速度 (Vx)、纵向表面应变 (εxx) 和表面应变率 (dεxx/dt，1 小时平均值) 来展示 1 mm 的预运动覆盖层厚度的实验结果（图. 3 和 4）。较厚的预运动覆盖层（5 毫米）的实验给出了类似的结构演变模式，因此没有详细描述（见图 DR1 和 DR2）。实验数据发表在 Ge 等人。(2019)。即时边缘倾斜 3.5° 的即时倾斜（实验 1；图 3A）触发了早期盆地范围内的薄皮变形，包括在高应变率（3-4 毫米/小时）下的上坡伸展和下坡收缩，随着系统接近，这种变形呈指数衰减重力稳定性（图 3A）。在实验的初始阶段，伸展域的宽度覆盖了盆地的 65%，围绕铰链区（50-60 厘米宽；图 3B 和 4A）。37 小时后，延伸部退回到盆地上部 40%（图 3B），并在实验结束时进一步减少到 20%（图 3B）。正常断层的连续上坡失活导致伸展域变窄（图 4A）。72 小时后，由于硅树脂的消耗和随后的焊接，上坡边缘附近的伸展断层也变得不活跃（图 4A）。早期收缩也广泛分布，约占盆地的 35%，主要集中在铰链带的下坡侧（图 3B）。收缩逐渐集中在三个主要褶皱和逆冲断层上，并在前 24 小时内向上坡迁移（图 4A），并在 72 小时后最终占据盆地的 50% 以上（图 3B 和 4A）。横截面显示了小的伸展结构，这些结构最初是活跃的并且分布广泛，然后在后期逐渐变得不活跃并被掩埋，当继续伸展仅发生在上坡边缘附近时（图4A）。运动学前覆盖层厚度增加（至 5 毫米）的模型具有类似的结构演变，但较小的 1GSA 数据存储库项目 2019389、盆地 1b 和 2b、图 DR1 和 DR2 以及表 DR1（比例关系）可在线获取： http://www.geosociety.org/datarepository/2019/，或应editing@geosociety.org 的要求。这项研究的基础数据是通过 Ge 等人的 Helmholtz Center Potsdam GFZ 数据服务公开发布的。(2019)，http://doi .org/10.5880/GFZ.4.1.2019.006。图 1. 现代被动边缘盆地的典型沉降曲线（热沉降主导）和本研究中应用的倾斜曲线。注意所有自然情况下的渐进沉降曲线。1—南海边缘（Lin et al., 2003）；2—摩洛哥边缘（Le Roy et al., 1998）；3—科特迪瓦-加纳边界（Clift et al., 1997）；4—西澳大利亚边际（Falvey and Mutter, 1981）；5——美国大西洋边缘（斯威夫特等，1987）；6—坎波斯盆地，巴西边缘（Beglinger 等人，2012 年）。虚线是即时和渐进倾斜曲线（trans