Abstract Discrepancies between geodetically and geologically estimated thrust fault slip rates are generally viewed as a methodological problem. Even when slip rate is steady over geological time, a discrepancy may exist because each method is sensitive to different deformation processes. However, this offers a tool to estimate the partitioning of convergence between footwall and hanging wall deformation, and therefore a way to discriminate among orogenic styles. Here we investigate one such discrepancy for the Shillong Plateau, a basement-cored contractional orogen within the Himalayan foreland basin. Using a regional block model to explain the modern geodetic velocity field and explicit uncertainty analysis of the geologic rates, we show that this discrepancy cannot be reconciled simply by invoking uncertainties in individual methods. Our results indicate that the Shillong Plateau is not an ongoing forward break of the Bhutan Himalayas, as was believed until recently. Instead, the observed inter-plate convergence and plateau uplift in this region may be driven primarily by an attempt of the negatively buoyant Indian passive margin lithosphere (the Surma Basin), south of the plateau, to initiate subduction. As a result, the uplift history of the plateau, which constrains the geologic rate, is significantly lower than expected given the geodetic convergence rate. We propose that this convergence is largely accommodated by the transport of the footwall into the mantle. This geodynamic scenario has important regional seismotectonic implications: (1) the cold and brittle sinking passive margin may have enabled the deep extent (∼30 km) and therefore large magnitude of the MW 8+ Shillong Earthquake of 1897; (2) the collapse of the Indian lithosphere into the mantle may have created the anomalously deep (∼20 km) Surma Basin; and (3) this subsidence may also drive accelerated post-Miocene westward propagation of the Indo-Burman Wedge. We propose that the Shillong Plateau is the only modern example of passive margin collapse, and can serve as a natural laboratory to study the earliest phase of subduction.

中文翻译：

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俯冲开始与西龙高原的崛起

摘要 大地测量和地质估计的逆冲断层滑动率之间的差异通常被视为方法问题。即使滑移率在地质时期内保持稳定，也可能存在差异，因为每种方法对不同的变形过程都很敏感。然而，这提供了一种工具来估计下盘和上盘变形之间的收敛划分，因此是一种区分造山风格的方法。在这里，我们研究了西隆高原的一个这样的差异，这是喜马拉雅前陆盆地内以基底为核心的收缩造山带。使用区域块模型来解释现代大地速度场和地质速率的显式不确定性分析，我们表明不能简单地通过调用个别方法中的不确定性来调和这种差异。我们的结果表明，西隆高原并不是不丹喜马拉雅山脉的持续前倾，直到最近才被认为是这样。相反，在该地区观察到的板块间辐合和高原隆升可能主要是由于高原以南的负浮力印度被动边缘岩石圈（苏尔马盆地）开始俯冲的企图。因此，限制地质速率的高原隆升历史明显低于给定大地收敛速率的预期。我们认为这种汇聚在很大程度上是由下盘向地幔的运输所适应的。这种地球动力学情景具有重要的区域地震构造意义：(1) 冷而脆的下沉被动边缘可能导致了 1897 年 MW 8+ 西隆地震的深度（约 30 公里），因此震级较大；(2) 印度岩石圈向地幔的坍塌可能产生了异常深（~20 公里）的苏尔马盆地；(3) 这种沉降也可能推动中新世后印缅楔加速向西传播。我们认为西隆高原是被动边缘塌陷的唯一现代例子，可以作为研究俯冲最早阶段的天然实验室。