Mud Diapir or Fault‐Related Fold? On the Development of an Active Mud‐Cored Anticline Offshore Southwestern Taiwan. Issue 9 (29th September 2022)

• Record Type: Journal Article
• Authors: Wang, Yu; Lin, Yunung Nina; Ota, Yoko; Chung, Ling‐Ho; Shyu, J. Bruce H.; Chiang, Hong‐Wei; Chen, Yue‐Gau; Hsu, Ho‐Han; Shen, Chuan‐Chou
• Is Part Of: Tectonics. Volume 41:Issue 9(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-09-29
• Subjects: mobile shale -- mud diapir -- marine terrace -- kinematic fault‐related fold model -- escape tectonics; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2022TC007234
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: There has been a controversial debate about the roles of diapiric folding versus fault‐related folding in mud‐cored anticlines. This study tackles this debate by looking into the structural developments in southwestern Taiwan. Among all the structures, the Liuchiu anticline is the only active structure exposed above sea level and therefore offers a unique opportunity to integrate land and marine data sets. On Liuchiu Island, we obtained the fold core geometry at the surface and identified three terrace groups with age constraints. In the offshore area, we obtained the fold limb geometry from seismic profiles. Given the tight fold shape and the asymmetric terrace tilting, we found that a hybrid fault tip fold‐fault propagation fold model can best explain the geologic and geomorphic features. In this regard, mud diapirism should be a secondary process, possibly along discrete patches of mobile shales to enhance fold amplification. Our model yields decreasing shortening rates from 0.7 mm/yr during the Plio‐Pleistocene to 0.375 mm/yr since 120 ka. The uplift rate however increases since the mid‐Holocene, which we tentatively attribute to larger mobile shale deformation triggered by earthquakes along the fault after the near‐fault fluids are drained. At the regional scale, we further suggest that the remaining plate convergence is absorbed by a few major structures west of Liuchiu Island. Plain Language Summary: Traditionally, many mud‐involved geological structures are believed to be associated with the vertical movement of mud diapirs, which form a column‐like structure uplifting the surface above. This idea, however, has been challenged by new studies, and the mechanisms to create these mud‐involved structures are under debate. Geological structures in southwestern Taiwan are considered to be governed by many mud‐cored anticlines due to the existence of a thick mudstone formation. Among these structures, the Liuchiu anticline is the only place that allows the integration of land and marine research data to examine the aforementioned debate. This study suggests that instead of diapiric deformation, fault‐related folding could be the main deformation mechanism, although deformation within shales of high fluid pressures (mobile shales) may still play important roles. Once the fluids are drained, the fault at depth may become capable of generating earthquakes, while the fluid‐rich mobile shales at shallow depth may continue to deform aseismically. This interpretation is supported by a local earthquake in 1865 and by a sequence of uplifted sea notches found along the coast. Finally, we suggest that tectonic shortening and lateral block motion occur on a few major faults west of Liuchiu Island. Key Points: Instead of columnar diapiric deformation, the geometry of the Liuchiu anticline can be mainly explained by fault‐related folding Model suggests a mean shortening rate of 0.70 mm/yr across the fold in the Plio‐Pleistocene and 0.375 mm/yr since 120 ka Mud‐cored anticlines may be capable of generating earthquakes after the shale fluids become drained along the fault