Development and evolution of folds, faults and joints within a thrust sheet in fold-thrust belt, Guoshing, central Taiwan
Date Issued
2016
Date
2016
Author(s)
Yeh, Chia-Hung
Abstract
Based on tectonic analysis on the well-exposed riverbed outcrops in the Guoshing area, central Taiwan, we elucidate and characterize the deformation structures, including joint, fault, and fold, and its evolution during the upward propagation of a thrust sheet in fold-thrust belt. We mainly focused on a section of continuous outcrops about 1 km long in the hanging wall of the Shuangtung Fault, a major thrust fault revealing a duplex structure in the western foothills of the Taiwan mountain belt. By detailed field investigation we established a local geological 3-D architecture of deformed Miocene strata, which is characterized by two to three duplex structures accompanied by multiple folds, which plunge exclusively toward the North, suggesting stress localization near the main Shuangtung fault zone. We conducted fault slip data and fracture analysis via Faultkin and T-tecto software. By further comparing the brittle fractures with fold structure, our results show that slickenside faulting occurred on multiple phase throughout the propagation of the thrust sheet on the hanging wall of the Shuangtung fault and that the multiple wave folding seemingly developed in the late stage during the exhumation of the thrust sheet. We are finally able to summarize the evolution of the deformation structures in Guoshing area as following: 1）one early extensional stress event, which makes syn-sedimentary faulting or joint; 2）three phases of compressional thrusting events; one is early conjugate thrust system with E-W compression which may indicate the beginning of thrusting when the strata of this thrust sheet still kept horizontal at depth of about 4-6 km（Stage 1）. Then, the hanging wall strata have been tilted through upward movement of the thrust sheet along the Shuangtung Fault. During this period, a late conjugate thrust system （with E-W to NE-SW compression）developed at the shallow depth of 2-4 km（Stage 2, 3）. We found that the late stage of slickenside faulting events is likely syn-folding with the multiple folds. Our observations and analyses also show that parts of the late stage thrusting slickenside followed the pre-existing fractures developed in the earlier stages. Comparing with regional pattern and strata attitude, it seems that the strata have not experienced block rotation. In addition, for each stages of compressional stress events, we found one mainly compressive direction and several secondary compressive direction, we interpret as following: numerous faulting （slickenside slip）events occurred within the hanging-wall thrust sheet during the evolution of the Shuangtung Fault system, which might represent several seismic cycles. We tend to interpret the variations on the orientation of the maximum principal stress axis to be aftershocks transient stress state under stress perturbation following the major earthquakes. We calculated the stress ratio for each stages of stress event, the stress ratio gradually increasing from Stage 1 to Stage 3, during the Shuangtung Fault evolution, averaging from 0.13 to 0.35. The results show that 1） strong compressive maximum principal stress （σ1）in the horizontal direction, 2） minimum principal stress in vertical direction （σ3=σv）, and 3）The second principal stress（σ2）always larger than the minimum principle stress（σ3=σv）. It suggested that there is still have obvious compressive lateral stress during the mountain building process.
Subjects
Western Foothills
fold and thrust belt
Shuangtung Fault
multiple phase fault-slip analysis
crustal stress
Type
thesis
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