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Subsurface structure and fault segmentation along the Median Tectonic Line active fault system, northwestern Shikoku, Japan



Subsurface structure and fault segmentation along the Median Tectonic Line active fault system, northwestern Shikoku, Japan



Zisin = Jishin 56(2): 141-155



The Median Tectonic Line active fault system is an about 360-km-long right-lateral strike-slip fault system composed of several active segments. Several researchers have presented segmentation models on the basis of fault geometry and faulting history. However, very few studies so far been made to understand the genesis of segment boundary with a consideration of the 3-D fault geometry system of a long strike-slip fault system. We conducted precise gravity survey and seismic reflection survey to assess 3-D fault geometry and segmentation of the Median Tectonic Line active fault system. A prominent low gravity anomaly in the southwestern Matsuyama Plain is recognized. This low gravity anomaly region reflects a half-graben structure of the basement. Thick sediments filling the graben inferred from the seismic reflection survey across the low gravity anomaly region. The Iyo, Shigenobu, Kitakata, and Kawakami faults surround the low gravity anomaly region. The Iyo and Kawakami faults in these faults array depict an extensional right overstep. The Shigenobu and Kitakata faults within the overstep area may have faulted mainly with vertical displacement in the southern part, as reactivation of existing fault that occurred during Late Cretaceous to Paleogene. This fact indicates that these faults are secondary fault accompanied by the formation of pull-apart basin. We, presently, lack of data that connect activity of these active faults with formation process of the low gravity anomaly region, even though above mentioned facts demonstrate that these active faults have resulted faulting and constructed extensional right overstep structure controlled by the basement structure which have formed about Pliocene. On the basis of the above results, the Median Tectonic Line active fault system of the northwestern Shikoku is divided into two segments, the Iyo segment and the Kawakami segment, recognizing pull-apart basin as the segment boundary, since this pull-apart basin region in a long strike-slip fault causes heterogeneity of faulting.

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Accession: 020158804

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