Deep structure and deformation history of the rapidly growing Tainan anticline, southwestern Taiwan
Abstract
A wealth of recent geodetic studies have shown that the Tainan anticline at the front of the southwestern Taiwan fold-and-thrust belt is growing rapidly by steady creep, consuming 1.5-2 cm/y shortening with an uplift rate up to 1.6 cm/y. This growing anticline lies within the Tainan metropolitan area leading to coherent InSAR and PSInSAR results (1995 to present). Other data include continuous GPS and leveling, shallow boreholes, trenching and dating of shallow horizons. In contrast, the underlying deep structure producing this surface deformation has not been well constrained. Here we present a new analysis of depth-converted petroleum seismic reflection profiles from the northern Tainan anticline and high-resolution topographic measurements, both of which we have correlated with geodetic and stratigraphic data to constrain the fold structure and history. We show that the Tainan anticline is a pure-shear wedge fault-bend-fold above a 35° west-dipping backthrust (the Houchiali fault) rooted on a nearly flat 3.6-km deep detachment. InSAR and leveling data reveal that the west limb is rotating, consistent with the gradient in uplift rate predicted by the shear fault-bend-fold theory. We apply recently developed techniques based on area of structural relief measurements to determine the shortening for each seismic horizon and its components (flexure, pure shear, simple shear and horizontal compaction). We determined the total horizontal shortening above the Houchiali fault to be 1 km +/- 0.1 km, dominated by pure shear and horizontal compaction (~30% of total shortening), with little flexural shortening. Horizontal compaction is indicated by horizontal gradients in seismic velocity combined with modeling of vertical gradients in shortening and is important only in the upper part of the fold, the lower part being overpressured. We have shown that the uppermost ~1.5 km strata are syntectonic growth strata and provide a quantitative history of shortening. We are constraining the timing of this shortening by tying the seismic lines to nannofossil biostratigraphy in nearby deep wells and to magnetostratigraphy and biostratigraphy in nearby surface exposures. Our preliminary estimate is that the Tainan anticline began growing a few hundred thousand years ago and appears to have experienced an increasing rate of shortening. In addition, the long-term rates are substantially slower than the current geodetic rates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T21F..03M
- Keywords:
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- 0935 EXPLORATION GEOPHYSICS / Seismic methods;
- 8005 STRUCTURAL GEOLOGY / Folds and folding