Wednesday, September 10, 2008
Meso-scale Structural Control of Western Taiwan Morphology Based on Integrated Analysis of Spatially Correlated Datasets.
Taiwan has been described by various models which each approximate an overall behavior of the orogen, yet no single posited model has succeeded in providing a synthesis of the entire suite of geophysical and geological data available across a wide range of observational scales. Along strike variability of structural styles, seismological activity, gravitational field strength and flexural response of the foreland have all been addressed at island encompassing scales in previous investigations; most existing models of Taiwan’s evolution fail to account for certain anomalous observations at intermediate scales. Spatial correlation of several such observations in Taiwan occurs between different data types, and coincides with recognized shifts in structural and depositional regimes, highlighting a region surrounding Taichung and Puli in Western Taiwan. This region exhibits a discrete structural style from the rest of the foreland, historically experiences large destructive earthquakes and is also spatially correlative with a -60 mGal gravity anomaly. The Taichung basin itself is composed primarily of Pleistocene synorogenic sediments, is larger and deeper than any other basin elsewhere on the island and does not appear to be isostatically compensated, however the reasons for this have not been fully explained in flexural models to date. The existence of the Puli Topographic Embayment additionally suggests that variations occur in one or more of the parameters which affect rock uplift and/or erosion in the region, such as the strength of the underlying decollement, rheology of the deforming wedge, or changes in either tectonic or surficial boundary conditions. We suggest that the Chinese Continental Margin, which exhibits a systematically variable distribution of pre-existing normal faults, is structurally incorporated into the orogenic wedge beneath the northern Hsueshan and northern Alishan ranges, but is resistant to incorporation beneath Taichung and Puli. This pattern of structural inversion and incorporation strongly contributes to the meso-scale along-strike variations in overall wedge geometry of Western Taiwan. These observations support our new hypothesis that the Puli topographic embayment is the result of the indentation of a basement low, buttressed to the north and south by basement highs, rather than a result of indentation by the Peikang basement high alone. The spatial correlation of anomalies further inboard of the active mountain-front nicely demonstrates the long-term affects of this anisotropic template being accreted and subsequently incorporated into the orogen. This new work allows for a more integrated understanding of the interactive processes which are responsible for both large-scale and meso-scale morphologies of the Taiwanese orogen.