ALONG-STRIKE VARIATIONS IN CRUSTAL ARCHITECTURE IN THE TAIWAN AND TACONIC COLLISIONAL OROGENS

In collisional settings, spatial variations in orogenic processes may be caused by the partial subduction of rift-related continental margin fracture zones, which are associated with fundamental lateral changes in crustal architecture. Partially subducted continental margin fracture zones have been inferred for both the Taiwan and Taconic orogens on the basis of a variety of geophysical and geological data, and broad similarities between the two collisional zones enable comparison.

The location, scale, and sense of offset of the fracture zone in Taiwan are well defined by magnetic anomaly data. The north-northwest-striking, left-offset fracture zone is approximately 70 km long and lies underneath the topographic boundary between the Hsuehshan Range to the northeast and Puli Basin to the southwest. Transpressional deformation is predicted by the low degree of obliquity between the fracture zone and northwest-trending relative plate convergence vector. The Eo-Oligocene Hsuehshan Range belt is an inverted rift basin, and, although the depocenter lies to the northeast of the fracture zone, the stratigraphic succession extends tens of kilometers to the south of the fracture zone. The interpreted geometry is supported by the distribution of earthquakes, which form a zone that corresponds with the edge of the partially subducted continental margin.

A west-northwest-striking, left-offset fracture zone of comparable scale and obliquity is inferred to lie in the Green Mountain massif directly southeast of Clarendon, Vermont. Like Taiwan, the fracture zone is not located in the vicinity of an abrupt change in stratigraphic thickness of synrift deposits. The Pinnacle Formation, which forms an inverted Neoproterozoic rift basin, decreases in thickness from several thousand meters in northern and central Vermont to several hundred meters near the Lincoln massif. Correlative units are relatively thin in the Green Mountain massif. These observations suggest that data in addition to synrift stratigraphy are needed to locate and understand the effects of continental margin fracture zones on orogenic processes in ancient collisions.