Origin and evolution of a melange: The active plate boundary and suture zone of the Longitudinal Valley, Taiwan

Juxtaposed against the remnant forearc basin sequences along thrust faults, the Lichi Melange of the Coastal Range of Taiwan is composed of exotic ophiolite and sedimentary blocks, metric in size, and coherent turbidite beds, all embedded in a sheared scaly argillaceous matrix. The Lichi Melange is controversial in origin, being interpreted either as a subduction complex, or as an olistostrome. Byseparating four main deformation levels within the Lichi Melange and adjacent sedimentary rocks, we establish detailed geological maps and structural profiles in two key areas of the Lichi Melange. We reconstruct also the evolution in cross-section and calculate the approximate minimum amount of shortening that corresponds to folding and thrusting in these areas. Our field studies suggest that the Lichi Melange most likely arose from the shearing of lower forearc sequences rather than from a subduction complex or an olistostrome. This conclusion is supported by the structural analysis, the clay mineral distribution, and some interfingering sedimentary relationships between the Lichi Melange and the lower Takangkou Formation. We also undertake a comprehensive tectonic analysis of the shear surfaces in the Lichi Melange. The direction of the maximum compressional stress that we obtain is N100°~ 120°E, compatible with that of plate convergence. During the most recent stage of collision, between the Eurasian plate (eastern Central Range of Taiwan) and the Philippine Sea plate (Coastal Range), a major fault zone developed along the innately weak zone of melange, further increasing the shear deformation pattern of the Lichi Melange. This Longitudinal Valley Fault separates the Eurasian plate and the Philippine Sea plate and is one of the most active faults in Taiwan. It can be considered as the present plate boundary in the Taiwan arc-continent collision terrane. According to our reconstruction, this plate boundary of the Longitudinal Valley originated as a submarine arc-prism boundary. (C) 2000 Elsevier Science B.V. All rights reserved.