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

Abstract

Juxtaposed against the remnant forearc basin sequences along thrust faults, the Lichi Mélange of the Coastal Range of Taiwan is composed of exotic ophiolite and sedimentary blocks, metric to kilometric in size, and coherent turbidite beds, all embedded in a sheared scaly argillaceous matrix. The Lichi Mélange is controversial in origin, being interpreted either as a subduction complex, or as an olistostrome. By separating four main deformation levels within the Lichi Mélange and adjacent sedimentary rocks, we establish detailed geological maps and structural profiles in two key areas of the Lichi Mélange. 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 Mélange 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 Mélange and the lower Takangkou Formation. We also undertake a comprehensive tectonic analysis of the shear surfaces in the Lichi Mélange. 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 mélange, further increasing the shear deformation pattern of the Lichi Mélange. 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.