LESSONS FROM THE MODERN ARC-CONTINENT COLLISION OF TAIWAN FOR CLEAVAGE AND MAP PATTERNS OF THE TACONIC OROGENY

Recent observations of cleavage patterns, strain histories, and kinematics across the Taiwan mountain belt depict two cleavage fronts that are spatially related to an orogen scale cleavage fan centered 10-15 km east of the synorogenic divide; this observation suggests that the pattern of cleavage development is a predictable consequence of orogenic stresses and kinematics and can be used to evaluate other orogenic systems. In Taiwan, continental crust within the collision is accreted in the prowedge facing Asia, but is advected eastward into the east-vergent retrowedge, where the most deeply exhumed rocks are exposed. Wedge mechanics predict a reversal in the direction of plunge of the principal compressive stress at the topographic divide between the opposing wedges. The easternmost cleavage front in this system marks the appearance of a slaty cleavage that consistently dips west as one traverses across the prowedge toward the divide; this cleavage is stable with respect to the stress field throughout the prowedge. The second cleavage front occurs with the appearance of a second crenulation cleavage within the retrowedge that dips westward toward the divide and forms the east side of the cleavage fan; thus, there is evidence for an abrupt change in stress orientation and unstable buckling of preexisting prowedge fabrics that relates to advection beneath the divide. Moreover, a sequence of cleavages can form during a single orogenic event. Now consider the fabrics of the Taconic orogeny in eastern NY and western MA: there is a series of imbricates with a cleavage front in the eastern Hudson Valley that marks the eastward appearance of slates that dip to the east. A second cleavage front at the MA border relates to buckling and backfolding of the pre-existing cleavage and thrusts. Based on this comparison with a modern analog, the subsurface projection of the synorogenic divide for the late Ordovician Taconic Orogeny can be roughly located on our current land surface.