‘Overturned’ marble layers: evidence for upward extrusion of the Backbone Range of Taiwan

The Backbone Range is part of the Taiwan Mountain Belt, which was formed due to the oblique collision between the Luzon Arc and the Eurasian passive continental margin from Plio–Pleistocene to the present. Stable isotope compositional profiles of thin marble layers from this mountain range show that marble layers from rock sections with an inverted metamorphic zonation were all overturned after the peak metamorphism and that marble layers from rock sections with a normal metamorphic zonation were not overturned. Exhumation folding seems to be the responsible process. An upward extrusion exhumation model is therefore postulated as the probable mechanism for the mountain building process. This model can well explain the fanning orientation of rock cleavage’s dipping in a regional scale as well as the metamorphic characteristics across the mountain range. The model also shows that the recorded stretching lineation is ‘secondary’ in nature, meaning the exact direction of rock movement during the initial exhumation stage in response to the oblique collision cannot be deduced.