'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 du e to the oblique collision between the Luzon Are and the Eurasian passive c ontinental margin from Plio-Pleistocene to the present. Stable isotope comp ositional profiles of thin marble layers from this mountain range show that marble layers from rock sections with an inverted metamorphic zonation wer e all overturned after the peak metamorphism and that marble layers from ro ck sections with a normal metamorphic zonation were not overturned. Exhumat ion folding seems to be the responsible process. An upward extrusion exhuma tion model is therefore postulated as the probable mechanism for the mounta in building process. This model can well explain the fanning orientation of rock cleavage's dipping in a regional scale as well as the metamorphic cha racteristics across the mountain range. The model also shows that the recor ded stretching lineation is 'secondary' in nature, meaning the exact direct ion of rock movement during the initial exhumation stage in response to the oblique collision cannot be deduced. (C) 2000 Elsevier Science B.V. All ri ghts reserved.