The Transition from Plastic to Brittle Strain in the Tananao Complex of Taiwan

The Taiwan mountain belt is due to the ongoing collision of the passive margin of Eurasia and the Luzon island arc. In spite of decades of research on the tectonic evolution of Taiwan, fundamental gaps in our understanding persist. For example, the geometry and kinematics of the structures that accommodate exhumation are poorly understood, yet such constraints are essential to mechanical models the orogen. Here we focus in on the eastern Central Range, the metamorphic core of Taiwan, documenting the youngest plastic deformation fabrics recorded in the Tananao complex and their relation to older deformation fabrics. Field observations spanning approximately 100 km along strike reveal that many exposures of the Tananao complex are dominated by a penetrative foliation (S2) commonly attributed to deformation (D2) that postdates burial-related compactive fabric development. S2 is parallel to the axial planes of cm- and dm-scale isoclinal folds. Locally, S2 is crosscut by a nonpenetrative foliation that is largely restricted to the cores of cm-scale folds, referred to as S3. Previous field research has independently documented a gently ~N-plunging mineral lineation attributed to late-stage deformation (L3) in the Tananao complex. A challenge in documenting the transition from down-dip stretching to along-strike stretching, is that the host foliations are commonly at low angles to one another. To address these relations in detail, we collected an oriented sample of pebbly meta-mudstone from along the Mugua River that records penetrative S2 with a clear down-dip stretching lineation, overprinted locally by L3. Strain analyses in the XZ and YZ planes of the dominant S2 fabric reveal oblate strain. Centimeter-scale oblate quartz pebbles are locally thinned approximately normal to the L3 direction that is recorded only in phyllosilicate-rich S3 planes sub-parallel to S2. These relations document the transition between two distinct strain regimes apparently operating at different structural levels. Our observations provide important constraints on the structures that accommodated deformation during changing conditions possibly at the onset of exhumation in the Taiwan mountain belt.