Collisional Reactivation of Rift Margin Fracture Zones, Taiwan and the Taconic Allochthon

Chapter I. Transverse Topographic Development due to the Reactivation of a Partially-Subducted Fracture Zone: The Southwest Hsüehshan Range, Central Taiwan Abstract — The southwest flank of the Hsüehshan Range is defined by a topographic break which cuts across regionally mapped structures in central Taiwan. The mountain front trends ~345°, slightly oblique to the Sanyi-Puli seismic zone which has been previously interpreted as a reactivated continental margin fracture zone. Structural data collected along the length of the topographic break reveal two populations of cross-cutting faults with distinct fault-zone materials and a series of southwest-plunging folds. Paleostress axes were reconstructed using the P-B-T axes, right dihedra, and Gauss paleostress methods. Stress inversion results yield an azimuth of maximum horizontal shortening for early-stage faults of 315°, subparallel to the current relative plate convergence vector, and 252° for late-stage faults, approximately normal to the topographic break. In addition, rock uplift rates were examined by extracting normalized steepness indices from streams with catchments ≥ 10 km2 within the Tachia, Peikang, and Mei River basins. With the incorporation of precise leveling data, zones of equal steepness coincide with increases in rock uplift rates from 5.15 ± 1.55 mm/yr in the Puli basin to 15.9 ± 1.88 mm/yr west of the Lishan fault zone, delineating a northwest-trending boundary consistent with the late-stage maximum compressive stress orientation. These results suggest that spatial variations in uplift rates across the topographic break are accommodated by the late-stage fault population. We propose that the formation of these late-stage structures is causally linked to the partial-subduction of a continental margin promontory and fracture zone. Chapter II. Spatial Variation of Slaty Cleavage and Stretching Lineation Orientation in the Taconic Allochthon, Vermont and New York Abstract — New and previously collected structural data were compiled to produce detailed maps of slaty cleavage (S2) and stretching lineation (L2) orientation in the Taconic slate belt. The area of study encompasses a ~360 km2 area between N43° 43’ and N43° 22’ along the Vermont and New York border. In total, ~1500 measurements of S2 were compiled at 120 sites and 145 oriented samples were analyzed at 90 sites to determine the orientation of L2. Kriging maps of the strike and dip of S2 and the rake and trend of L2 were constructed from these data. The kriging maps delineate three structural domains previously recognized from along-strike changes in the orientation of the axial traces of F2 folds. The boundaries between domains are transitional; for domains 1 and 2 the boundary is located at approximately N43° 34’ and for domains 2 and 3, the boundary is located at approximately N43° 26’. In domain 1, the mean orientations of S2 and L2 are 013, 31 E and 111, 29, respectively. The mean rake of L2 on S2 is 82° from the south. In domain 2, the mean orientations of S2 and L2 are 354, 42 E and 124, 36, respectively. The mean rake of L2 on S2 is 50° from the south. In domain 3, the mean orientations of S2 and L2 are 013, 32 E and 111, 35, respectively. The mean rake of L2 on S2 is 82° from the south. The kriging maps show a correlation between the strike of S2, dip of S2, and rake and trend of L2 along strike of the entire study area. The central part of domain 2 shows the most extreme values of strike, dip, rake, and trend. The overall change in S2 and L2 orientation from north to south has been previously interpreted as resulting from transport of domain 2 across an Iapetan transform fault and related along-strike variation in orientation of the shear zone in which S2 and L2 formed