Understanding block rotation along strike-slip fault zones in Yunnan (China): paleomagnetic and structural approach.

Abstract

Data from this study report on the paleomagnetism of sedimentary and volcanic rocks cropping out near the Gaoligong and Ailao-Shan Red River Shear Zones. Fifty paleomagnetic sites were analyzed collecting 503 samples, during the first year of Ph.D., at variable distances (up to ca. 25 km) from mylonites exposed along the Gaoligong fault. Jurassic-Cretaceous red bed sites yield systematic CW rotations with respect to Eurasia reaching the peak values of 176 degree close to the fault, and progressively decrease moving eastward, up to be virtually annulled ca. 20 km E of mylonite contact. West of the Gaoligong fault, Pliocene-Holocene sites from the Tengchong volcanic field do not rotate. Thus, data show that the Gaoligong Shear Zone activity yielded significant CW rotations that were likely coeval to the main Eocene-Miocene episodes of dextral fault shear. The Gaoligong zone rotation pattern conforms to a quasi-continuous crust kinematic model, and shows blocks of less then or equal 1 km size close to the fault, which become bigger moving eastward. Rotation and width values of the rotated-deformed zone translate to a 230-290 km Gaoligong Shear Zone dextral offset, which shows that fault shear plays a significant role in Indochina CW block rotation.During the second year of Ph.D., forty-four Triassic-Cretaceous sites (425 samples) were collected at both sides of the Ailao-Shan Red River Shear Zone (ARRSZ), within the Chuandian, Lanping and Northern Simao blocks. Nearly all sites yielded measurable and stable magnetization components, but magnetization acquisition timing was different in the three blocks. Sites from the Chuandian block show a normal polarity and were remagnetized after folding. In the northern Simao block the magnetization was acquired before folding (about 33 Ma ago), but the ubiquitous normal polarity in Jurassic-Cretaceous sites suggests a pre-folding magnetic overprint. The data show variable and different rotation that do not display evidence of a rigid block rotation, but suggest that the northern Simao block is made of small (few km size) sub-blocks rotating CW, separated by non-rotating domains of similar size. Finally, a high-temperature (640-680 degree C) magnetization component suggests a similar rotational behaviour (CW-rotating and non-rotating sub-blocks) in the centre of the Lanping block. Conversely, a 300-640 degree C component was later acquired at 28% unfolding and subsequently underwent no rotation. The sites close (less than 25 km) to the ARRSZ yield great rotations of nearly 180 degree, which confirm past occurrence of significant strike-slip shear along the ARRSZ itself. Conversely, sites located at 10-15 km distance from the Chongshan Shear Zone show ca. 90 degree CCW rotations that imply a left-lateral shear along the fault zone, consistently with recent geological evidence. Summarizing, data from my Ph.D. study, together with previous evidence of rotations documented both near the fault zones and within the blocks themselves, show that crustal deformation of the Yunnan is extremely complex and still puzzling. The Baoshan and Lanping-Simao blocks underwent strong internal deformation and were likely fragmented in smaller independent sub-blocks whose kinematics and tectonics are still a matter of speculation.