Tectonics and Paleomagnetic Rotation Pattern of Yunnan (24°N–25°N, China): Gaoligong Fault Shear Versus Megablock Drift

Abstract

A wealth of paleomagnetic data from Yunnan (China) showed in the past a predominant post-Cretaceous clockwise (CW) rotation pattern, mostly explained invoking huge (hundreds of kilometer wide) blocks, laterally escaping (and/or rotating) due to India-Asia collision, separated by major strike-slip shear zones. Here we report on the paleomagnetism of the outcrops close to the Gaoligong dextral shear zone. Fifty paleomagnetic sites (503 samples) were sampled at variable distances (up to~25 km) from mylonites exposed along the fault. Eighteen Jurassic-Cretaceous red bed sites yield systematic CW rotations with respect to Eurasia that peak at maximum (176°) close to the fault, and progressively decrease moving eastward, up to be virtually annulled~20 km east of mylonite contact. West of the fault, 15 Pliocene-Holocene sites from the Tengchong volcanic field do not rotate. Thus, our data show that Gaoligong shear zone activity yielded significant CW rotations that were likely coeval to the main Oligo-Miocene episodes of dextral fault shear. The Gaoligong zone rotation pattern conforms to a quasi-continuous crust kinematic model and shows blocks of ≤1 km size close to the fault that enlarge moving eastward. Rotation values and width of the rotated-deformed zone translate to a 230-290 km Gaoligong shear zone dextral offset. Our work shows that fault shear plays a significant role for Indochina CW rotation occurrence. However, significant rotations at distances >30 km from main faults were also documented, so that additional tectonics-whose relative relevance has not been elucidated yet-must contribute as well to the rotation pattern.Citation:

Paleomagnetic Evidence for 25–15 Ma Crust Fragmentation of North Indochina (23–26°N): Consequence of Collision With Greater India NE Corner?

“…Considering an average 5 cm/year India‐Indochina convergence (Molnar & Tapponnier, ), and an oblique 40° subduction/collision from 32 to 15 Ma (thus assuming to have occurred mostly before the 20° CW Indochina rotation) translates into a ~550 km plate‐parallel dextral shear. This estimate is higher than the 230–290 km dextral displacement calculated by Pellegrino et al () for the Gaoligong fault on the basis of paleomagnetic data, implying that dextral shear parallel to the trench also occurred along other subparallel fault(s). After ~15 Ma, onset of southward crustal drift from Tibet changed the geodynamics of north Indochina, and dextral strain partitioning jumped farther west, along the Sagaing fault.…”

“…Such strike‐slip‐related crustal block rotations have been widely documented elsewhere (e.g., Beck, ; Hernandez‐Moreno et al, , ; Kimura et al, , ; Piper et al, ; Randall et al, ; Ron et al, ). Within north Indochina, Pellegrino et al () showed that crust east of the Gaoligong shear zone was broken in ≥1 km wide crustal blocks that experienced CW rotations of about 180° and that rotations ended at a ~20 km distance from the shear zone. They suggested that rotation pattern is consistent with a quasi‐continuous kinematic model (e.g., Sonder et al, ) and proposed a 230–290 km dextral offset for the Gaoligong shear zone based on rotation values and width of the block rotation zone.…”

“…This study showed that—in the vicinity of the Gaoligong shear zone—the crust is broken in 1 km size crust fragments, conforming to a quasi‐continuous crust deformation model (Sonder et al, ). However—as also Pellegrino et al () admit—“blocks” between major shear zones are up to 200 km wide (in case of the north Simao domain), implying that fault shear cannot be the sole mechanism for the widespread CW rotations documented in northern Indochina.…”

“…It is interpreted as the present-day eastern boundary of SE extruding Tibetan crust, diverted to the south due to collision with rigid crust of the Sichuan Basin and South China Block (Figure 1). Indochina relative to the ductile 55-20 Ma and fragile present-day deformation show a clear tectonic regime change, from a roughly E-W shortening direction (in present-day coordinates) during early-middle Cenozoic times to the N-S shortening direction evidenced at present by both active fault kinematics and focal mechanisms of major-mostly strike-slip-earthquakes (Ekström et al, 2012;Pellegrino et al, 2018;Socquet & Pubellier, 2005;Zhang et al, 2017;Zhang, Zhang, Chang, et al, 2012). This observation, coupled with GPS data showing a southward drift of north Indochina at a~2 cm/year rate (Gan et al, 2007;Liang et al, 2013) and with evidence for hot-and likely partially molten-lower crust below E-SE Tibet (Nelson et al, 1996;Yao et al, 2008), suggests that active north Indochina deformation is driven by lower crust flowing southeastward in-between the rigid crustal buttresses represented by the East Himalayan Syntaxis and the Sichuan basin ( Figure 1; Royden et al, 1997Royden et al, , 2008.…”

“…In Figure 2, we show rotation estimates from north Indochina and the Chuandian domain with respect to stable Eurasia, reevaluated using reference Eurasia paleopoles by Torsvik et al (2012) for sites in 170 to 140 Ma values, and the most recent East Asia-based poles summarized by Cogné et al (2013) for sites in 130 to 10 Ma values. Data (and the complete list of papers) from the Lanping, north Simao, and Chuandian domains are given in Table S1 (see supporting information), while revised data from the Baoshan and Tengchong domains were recently reported by Pellegrino et al (2018). Figure 2 shows a general predominance of variable magnitude CW rotations from Jurassic-Oligocene rocks, and Neogene sediments and deposits show barely significant rotations.…”