Structural constraints on the timing of left-lateral shear along the Red River shear zone in the Ailao Shan and Diancang Shan Ranges, Yunnan, SW China

Abstract

The >1000-km-long Oligocene-Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene-active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea, has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene-Early Miocene leftlateral shear and localized partial melting (leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strike-slip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by K-feldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (~239-243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (~26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (~35 Ma).Later biotite leucogranites intruded the orthogneisses and migmatite host rocks before a signifi cant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (~500-550 °C) for the ALS and lower temperatures (~250-150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated fi nal exhumation of the Red River gneisses. Using published U-Th-Pb and 40 Ar/ 39 Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed leucogranites (31.9-24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ~17 Ma.

Crustal–lithospheric structure and continental extrusion of Tibet

“…A simple test of this theory would be that U-Pb monazite ages for peak metamorphism and magmatism would all have to date strike-slip motion along the faults. U-Pb monazite ages obtained from metamorphic rocks along the Karakoram Fault are Cretaceous (Streule et al 2009) and preliminary zircon ages from metamorphic rocks along the Red River Fault are Triassic in age (Searle et al 2010c), strongly suggesting that metamorphism was earlier than, and unconnected to shearing along the fault. Because the crust beneath western Tibet is up to 90 km thick (Wittlinger et al 2004;Rai et al 2006), these faults would have to penetrate these depths and remain coherent, an unlikely scenario given the high geothermal gradient and great depth.…”

“…This view suggests that the metamorphism and most of the granites were formed prior to strike-slip faulting and not by shear heating, and that the faults are purely crustal features where localized strain at the surface dissipates into a wide zone of distributed ductile shear at depth. This view has been promoted both for the Karakoram Fault in particular (Searle 1996;Searle et al 1998;Murphy et al 2000;Phillips et al 2004;Robinson 2009a,b) and for the Red River Fault Searle 2006Searle , 2007Yeh et al 2008;Searle et al 2010c).…”

Where was the Ailaoshan Ocean and when did it open: A perspective based on detrital zircon U–Pb age and Hf isotope evidence

“…faults (Tapponnier et al, 1990;Leloup et al, 2007;Searle et al, 2010). Among these faults, the Ailaoshan has divided the Ailaoshan belt into two major lithotectonic successions ( Fig.…”

Petrology, geochemistry, and origin of metamorphosed mafic rocks of the Trans Vietnam Orogenic Belt, Southeast Asia

“…Mylonitized metamorphic rocks found in this shear zone exhibit structures which indicate left -lateral (sinistral) shearing, probably in relation to far -field collision of the Indian subcontinent with the Eurasian continent during the Eocene (e.g., Leloup et al, 1993). Furthermore, it has been suggested that the regional metamorphism in the Cangshan Mountains area was related to an earlier (Permo -Triassic) continental collision event involving the South China and Indochina cratons (e.g., Osanai et al, 2008;Searle et al, 2010). Metamorphic rocks of the Cangshan Mountains contain mineral assemblages that indicate greenschist facies to amphibolite facies metamorphic conditions.…”

“…Parts of the Red River Shear Zone experienced ultrahigh -temperature (UHT) conditions, as inferred from the coexistence of Grt, Crn, and Qtz in Kybearing Grt -Crn -Sil -Spl gneiss , 1998;Leloup et al, 2001;Gilley et al, 2003). Some older ages, between 220 and 265 Ma, have also been reported for rocks from this shear zone (e.g., Carter et al, 2001;Gilley et al, 2003), possibly related to older continental collision event, as suggested by Osanai et al (2008) and Searle et al (2010).The metamorphosed mafic rocks of the Red River Shear Zone analyzed in this study were collected from the Song Hong and Xuan Dai groups (Fig. 2).…”