Miocene structural reorganization of the South Tibetan detachment, eastern Himalaya: Implications for continental collision

Abstract

In the eastern Himalaya (Bhutan), there are two distinct top-down-to-the-north segments of the South Tibetan detachment system. The outer segment is a diffuse ductile shear zone preserved as klippen in broad open synforms. New age constraints show that it was active until at least ca. 15.5 Ma and cooled by ca. 11.0 Ma, as constrained by sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology of magmatic zircon and 40 Ar/ 39 Ar thermochronology of muscovite in ductilely deformed leucogranite sills. The inner segment is a ductile shear zone active at least until ca. 11.0 Ma (constrained by SHRIMP U-Pb geochronology of magmatic zircon) and overprinted by more recent brittle faulting. These age constraints indicate that ductile deformation continued on the South Tibetan detachment more recently in the eastern Himalaya than in central and western parts of the orogen. These improved constraints on timing of South Tibetan detachment segments allow for a more detailed reconstruction of continental collision in the eastern Himalaya in which the outer South Tibetan detachment segment was abandoned in the mid-Miocene and passively transported southward in the hanging wall of the Main Himalayan thrust (the basal detachment of the orogen), while top-to-the-north ductile to brittle shearing continued on the inner South Tibetan detachment segment. Hinterland stepping of the South Tibetan detachment to maintain an orogenic critical taper (frictional wedge model) is a possible mechanism for this tectonic reorganization of the South Tibetan detachment during the Miocene. However, our data combined with published geochronologic data for the eastern Himalaya demonstrate that foreland translation and exhumation of a midcrustal dome (viscous wedge model) is the more tenable mechanism.

Geologic Map of Bhutan

“…Other compiled geologic maps include: 1) Chakungal et al (2010), which covers part of northwest Bhutan near the town of Laya; 2) Wu et al (1998), which covers the northwest corner of Bhutan and surrounding areas in southern Tibet, and focuses on the northeast-striking Yadong cross-structure; 3) Edwards et al (1999), which covers part of north-central Bhutan and surrounding areas in southern Tibet; 4) the map of western Arunachal Pradesh published in Yin et al (2010), which was used to locate the Lum La window; 5) Warren et al (2011), which was used to locate the Laya thrust in northwest Bhutan; and 6) unpublished data from a newly-updated version of Djordje Grujic's simplified geologic map of Bhutan (see previous versions in Grujic et al (2002); Hollister and Grujic (2006); Kellett et al (2009)), which was used to locate the South Tibetan detachment and Kakhtang thrust in northeast Bhutan and westernmost Arunachal Pradesh.…”

“…Unit divisions, structure and stratigraphic contact locations, and rock unit descriptions on our map benefit greatly from recent studies that describe the structural geometry, metamorphic grade, and deformation history of GH rocks in Bhutan (Chakungal et al, 2010;Daniel et al, 2003;Davidson et al, 1997;Gansser, 1983;Grujic et al, 1996;Hollister and Grujic, 2006;Kellett et al, 2009;Long and McQuarrie, 2010;Long et al, 2011a;Swapp and Hollister, 1991;Warren et al, 2011). Recent studies that present new detrital geochronology data Richards et al, 2006) provide valuable constraints on the age range of deposition of sedimentary protoliths of GH rocks and the age of intrusive rocks within the GH section.…”

“…Hughes et al, 2011;Kellett et al, 2009;Long and McQuarrie, 2010). Areas that could benefit the most from detailed mapping include the Dang Chu klippe and Black Mountain region, as well as the area north of the Ura klippe.…”

“…In the Ura area, further work distinguishing between metasedimentary rocks of the GH section and TH strata would be beneficial. For example, note that significant differences exist in the map pattern of the Ura klippe between maps in Long and McQuarrie (2010) and Kellett et al (2009;. Kellett et al (2009; interpreted a lower structural position for the South Tibetan detachment than Long and McQuarrie (2010), and mapped a large area of rocks as TH that Long and McQuarrie (2010) interpreted as GH metasedimentary rocks.…”

Cambrian rocks and faunas of the Wachi La, Black Mountains, Bhutan

“…1b) (Gansser, 1983;Grujic et al 2006). Several Bhutanese outliers, including the Tang Chu, are reportedly underlain by thrust faults and are thus klippen (Grujic et al 2006;Kellett, Grujic & Erdman, 2009, but also see Long & McQuarrie, 2010). Details of the relationship between the rocks in the Bhutanese outliers and other parts of the Himalaya have not been investigated to date.…”