Limited underthrusting of India below Tibet: 3He/4He analysis of thermal springs locates the mantle suture in continental collision

Abstract

During continent–continent collision, does the downgoing continental plate underplate far inboard of the collisional boundary or does it subduct steeply into the mantle, and how is this geometry manifested in the mantle flow field? We test conflicting models for these questions for Earth’s archetypal continental collision forming the Himalaya and Tibetan Plateau. Air-corrected helium isotope data (3He/4He) from 225 geothermal springs (196 from our group, 29 from the literature) delineate a boundary separating a Himalayan domain of only crustal helium from a Tibetan domain with significant mantle helium. This 1,000-km-long boundary is located close to the Yarlung-Zangbo Suture (YZS) in southern Tibet from 80 to 92°E and is interpreted to overlie the “mantle suture” where cold underplated Indian lithosphere is juxtaposed at >80 km depth against a sub-Tibetan incipiently molten asthenospheric mantle wedge. In southeastern Tibet, the mantle suture lies 100 km south of the YZS, implying delamination of the mantle lithosphere from the Indian crust. This helium-isotopic boundary helps resolve multiple, mutually conflicting seismological interpretations. Our synthesis of the combined data locates the northern limit of Indian underplating beneath Tibet, where the Indian plate bends to steeper dips or breaks off beneath a (likely thin) asthenospheric wedge below Tibetan crust, thereby defining limited underthrusting for the Tibetan continental collision.

Keywords: Indian lithosphere; Tibetan plateau; continental collision; mantle helium; structural seismology.

Fig. 1.

Alternative geometries of continental collision, and location map. (A and B) Cartoons defining “underthrusting,” “subduction,” and “mantle suture” [boundary of Indian lithosphere with (A) Tibetan lithosphere or (B) asthenosphere]. Blue: cratonic lithosphere (cold); red: asthenosphere (hot). (C) Map (same area as Fig. 3) locating cross-section in Fig. 4 (yellow swath) and previous seismic interpretations: red: mantle suture from body-wave tomography (3); green: crustal front from receiver-functions (7); blue: northern limit of 240-km-thick Indian lithosphere produced by pure-shear shortening (2) intended to reproduce lithospheric thickness mapped by Rayleigh-wave tomography (8). Thin gray lines: sutures/major faults, south to north. MFT, Main Frontal Thrust; STD, South Tibet Detachment; YZS, Yarlung-Zangbo Suture; BNS, Banggong-Nujiang Suture; JRS Jinsha River Suture and major strike-slip faults ATF (Altyn Tagh), KF (Kunlun), and KKF (Karakoram). Thick gray lines: active rifts: YR, Yare; TG, Thakkola graben; LG, Lunggar graben; TY, Tangra-Yumco; PX, Pumco-Xainza; YG, Yadong-Gulu; CS, Cona-Sangri. Colored circles and triangles correspond to geologic terranes and match symbols in Fig. 2 and SI Appendix, Figs. S2 and S5.

Fig. 2.

Isotopic data by Tibetan terrane. R/R_A plotted against (He/Ne)_sample/(He/Ne)_{ASW, Field T}. Circles: our “high-quality” data separated by a vertical line at (He/Ne)_sample/(He/Ne)_{ASW, Field T} = 10 from hexagons: our “uncertain-quality” data; triangles: all “high-quality” data from other authors. Locations: blue: Tarim and Qaidam Basins; orange: Qiangtang terrane (includes one Songpan-Ganzi spring); white: Lhasa terrane; gray: Tethyan Himalaya; black: Greater, Lesser, and Sub-Himalaya; crosses indicate water samples. Data are superimposed on mixing curves of three end-member components [mantle = 8R_A and 10⁵(He/Ne)_ASW, crust = 0.02R_A, 10⁵(He/Ne)_ASW, and ASW = 0.985R_A (black square)]. Our “null,” “intermediate,” and “mantle” designations are separated at 0.5% and 1% mantle contributions (corresponding to R/R_A = 0.06 and 0.1 at high He/Ne values); red, yellow, blue, and green arrows and lettering correspond to colors in Fig. 3. SI Appendix, Fig. S2 shows these data after air correction (plotting R_C/R_A instead of R/R_A). All plotted data are listed in Dataset S1 but only “high-quality” points are plotted in Fig. 3.

Fig. 3.

Distribution of “mantle” and “null” samples. (A) ³He/⁴He isotope data (R_C/R_A); circles and triangles as in Fig. 2, now color-coded red “mantle” samples >1R_A; yellow “mantle” samples >0.1R_A; green “intermediate” samples >0.06R_A; blue “null” samples ≤0.06R_A. Mantle suture: preferred: thin solid white line shows 10-km-wide helium boundary that 1) separates 99% of “mantle” samples from 96% of “null” samples, dashed where data are sparse, 2) separates 65-to-100-km deep earthquakes (black stars) from Quaternary volcanoes (narrow red triangles), and 3) is further constrained to trend 105° orthogonal to plate-convergence vector (white arrows) and be offset only at active rifts (heavy black lines). Alternate interpretation of helium transition shown as 75-km-wide transparent white line. Small white squares: seismic observations of Moho offsets and abrupt upper-mantle velocity transitions (lines linking squares). Yellow lines: sutures/major faults; black lines: normal faults; blue/green/red geophysical boundaries as in Fig. 1C. (B) R_C/R_A as a function of distance from the preferred 10-km-wide helium boundary (using northern dashed line in A and color-coded by terrane affinity as in Figs. 1C and 2). All samples in the blue “India” null quadrant, yellow “Asia” mantle quadrant, or green intermediate band fit our preferred model. (C) Measured abundance of [³He] vs. [⁴He] color-coded as in main map, showing clear bimodality (two parallel but clearly separated trend-lines). (D) Seismic body-wave tomography, dVp/Vp at 100 km depth (3), same area as A, overlain by preferred helium boundary (white line). (E) As in D but showing adjoint tomography, dVs/Vs at 80 km depth (6). For additional data sources see SI Appendix, Fig. S7.

Fig. 4.

True-scale cross-section of Tibet at ∼88° to 91°E along yellow swath in Fig. 1C. All solid gray lines have been geophysically imaged (23). Mantle suture is interpreted vertically below the boundary between “null” and “mantle” helium domains and its delineation is the main contribution of this paper. Black arrows show relative motion of Indian crust (green) and lithosphere (blue) with respect to fixed Asia (gray). Dip of subducting slab is not constrained by helium studies and likely varies along strike (37, 38), shown here with 5° to 30° dip based on seismic attenuation (1, 39) and tomographic studies (6, 35). Location of the crustal front (northern limit of Indian crust immediately above the Tibetan Moho) varies along strike with respect to the mantle suture, sometimes north of the mantle suture (5) (as shown here), elsewhere coincident with (7), or possibly south of (32, 35), the mantle suture. MHT, Main Himalayan Thrust.