U-Pb zircon geochronology of major lithologic units in the eastern Himalaya: Implications for the origin and assembly of Himalayan rocks A. Alexander G. Webb1,†, An Yin2, and Chandra S. Dubey3 1Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA 2Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095, USA 3Department of Geology, Delhi University, Delhi-110007, India ABSTRACT The ca. 280–220 Ma detrital zircons of the Indian basement and cover sequences (Argand, Late Triassic strata are derived from an arc 1924; Heim and Gansser, 1939; Dewey and Models for the origin and deformation of developed along the northern margin of the Bird, 1970; Le Fort, 1975). Himalayan rocks are dependent upon geo- Lhasa terrane. Detritus from this arc was de- metric and age relationships between major posited on the northern margin of India dur- Assembling the Himalaya units. We present fi eld mapping and U-Pb ing India-Lhasa rifting. Along-strike hetero- dating of igneous and detrital zircons that es- geneity in Main Central thrust footwall Structural evidence provided the fi rst intima- tablish the lithostratigraphic architecture of chronostratigraphy is indicated by detrital tion that the kinematic evolution might have the eastern Himalaya, revealing that: (1) the zircon age spectrum differences from central involved complexities beyond thrust tectonics. South Tibet detachment along the Bhutan- western to far western Arunachal. Nonethe- Recognition of top-to-the-north shear structures China border is a top-to-the-north ductile less, the Late Proterozoic rocks in the Main along the gently north-dipping GHC-THS con- shear zone; (2) Late Triassic and Early Cre- Central thrust hanging wall and footwall in tact at the range crest (e.g., Caby et al., 1983; taceous sedimentary samples from the north- far western Arunachal can be correlated to Burg et al., 1984) quickly led to acceptance of a ern Indian margin show a similar age range each other, and to previously analyzed rocks new paradigm—that this contact was defi ned by of detrital zircons from ca. 3500 Ma to ca. in the South Tibet detachment hanging wall a top-to-the-north low-angle normal fault sys- 200 Ma, but the Late Triassic rocks are distin- to the west and in the Indian craton to the tem with tens or even hundreds of kilometers of guished by a signifi cant age cluster between south. These fi ndings are synthesized in a slip (e.g., Searle, 1986; Burchfi el et al., 1992). ca. 280 and ca. 220 Ma and a well-defi ned reconstruction showing Late Triassic India- This fault system is termed the South Tibet de- age peak at ca. 570 Ma, (3) an augen gneiss Lhasa rifting and Cenozoic eastern Hima- tachment. Current models for the assembly of in the South Tibet detachment shear zone layan construction via in situ thrusting of the Himalayan units focus on the emplacement in southeast Tibet has a Cambrian–Ordo- basement and cover sequences along the of the GHC along the South Tibet detachment vician crystallization age, (4) Main Central north Indian margin. and the Main Central thrust, which emplaced thrust hanging-wall paragneiss and footwall the GHC southward over the LHS. quartzites from the far western Arunachal INTRODUCTION The fi rst kinematic model proposed for this Himalaya share similar provenance and Late emplacement is wedge extrusion, in which the Proterozoic maximum depositional ages, and Simple questions of Himalayan geology— GHC extruded southward between the other (5) Main Central thrust footwall metagray- i.e., where does Himalayan material originate, two units as a northward-tapering wedge (Fig. wacke from the central western Arunachal and how was it assembled?—may have surpris- 2A; Burchfi el and Royden, 1985). Some recent Himalaya has a Paleoproterozoic maximum ingly complex answers. The geometric frame- workers associated these kinematics with criti- depositional age, indicated by a single promi- work of the Himalayan orogen was established cal taper–Coulomb wedge theory (e.g., Robin- nent age peak of ca. 1780 Ma. Recent work by Heim and Gansser (1939), who divided it son et al., 2006; Kohn, 2008; Zhang et al., 2011), in the eastern Himalaya demonstrates that into a generally north-dipping stack of three which suggests that normal faulting may occur in the early-middle Miocene, the Himalayan units separated from foreland detritus to the during collapse of overthickened thrust wedges crystalline core here was emplaced south- south and Asian plate rocks to the north. These (e.g., Davis et al., 1983; Dahlen, 1990). In the ward between two subhorizontal shear zones units are now termed the Lesser Himalayan Se- second kinematic model, channel fl ow–focused that merge to the south. A proposed subse- quence (LHS, at the base), the Greater Himala- denudation, the GHC represents partially mol- quent (middle Miocene) brittle low-angle yan Crystalline complex (GHC, in the middle), ten lower to middle crust that tunneled south- normal fault accomplishing exhumation of and the Tethyan Himalayan Sequence (THS, at ward during the Eocene–Oligocene (Fig. 2B), a these rocks along the range crest can be pre- the top) (Fig. 1; e.g., Yin, 2006). Early models process driven by the lateral pressure gradient cluded because new and existing mapping for the origin and assembly of these units were created by the gravitational potential difference demonstrates only a ductile shear zone here. straightforward, suggesting that as the southern between the Tibetan Plateau and its margins front of the India-Asia collision, the Himalayan (e.g., Beaumont et al., 2001, 2004; Godin et al., †E-mail: [email protected] orogen was generated via in situ thrusting of 2006). Subsequently, this channel was exhumed GSA Bulletin; March/April 2013; v. 125; no. 3/4; p. 499–522; doi: 10.1130/B30626.1; 12 fi gures; 2 tables; Data Repository item 2012341. For permission to copy, contact [email protected] 499 © 2013 Geological Society of America Webb et al. 7070°EoE 8080°EoE 9090°EoE N 3030°NoN China Figure 4A India Bhutan 30°N Myanmar Pakistan India Bangladesh (Burma) Tethyan Himalayan Sequence: Quaternary Main Central thrust (MCT) < Precambrian - Cambrian Late Cretaceous - Tertiary < Ordovician - Mesozoic South Tibet detachment (STD) (includes Pulchauki Group) Asian and Indo-Burman plate rocks Greater Himalayan Crystalline Great Counter thrust complex Indian Craton Main Central thrust - South Tibet detachment Ophiolite / Ophiolitic Lesser Himalayan Sequence intersection line (white dash = buried, black melange dash = eroded) Figure 1. Simplifi ed geological map of the Himalaya, based on Webb et al. (2011b, and references therein). by enhanced erosion (in response to a climatic nating top-to-the-north and top-to-the-south nel tunneling” because of the proposed early change) across a narrow zone where precipita- shear along the South Tibet detachment (e.g., Miocene timing, and because the leading edge tion was focused along the topographic front of Hodges et al., 1996; Mukherjee and Koyi, 2010) of the modeled GHC was not exhumed by ex- the orogen (e.g., Beaumont et al., 2001; Hodges could be accommodated by slip transfer along a trusion. Rather, a localized, hinterland portion et al., 2001; Clift et al., 2008). second thrust wedge geometry across the GHC of the GHC and the early Miocene “lower” In both wedge extrusion and channel fl ow– hinter land (Webb et al., 2007). South Tibet detachment was extruded between focused denudation models, the Main Central Some workers have noted similarity be- the low-angle normal fault, termed the “upper” thrust and South Tibet detachment were active, tween tectonic wedging and channel tunneling South Tibet detachment, and an out-of-sequence surface-breaching faults during early-middle processes , as both feature southward motion thrust (the Kakhtang thrust). Restricted low- Miocene GHC emplacement. However, the of the GHC between two subhorizontal shear angle normal fault motion may be consistent South Tibet detachment map pattern in the west- zones that merge to the south (Kellett and Godin , with critical taper theory, as discussed above. ern and Bhutan Himalaya (Yin, 2006) and fi eld, 2009; Larson et al., 2010a). Modes of inter- and Alternatively, such faulting could result from structural, and U-Pb geochronologic studies in intra-unit shearing may indeed correspond. sub hori zontal shear tractions applied to the base the western and central Himalaya (Webb et al., Nonetheless, the kinematic models of channel of the Himalayan brittle upper crust by fl ow 2007, 2011a, 2011b) indicate that the South fl ow–focused denudation and tectonic wedging of middle crust (driven southward in response Tibet detachment intersects with the Main Cen- can be distinguished by two criteria: timing and to the lateral pressure gradient that varies with tral thrust at the leading edge of the GHC. This extrusion. In the fi rst model, proposed chan- topog raphy; Yin, 1989). frontal tip of the GHC is locally preserved; nel tunneling occurs in the Eocene–Oligocene, south of this tip, the THS is thrust directly atop preceding the Miocene surface emplacement Origins of Himalayan Material the LHS along the Main Central thrust. These of the GHC via channel fl ow coupled to extru- observations led to a third kinematic model— sion (e.g., Beaumont et al., 2001; Hodges et al., The fi rst model to include non-Indian mate- that the GHC was emplaced in the early-middle 2001; Godin et al., 2006). In contrast, proposed rial in the Himalayan orogen was the early chan- Miocene via tectonic wedging (Fig. 2C; Yin, tectonic wedging occurred in the Miocene nel fl ow concept of Nelson et al. (1996). These 2006; Webb et al., 2007). Model kinematics and accomplished emplacement of the GHC workers speculated that the GHC might repre- are analogous to thrust tectonics of the frontal at depth, without extrusion (Yin, 2006; Webb sent Tibetan middle crust that was thickened, Canadian Cordillera (Price, 1986): The South et al., 2011b).