CONTRACTION AND EXTENSION IN CONVERGENT OROGENS NORTH HIMALAYAN GNEISS DOMES: STRUCTURAL, PETROLOGIC, AND GEOCHRONOLOGIC ANALYSES OF MABJA DOME

Transcription

1 CONTRACTION AND EXTENSION IN CONVERGENT OROGENS NORTH HIMALAYAN GNEISS DOMES: STRUCTURAL, PETROLOGIC, AND GEOCHRONOLOGIC ANALYSES OF MABJA DOME PROJECT DESCRIPTION 1. Introduction The Himalayan orogeny records the continent-continent collision and continued convergence between India and Asia since Eocene time. This extraordinary geologic event has resulted in the ~70-80 km thick crust and high elevation of the Himalaya and Tibetan Plateau. The timing and mechanisms of formation of overthickened crust and high elevations are hotly debated topics that have resulted in a number of theories, including underthrusting of India, lower crustal flow, distributed or discrete intracontinental shortening, lithospheric delamination, continental extrusion, and combinations of these (Argand, 1924; Zhao and Morgan, 1985; Dewey and Burke, 1973; England and Houseman, 1988; Tapponnier et al., 1982; Harrison et al., 1992; and many others). These models make predictions about the evolution of the Himalaya and Tibetan plateau that can be tested by examining the geometry, kinematics, and timing of deformational, intrusive, metamorphic, and exhumation events. Yet, our ability to test these models is limited because our knowledge of the geologic history of the Tibetan Plateau is still in its infancy. One of the more prominent features within southern Tibet is the belt of North Himalayan gneiss domes, metamorphic and plutonic culminations that lie south of the Indus-Tsangpo Suture Zone (ITSZ), north of the Southern Tibetan Detachment System (STDS), and along the axis of the North Himalayan antiform within the Tethyan Himalaya (Fig. 1). Gneiss domes are found in orogenic belts worldwide and are typically composed of a core of granitic migmatites or gneisses structurally overlain by a mantle of high-grade metasedimentary rocks (e.g. Eskola, 1949). The origin of gneiss domes is a world-class problem and is commonly attributed to three processes: diapirism (e.g. Ramberg, 1980), crustal shortening (e.g. Brun, 1983; Ramsay, 1967), or crustal extension (e.g. Brun and Van Den Driessche, 1994; Miller et al., 1992). Each of these processes has been proposed as the mechanism by which the North Himalayan gneiss domes formed and each has distinctly different implications for the tectonic evolution of southern Tibet and the High Himalaya. Le Fort and coworkers (Le Fort, 1986; Le Fort et al., 1987) suggested a diapiric origin for the North Himalayan domes, whereby thrusting along the Main Central Thrust (MCT) of hot portions of the Tibetan Slab over weakly metamorphosed sediments resulted in large-scale release of fluids that rose above the MCT and induced anatexis. Doming was suggested to result from compressivestress induced undulations enhanced by the buoyancy of anatectic melts (Le Fort, 1986; Le Fort et al., 1987). Harrison et al. (1997), on the basis of numerical simulations, suggested that the anatectic leucogranites were products of deformational heating along the Himalayan decollement. They implied that these magmas were hot enough and of low enough viscosity to rise diapirically into the middle crust. In contrast, Burg et al. (1984) argued that the Kangmar Dome, one of the North Himalayan gneiss domes (Fig. 1) formed by a thrust duplex at depth on the basis of reconnaissance observations of S-verging folds in the upper part of the metasedimentary carapace, the reorientation of fold axes to a N S trend in the lower part of the carapace, and increasing shear strain with structural depth. More recent reconnaissance studies in the Kangmar Dome (Chen et al., 1990) documented a top-to-the-n mylonitic fabric in the high-grade metasedimentary rocks and orthogneiss. In addition, Chen et al. (1990) interpreted the contact between the orthogneiss core and metasedimentary mantle as an extensional detachment fault, similar to those associated with C-1

2 C-2

3 metamorphic core complexes of the western US, and argued that displacement along the Kangmar detachment was northward, the same as the STDS. They concluded that like the STDS, the Kangmar Dome is an extensional feature that formed in response to gravitational collapse of the Himalayan topographic front. On the basis of new detailed geologic mapping, structural analysis, metamorphic petrology, and thermochronology in the Kangmar Dome, Lee et al. (1999, in review a, b) suggested that the domal form resulted from contraction followed by extension, and ending with exhumation by thrust faulting and erosion. Lee and colleagues interpreted the formation of the extensional fabrics as a result of vertical thinning and horizontal extension at mid-crustal depths as a consequence of maintaining a stable wedge geometry. Subsequent doming during the middle to late Miocene was attributed to thrusting upward and southward over a north-dipping ramp along the Gyirong- Kangmar Thrust fault system (GKT) (Fig. 1) above cold Tethyan sediments. If true, these relations imply that middle to late Miocene thrusting in the Kangmar Dome region was synchronous with normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault (RZT) (Fig. 1), leading Lee et al. (in review a) to hypothesize a kinematic link among these structures and simultaneous contractional and extensional deformation within southern Tibet. This exciting new kinematic picture of the evolution of the Himalaya and southern Tibet needs to be confirmed. To test this hypothesis, we propose to characterize the deformational, metamorphic, and exhumation history of the Mabja Gneiss Dome, located ~150 km west of the Kangmar Dome (Figs. 1 & 2), because preliminary studies suggest that it exposes similar deformational and metamorphic histories as in the Kangmar Dome. However, the Mabja Dome exposes migmatites, syn- to post-tectonic pegmatites, and syn(?)- to post-tectonic granites (Lee et al., 1998) suggesting a diapiric origin or diapirism accompanied by a significant component of regional deformation. This proposal seeks funding for a two-year period to conduct an integrated field-based investigation of the Mabja Gneiss Dome in collaboration with Dr. Wang Yu, of the Institute of Geology, State Seismological Bureau, Beijing, China. Our primary objective is to document and characterize the geometry, kinematics, magnitude, and timing of brittle and ductile deformation, the pressure and temperature conditions of high-grade metamorphism during deformation, the timing of metamorphism and magmatic activity, and the cooling history in the Mabja Dome in order to assess whether it s origin is contractional, diapiric, extensional, or some combination of these. In the ideal end-member setting, each of these postulates make specific and testable predictions: (1) If the dome formed as a result of contraction, we expect to document: a unidirectional stretching lineation and an up-dip sense of shear; structural duplication across major faults or shear zones; that metamorphism was the product of conductive relaxation of isotherms; that magmatism was not necessarily genetically linked to deformation; and that cooling ages (exhumation) are asymmetric across the dome and decrease down structural dip in a direction opposite to the sense of shear. (2) If, on the other hand, the dome formed as a result of diapirism, we expect to document: a radially oriented stretching lineation and down-dip sense of shear; no structural duplication or omission; spatial and temporal association of magmatism, metamorphism, and deformation; and symmetrically distributed cooling rates that decrease from rapid to slow, indicative of cooling following pluton emplacement. (3) Finally, if the dome formed as a result of extension, we expect to document: a unidirectional stretching lineation and down-dip sense of shear; C-3

4 structural omission across major faults or shear zones; that magmatism was not necessarily genetically linked to deformation; and that cooling ages (exhumation) are asymmetric across the dome and decrease down structural dip in the direction of shear. Documenting and characterizing the processes that created these gneiss domes have important implications for mechanisms proposed for development of overthickened crust and high elevation in the Himalaya and Tibetan plateau. For example, if these domes are the result of crustal contraction, quantifying the magnitude and geometry of contraction involved in their exhumation may yield better constraints on the degree of deformation partitioning between crustal contraction and lateral extrusion, both of which accommodate convergence between Asia and India (e.g. Tapponnier et al., 1982). Furthermore, if we can constrain the timing of this contractional deformation, we can assess whether slip along the STDS and contractional deformation within the gneiss domes occurred contemporaneously or sequentially during the Miocene. If, on the other hand, these domes are diapirs, documenting the geometry and timing of diapirism will lead to a better assessment of Harrison et al. s (1997) numerical model for their generation by shear heating along the Himalayan decollement and subsequent anatexis. In order to test existing mechanisms of gneiss dome development, or to formulate new mechanisms, we need a much better understanding of the structure, kinematics, metamorphic conditions, and timing of deformational, intrusive, and metamorphic events within the core, the mantle of high grade metamorphic rocks, and the surrounding lower grade rocks. Finally, data from our studies, integrated with results from seismic reflection investigations in Tibet by other workers (e.g. INDEPTH), will provide an excellent opportunity to document the space-time evolution of gneiss dome formation within the tectonic framework of southern Tibet. Distinguishing the process that led to unroofing of these domes has important implications for the tectonic evolution of the Tibetan Plateau. 2. Geologic Setting A series of plutonic and metamorphic culminations referred to as the North Himalayan Gneiss domes are exposed within the Tethys Himalaya, just south of the ITSZ (Fig. 1). This region is underlain by a miogeoclinal sedimentary sequence deposited upon the passive northern margin of the India continent. This marine sedimentary sequence is nearly continuous in age from Cambrian to Eocene (Gansser, 1964; Le Fort, 1975), with the Eocene marine sedimentary rocks probably marking an upper bound on the timing of the India-Asia continental collision. The zone is structurally complex, exhibiting Cretaceous to Holocene contractional and extensional structures in a variety of orientations (e.g. Armijo et al., 1986; Burg and Chen, 1984; Le Fort, 1975; Quidelleur et al., 1997; Ratschbacher et al., 1994; Searle, 1983). The oldest structures are S-directed thrust and fold nappes of Paleocene age that are probably related to obduction of an ophiolite (Ratschbacher et al., 1994; Makovsky et al., 1999). Younger, Eocene to Miocene, structures include shallow-dipping, S-directed thrusts and steeply dipping N-directed backthrusts (Burg and Chen, 1984; Coward et al., 1988; Quidelleur et al., 1997; Ratschbacher et al., 1992, 1994, Searle et al., 1988; Yin et al., 1994). Imbrication of the Tethyan Himalaya is thought to be Miocene (Ratschbacher et al., 1994). Pliocene to Recent N S striking grabens (e.g. Armijo et al., 1986) appear to be the result of gravitational collapse of overthickened crust and high elevations. The E W direction of extension is attributed to eastward extrusion along the Pacific margin (e.g. Molnar and Lyon-Caen, 1989; Molnar and Tapponnier, 1975; Molnar and Tapponnier, 1978; Tapponnier et al., 1982) and has been linked to a postulated abrupt increase in surface elevation of the plateau as a consequence of possible convective removal of the lower continental lithosphere (e.g. England and Molnar, 1993). C-4

5 3. North Himalayan Gneiss Domes 3.1 Introduction The North Himalayan Gneiss Domes are cored by an orthogneiss mantled by high-grade metasedimentary rocks intruded by middle to late Miocene two-mica granites (Burg et al., 1984; Chen et al., 1990; Lee et al., 1998, 1999, in review a, b; Maluski et al., 1988; Scharer et al., 1986). Summaries of reconnaissance studies in the most-accessible of these domes, the Kangmar Dome, by Burg et al. (1984) and Chen et al. (1990), are provided in the Introduction (see section 1). Below, we present a summary of our previous multidisciplinary investigations within the Kangmar Dome, the most accessible of the domes, and preliminary studies within the Mabja Dome. The primary contribution of this research is documentation of the space-time evolution of gneiss dome formation within an overall convergent setting, and assessment of the implications for the tectonic evolution of the Tibetan plateau. This research was funded by National Science Foundation grant EAR , and was conducted in collaboration with Drs. Wang Yu and Chen Wenji, Institute of Geology, State Seismological Bureau, Beijing, China. 3.2 Kangmar Dome The Kangmar Dome lies in the hangingwall of the GKT (Fig. 1) (Burg and Chen, 1984; Lee et al, in review a; Ratschbacher et al., 1994; Wu et al., 1998) and is cored by a Cambrian orthogneiss (Lee et al, in review a, b; Scharer et al., 1986) that is mantled by medium- to high-temperature/moderate pressure metapelites; the grade of metamorphism decreases upsection. Two dominant deformational events are preserved within the Kangmar Dome. The older event, D1, is best exposed at high structural levels within unmetamorphosed to low-grade metamorphic rocks on the northern and southern flanks of the dome. This deformational event resulted in ENE-WSW trending, tight to isoclinal, S-vergent F1 folds of S0 with an associated moderately to steeply NNW-dipping axial planar foliation, S1. The structural fabrics associated with the second deformational event, D2, are manifested at higher structural levels as a series of open, recumbent, NS-trending folds of bedding and the S1 foliation. With increasing structural depth, bedding and the S1 foliation are first crenulated by a spaced, subhorizontal S2 foliation and finally transposed parallel to a mylonitic S2 foliation. Associated with the high strain foliation is a N S stretching lineation. The S2 foliation dips moderately north on the north flank and moderately south on the south flank, defining the domal form. Shear sense within the orthogneiss and high grade metasedimentary rocks during formation of S2 varies from top-to-the-s on the south dipping flank to top-to-the-n on the north dipping flank of the Kangmar Dome; the central part of the dome exhibits opposing shear sense indicators or symmetric fabrics. The contact between the orthogneiss and the overlying metapelites ranges from a knife-sharp, concordant boundary with no mesoscopic evidence of brittle deformation, to a locally discordant contact with an up to 1 m wide zone of fault gouge and breccia developed within crushed and sheared schist; there is no evidence of brittle deformation within the underlying orthogneiss. We attribute this local deformation to the rheological contrast between the schist and orthogneiss because there is no evidence for structural omission across this contact. Furthermore, there is no discontinuity in mica 40 Ar/ 39 Ar ages across this contact (see below), supporting our field interpretation. Elsewhere in the dome faults are scarce. Pelites within the metasedimentary carapace record prograde metamorphism, with successive chloritoid-in, chloritoid-out/garnet-in, staurolite-in, and kyanite-in isograds toward the orthogneiss. The isograds cut across mapped units and structures, defining a thermal high northeast of the center of the dome; this requires that the orthogneiss exposed in the core of the dome was not the heat C-5

6 source responsible for metamorphism. Microtextures, including crenulated quartz and opaque inclusion trails within garnet porphyroblasts that we interpret as preserved S1 foliation crenulated by S2, indicate that these minerals grew after the D1 deformation and prior to or during the early stages of D2 deformation. Peak conditions reveal an increase in temperature from ~445 C in garnetzone rocks to ~624 C in staurolite+kyanite zone rocks. Significantly, peak pressures increase northward across the dome from ~660 MPa to MPa, with the highest pressures preserved on the north flank of the dome. In addition, these data indicate an apparent gradient in pressure of ~125 MPa/km, well in excess of the expected gradient of 27 MPa/km for supracrustal rocks with an average density of 2700 kg/m 3. This vertically shortened pressure gradient requires a factor-of-five subhorizontal stretching after the pressure gradient was frozen in. 40 Ar/ 39 Ar dating of mica and K-feldspar from the orthogneiss and metasedimentary rocks along a transect parallel to the N S trending stretching lineation shed considerable light on the cooling history. Muscovite generally yielded plateaus or slightly disturbed spectra, with ages of 12.2 Ma in schists on the northern flank of the dome that increase to ~15 Ma across the central and southern part of orthogneiss and overlying schist. Biotite yielded plateaus or slightly more disturbed spectra, with ages that are slightly younger than muscovite in the schist, but older than muscovite within the orthogneiss. Six K-feldspars from the orthogneiss yielded complex spectra with old apparent ages at the lowest temperature steps, followed by ages that climb gradually from ~10 Ma to ~11 Ma over the first 45-65% of 39 Ar released. Over the last 40-60% of the 39 Ar released, all six samples exhibit complex age spectra characterized by ages that climb steeply and erratically indicating incorporation of excess argon; ages as old as Ma occur at the high temperature steps. Diffusion modeling (Lovera et al., 1989) of these data indicates relatively rapid cooling (~10-30 C/Ma) from as high as ~350 C to as low as ~250 C between 11.5 Ma and 10.0 Ma synchronously across the dome. Six apatite separates, from the same rock samples that provided the potassium feldspar separates, yield fission track ages ranging from 4.1±1.9 to 7.9±3.0 Ma (±1 σ). These ages are indistinguishable, indicating that the dome continued to cool symmetrically through approximately 120 C at 5.5 Ma, the mean age for all samples. The cooling rate between muscovite closure and apatite retention was rapid, C/Ma, consistent with the calculated K-feldspar cooling histories. There are five notable observations that fall from the thermochronology data (Lee et al., in review a). First, mica ages increase down section, compatible with cooling from below due to underthrusting of colder rock. Second, mica ages young northward within a given structural horizon, suggesting that the north flank of the dome resided at slightly deeper structural levels, compatible with the northward increase in peak metamorphic pressures and temperatures within a given structural horizon. Accordingly, if we assume subhorizontal isotherms, this relation implies that exhumation at the northern end of the dome through the 370 C to 335 C isotherms occurred 2-3 m.y. after the southern flank of the dome. Implicit in this interpretation is that the dome was tilted northward and exhumed southward. Third, there is no discontinuity in mica ages across the contact between the orthogneiss and overlying schist, supporting our field interpretation that there is no structural omission across, and little brittle motion along, this contact. We can not, however, rule out the possibility that the orthogneiss and metasedimentary rocks had different cooling histories following peak metamorphism but similar histories (i.e. cooling below ~ C) following juxtaposition due to faulting (e.g. Chen et al., 1990). Fourth, the uniform cooling histories derived from the K-feldspar and apatite data suggest that the dome was symmetrically exhumed between approximately 11 Ma and 5.5 Ma. Fifth, the cooling rate of about C/Ma, from the closure temperature for muscovite to the annealing temperature for apatite, appears to have been relatively constant across the dome and reflects both refrigeration and exhumation. C-6

7 The structural history and PT conditions we have documented in the Kangmar Dome lead to the following tectonic history. The miogeoclinal section exposed within the Kangmar Dome was thickened and buried by distributed folding during D1 deformation, such that staurolite-kyanite zone rocks, the bottom of the section, were buried to ~30 km depth. We attribute metamorphism to the conductive relaxation of isotherms, because microtextural evidence indicates peak metamorphism occurred after D1 and prior to or during the early stages of D2, and there is no field evidence for a magmatic heat source. Following peak metamorphism, the rocks were stretched subhorizontally by a factor of about five, collapsing the apparent isobars to ~20% of their original thickness. The dome was subsequently tilted southward ~3 ; the northern end of the dome was lifted about 1 km relative to the southern end. Finally, the core was domed 2-3 km upward relative to the outside to create the domal form. The S2 foliation, isobars, isotherms, and mica 40 Ar/ 39 Ar isochrons are domed, but K-feldspar 40 Ar/ 39 Ar isochrons are not, implying that doming occurred at temperatures of ~ C at about 11 Ma. In summary, our work has documented a first phase of deformation characterized by N-S contraction leading to thickening in the Kangmar Dome region, followed by thermal re-equilibration and peak metamorphism, which in turn was followed by a second phase of deformation characterized by horizontal extension, vertical thinning, and ending with a third phase of deformation characterized by doming and exhumation. In addition to the data we have collected from the Kangmar Dome, there are two important regional relations that bear on its development and its role in the tectonic evolution of southern Tibet. First, INDEPTH seismic reflections have been interpreted to show a ~35 km high antiformal duplex in the hangingwall of a crustal ramp along the Main Himalayan Thrust (MHT) beneath the core of the Kangmar Dome (Hauck et al., 1998). Second, surface geologic mapping indicates that the Kangmar Dome lies in the hangingwall of the N-dipping GKT (Burg and Chen, 1984; Lee and Dinklage, unpubl. mapping; Ratschbacher et al., 1994; Wu et al., 1998), a fault not imaged by the INDEPTH seismic data. Our data, along with these regional relations, rule out a simple metamorphic core complex, diapir, or thrust-duplex origin for the Kangmar gneiss dome for three salient reasons: D1 fabrics indicate contractional deformation, D2 fabrics indicate extensional deformation, and the mica cooling ages suggest underthrusting of a cold slab. One way to explain alternating contraction and extension is to consider the hanging wall of the MHT as a southward-tapering orogenic wedge (e.g. Platt, 1986). Lee et al. (in review a) suggest that D2 extensional fabrics formed as a consequence of increased wedge thickness and decreased rock strength. Underplating of large amounts of material into the core of the antiform that overlies the N-dipping crustal ramp along the MHT would have caused an increase in wedge thickness. The rheologically weakened middle crust could then have thinned vertically and stretched horizontally, resulting in the development of the subhorizontal D2 fabrics, and decreasing wedge thickness. Continued underplating at depth and extension at midcrustal levels could have transported Kangmar rocks to shallower crustal levels. We interpret rapid cooling (25-70 C) at 15 to 11 Ma as refrigeration by underthrusting of cold Tethyan sediments. Because mica cooling ages young northward within a single structural horizon, this implies that Kangmar rocks were captured in the hangingwall of a north-dipping thrust fault, the GKT, and uplifted southward during the middle Miocene. Subsequent movement of these rocks up and over a N-dipping ramp along the GKT at ~11 Ma and temperatures of C resulted in doming of the S2 mylonitic foliation, the metamorphic isobars and isotherms, and mica 40 Ar/ 39 Ar isochrons. Symmetric cooling of the dome from approximately 300 C to 120 C between approximately 11 and 5.5 Ma implies that rapid exhumation (~20-40 C/Ma) due to erosion followed thrust faulting. Because the timing of thrusting along the Gangdese and RZT thrust fault systems to the north is estimated to be late Oligocene to early Miocene and early to late Miocene, respectively (Yin et al., C-7

8 1994; Ratschbacher et al., 1994; Quidelleur et al., 1997), and initial slip along the MCT is estimated to be late Oligocene to early Miocene (Hubbard and Harrison, 1989; Harrison et al., 1995; Coleman and Parrish, 1995), these relations suggest that vertical thinning and horizontal extension in the Kangmar region may have been concurrent with contractional deformation to the north and south. In addition, geochronologic studies reveal that slip along the STDS in the Wagye La and Khula Kangri areas (Fig. 1), south and southeast, respectively, of the Kangmar Dome, occurred at about 12 Ma (Edwards and Harrison, 1997; Wu et al., 1998). The Ma cooling history we have documented in the Kangmar Dome is synchronous with this, implying that normal slip along the STDS was accompanied by contraction in the hangingwall of the STDS. This raises the possibility that normal slip along the STDS was accompanied by thrust faulting in the Kangmar region and along the RZT, suggesting a kinematic link among these three structures (Lee et al., in review a). 3.3 Mabja Dome The regional setting of the Mabja Dome is similar to that of the Kangmar Dome (Fig. 1). The Mabja Dome is exposed within the core of the North Himalayan antiform and lies in the hangingwall of the GKT. Our preliminary studies within the west-central part of the Mabja Dome (Fig. 2) reveal that its geologic history is also similar to the Kangmar Dome (Lee et al., 1998b). However, significant differences suggest that the Mabja Dome did not form by thrusting of the core southward over a north-dipping ramp along the GKT (e.g. Lee et al., 1999, in review a). The deepest structural levels of the Mabja Dome are underlain by a K-feldspar augen biotite orthogneiss and incipient migmatites, which are in turn are overlain by high-grade metasedimentary rocks and granitic orthogneisses (Fig. 2). The grade of metamorphism, defined by sillimanite-in, kyanite-in, staurolitein, garnet-in, and chloritoid-in isograds, decreases upsection and dies out at the highest structural levels, where unmetamorphosed clastic rocks are exposed. Mesoscopic fabrics indicate that peak metamorphism and isograd development occurred after D1 deformation and prior to or during the early stages of D2 deformation. Preliminary studies have identified two major penetrative deformational events within the Mabja Dome. The first deformational event, D1, exposed at structural levels above the garnet-in isograd, resulted in WNW ESE trending, open to tight to isoclinal F1 folds of S0 and an associated moderately NE-dipping axial planar foliation, S1. The second event, D2, exposed at structural levels below the garnet-in isograd, crenulated bedding and S1 foliation at high structural levels and transposed bedding and S1 into parallelism to a high strain mylonitic foliation, S2, at deeper structural levels. Associated with the S2 foliation is a ~N S stretching and mineral alignment lineation. On the basis of our mapping to date, the S2 foliation is somewhat domed across the area: it dips moderately to steeply SSW on the southern flank of the dome and moderately NW on the northern flank. Mesoscopic kinematic indicators associated with the S2 mylonitic foliation record top-to-the-s shear on the south-dipping flank and both top-to-the-n and top-to-the-s shear on the north-dipping flank of the Mabja Dome. The K- feldspar augen orthogneiss exhibits predominantly symmetric fabrics. Faults are scarce, and there is no evidence for significant offset of units or metamorphic isograds throughout the region mapped to date. A pegmatite dike swarm and 2 two-mica granites have also been mapped. The dike swarm, exposed at fairly deep structural levels, appears to have been emplaced during or after the D2 deformation. The two-mica granites are undeformed and cut across isograds, unit contacts, and the D2 structural fabrics, indicating that they were emplaced after the D2 deformation. Andalusite in the contact aureole of one of these plutons implies emplacement at relatively shallow depths. C-8

9 Reconnaissance suggests that at least one additional syn(?)- to post-tectonic granite is exposed along the western flank of the dome. U/Pb geochronology by Scharer et al. (1986) on monazite yielded ages of 9.2±0.9 Ma and 9.8±0.7 Ma from slightly deformed granites (Maluski et al., 1988) within the Mabja dome; unfortunately, the location of these samples was not reported. 40 Ar/ 39 Ar thermochronology on biotite and muscovite from both the granitic rocks and orthogneissic rocks from the Mabja Dome yielded disturbed spectra with total gas ages of 6-8 Ma, although the location of these samples was also not reported (Maluski et al., 1988). Our preliminary U/Pb geochronology on zircons from one of the orthogneiss bodies shows that it is Paleozoic and represents basement similar to that seen in Kangmar Dome. Preliminary U/Pb geochronology on igneous monazite from one of the posttectonic 2-mica granites yielded an intrusive age of 14.5±0.1 Ma (Fig. 2) (Lee et al., 1998b) indicating that D2 deformational fabrics and the domal form developed prior to the middle Miocene. The striking similarity of the structural geology and metamorphic history between the Mabja and Kangmar domes implies that they formed by the same process: contraction, followed by extension, and ending with exhumation by thrust faulting and erosion (Lee et al., 1999; in review a). However, because migmatitic rocks, syn- to post-tectonic pegmatites, and syn(?) to post-tectonic granites are exposed within the Mabja Dome, but not in the Kangmar Dome, we cannot yet rule out a diapiric origin or diapirism accompanied by regional deformation. 4. Proposed Research 4.1 Introduction The results of our preliminary geologic mapping and structural studies within the Mabja Dome provide plausible evidence for a combination of contractional and extensional deformation leading to the domal form. Furthermore, these studies are compatible with, but do not prove, our hypothesis that doming occurred as a consequence of thrust faulting contemporaneously with normal slip along the STDS. However, because these preliminary studies raise a number of questions and because critical areas have not been mapped, we cannot yet rule out a diapiric origin or diapirism accompanied by a significant component of regional deformation (contraction and/or extension). Our proposed research focuses on documenting the mechanism by which the Mabja Dome formed by a combination of geologic mapping, detailed structural, kinematic, metamorphic petrology, geochronologic, and thermochronologic investigations. Our proposed field-based investigations will provide a three-dimensional view of the nature, geometry, and kinematics of ductile and brittle deformation of plutonic rocks and country rocks, and relative ages of pluton emplacement, deformation, and metamorphism. Metamorphic petrology will provide quantitative constraints on peak temperatures and pressures prior to exhumation. U/Pb geochronology and 40 Ar/ 39 Ar thermochronology will provide age constraints on the timing of pluton emplacement, peak metamorphism, brittle and ductile deformation, and cooling histories. This strategy will document a time-integrated view of gneiss dome development within an overall lithospheric convergent setting. Furthermore, it will allow us to test our hypotheses that normal slip along the STDS was accompanied by contraction in the hangingwall of the STDS and that there is a kinematic link among the North Himalayan gneiss domes, the STDS, and the RZT. Finally, if we confirm our hypotheses, our research will provide much needed geologic data for characterizing the mechanisms of formation of the Tibetan plateau. 4.2 Geologic Mapping, and Structural and Kinematic Studies C-9

10 With the exception of the Kangmar Dome (e.g. Burg et al., 1984; Chen et al., 1990; Gans et al., 1998; Hacker et al., 1998; Maluski et al., 1988; Scharer et al., 1986; Wang et al., 1997; Lee et al., 1998a, 1999, in review a, b), little is known about the geology of the North Himalayan Gneiss Domes. Preliminary mapping within the Mabja Dome has established that its geology, structure, and metamorphic histories are similar to the Kangmar Dome, but there are important differences, including exposures of migmatites, syn- to post-tectonic pegmatites, and syn(?)- to post-tectonic granites (Lee et al., 1998b). In addition, most of the dome remains unmapped, including such critical areas as the core, the western flank around a syn(?)- to post-tectonic granite, the southern flank where the GKT is exposed(?), and the northeastern part of the dome (Fig. 2). Mapping these areas may reveal crucial field relations that will shed light on the evolution of Mabja Dome. We will undertake geologic mapping at 1:50,000 scale on topographic maps and aerial photographs, and collect structural and kinematic data to constrain the nature, geometry, kinematics and magnitude of deformation, and timing of magmatism relative to deformation and metamorphism. Such an approach will provide the necessary framework for understanding the nature of deformation with respect to the mechanisms proposed for gneiss dome formation, for understanding the results of our thermobarometric, geochronologic, and thermochronologic studies, and for understanding the nature of deformation in this area with respect to the STDS in particular, and the Tibetan plateau in general. The primary goals of this aspect of our research are to: (1) Map the core of the dome to determine if a syntectonic granitic body and/or an extensive zone of migmatites are exposed. Is the core dominated by a plutonic body? If so, what is the age of emplacement of this pluton relative to peak metamorphism and deformation? Or, is the core dominated by migmatites, and, if so, are the migmatites the source of the pegmatites exposed at higher structural levels? (2) Map metamorphic isograds to ascertain whether they are related to structural or plutonic features. Is the metamorphism the result of conductive relaxation of isotherms or is it the result of convection triggered by the emplacement of plutons? If the latter, is the source of the heat exposed in the core of the dome? (3) Map the southern flank of the Mabja Dome to discover if it lies in the hangingwall of a northdipping thrust fault, similar to the Kangmar Dome which lies in the hangingwall of the northdipping GKT (Burg et al., 1984; Lee and Dinklage, unpubl.; Ratschbacher et al., 1994). (4) Map the western part of the dome to document whether the granite pluton exposed there is syn- or posttectonic. As part of these studies, we will also: (a) Create a detailed (1:50,000 scale) geologic map of the dome. (b) Create a detailed map of foliation and lineations orientations to determine the threedimensional geometry and kinematics of foliations and lineations within the Mabja Dome. Preliminary mapping and structural studies suggest that Mabja Dome possesses two major penetrative deformational fabrics (Lee et al., 1998b). However, whether the mylonitic foliation is domed and the stretching lineation unidirectional is not known. If the stretching lineation is unidirectional, then this suggests a tectonic origin. If however, the stretching lineation is not unidirectional, as has been documented in the Kigluaik gneiss dome, Alaska (Amato et al., 1994; Calvert et al., in press), then this suggests a diapiric origin. (c) Collect structural and kinematic data on finite strain and shear within the penetratively deformed rocks using conventional mesoscopic and microscopic investigations. In addition, complete electron back-scatter diffraction (EBSD) studies of selected, chiefly monomineralic, samples to assess sense of shear through asymmetrical lattice and shape preferred orientations, as well as the contributions of dislocation glide and creep relative to cataclasis and grain-boundary sliding through variations in preferred orientation strength. Is the bulk strain coaxial or non-coaxial? If non-coaxial, is there a consistent sense of and direction of shear across this dome, suggesting a tectonic origin? Or, is the direction of shear down-dip and C-10

11 radially oriented, compatible with a diapiric origin? What is the magnitude of finite strain within the tectonites of this dome and how does it vary in three dimensions? (d) Document field evidence for timing of magmatism relative to penetrative deformation and metamorphism to address whether metamorphism was the result of conduction or advection. 4.3 Metamorphic Petrology Quantitative thermobarometry investigations within the Kangmar Dome provided critical information on the tectonic evolution of the dome. Most importantly, not only were the rocks buried to depths of 30 km, but the apparent pressure gradient is ~125 MPa/km, well in excess of the expected gradient of 27 MPa/km. Development of such a subvertically shortened pressure gradient requires a factor of five subhorizontal stretching after the pressure gradient was frozen in. The PT data, along with structural data, imply a tectonic history of burial by contraction, followed by peak metamorphism, subvertical thinning and tilting, and ending with doming by thrust faulting (see section 3.2). Quantitative thermobarometry should place similarly strong constraints on the tectonic evolution of the Mabja dome. We will undertake petrographic examination of pelitic rock samples to document the prograde sequence of mineral assemblages that define a series of isograds. Are the isograds concentric, as documented in the Kangmar Dome? In addition, petrographic examination will establish the relative ages between mineral growth and deformational events did peak metamorphism occur during the early stages of D2 deformation, as documented in the Kangmar Dome? On the basis of these studies, we will select 10 samples that provide three-dimensional coverage of the dome for detailed thermobarometry. Each sample will be examined carefully with back-scattered electron imaging and element mapping prior to quantitative line scans of selected mineral groups. Temperatures and pressures will be calculated with well-calibrated thermobarometers (e.g., garnet biotite (Ferry and Spear, 1978; Hodges and Spear, 1982), garnet aluminumsilicate quartz plagioclase (Ghent, 1976; Ghent et al., 1979)), using principally Thermocalc (Holland and Powell, 1998), supplanted with a healthy dose of geologic sense regarding the application of such calculations in light of reaction textures and mineral zoning. If we establish that peak pressures and temperatures developed asymmetrically across the dome for example the north flank of the dome was deeper and hotter and were not spatially associated with a pluton, such relations would support conductive relaxation of isotherms and a contractional origin for the dome. If, on the other hand, we show that peak pressures and temperatures are symmetrically disposed around a pluton exposed within the core of the dome, this finding would support an advective source for metamorphism and a diapiric origin for the dome. 4.4 Geochronology and Thermochronology 4.4a Introduction A critical aspect of testing the various models proposed for the formation of the Himalayan gneiss domes, as well as addressing questions concerning the evolution of the Tibetan plateau, involves acquiring chronological constraints on intrusive, metamorphic, and deformational events. We propose to use U/Pb and 40 Ar/ 39 Ar methods to date igneous and metamorphic rocks in the Mabja Dome to build on our reconnaissance U/Pb geochronology and as well as the reconnaissance U/Pb geochronology and 40 Ar/ 39 Ar thermochronology of Scharer et al. (1986) and Maluski et al. (1988), respectively. Just such an integrated geochronologic and thermochronologic study at Kangmar Dome provided absolute age constraints on the timing of intrusive, metamorphic, and deformational events, and can provide a T-t history ranging from temperatures as high as ~800 C (approximate temperature at which zircon crystallizes) to as low ~150 C (closure temperature for the smallest C-11

12 domains in K-feldspar) (e.g. Heizler et al., 1988; Lovera et al., 1989). Provided we can eliminate cooling mechanisms such as fluid convection and/or lateral heat flow, calculated cooling rates may either reflect tectonic denudation resulting from motion along shear zones and faults (asymmetric cooling) or cooling following emplacement of plutonic bodies (symmetric cooling). 4.4b Zircon and Monazite U/Pb Geochronology In the Mabja Dome at least three two-mica granites are exposed, two of which have been partially mapped (Fig. 2). The relative age relations between granite emplacement and the deformational events, for at least these two plutons, indicate that the plutons were emplaced after the D2 deformation. Our proposed geologic mapping and structural studies will establish how many additional plutons might be present in this dome, especially in the core of the dome, and most importantly, their emplacement ages relative to formation of the deformational fabrics and the growth of metamorphic minerals. Geochronologic studies of these rocks will provide some of the most critical data on the timing of deformational and metamorphic events and thereby allow us to place these events into a regional tectonic context. To establish absolute ages for pluton emplacement and place age constraints on deformational structures and metamorphic events, we propose to use U/Pb geochronology on igneous zircon and monazite to determine crystallization ages of four plutonic rocks and on metamorphic monazite from four metasedimentary rocks to constrain the age of peak metamorphism. U/Pb ages are critical because they will establish whether metamorphism and plutonism were synchronous and, along with our mapping and petrologic studies, assess whether plutonism was the heat source for metamorphism. We were unable to solve this problem at the Kangmar Dome because Tertiary plutons are not exposed there. Secondly, on the basis of numerical simulations, it is plausible that the North Himalayan leucogranites were diapiric in origin (Harrison et al., 1997). If true, then the location of the domes within a belt of leucogranites is merely coincidence if our ramp model is correct. Therefore, establishing the age of pluton emplacement, metamorphism, and exhumation (see section 4.4c below) is important for differentiating among these different mechanisms. For example, if peak metamorphism and pluton emplacement were not related in time and space, then a diapiric origin seems unlikely. Furthermore, if we establish that the age of formation of the Mabja Dome is ~17-12 Ma, then we will have established a temporal link to normal slip along the STDS (Edwards and Harrison, 1997; Wu et al., 1998; Murphy and Harrison, 1999), which would support our working hypothesis. Finally, these studies will also establish the duration of Tertiary magmatism. If the plutons were emplaced over a several m.y. period, this will have different implications than if the plutons were emplaced during a single episode following D2 deformation. Inheritance and/or Pb loss are problems common among the attempts to use U/Pb geochronology to date refractory accessory minerals from the High Himalayan leucogranites. To address this potential problem in the Mabja Dome, we will analyze different fractions utilizing a combination of abrasion techniques and partial dissolution experiments (McClelland and Mattison, 1996) to establish and remove the effects of inheritance and/or Pb loss. 4.4c 40 Ar/ 39 Ar Thermochronology Reconnaissance 40 Ar/ 39 Ar thermochronology on biotite and muscovite from both the granitic rocks and orthogneissic rocks from the Mabja Dome yields disturbed spectra with total gas ages of 6-8 Ma (Maluski et al., 1988). Because the locations of these samples are unknown, we do not know whether these ages reflect cooling following the emplacement of a post-tectonic pluton, cooling due to exhumation along a thrust ramp, or cooling due to buoyancy driven exhumation. What were the timing and rates of cooling across the dome? Do the timing and cooling rates vary spatially? In the C-12

13 Kangmar dome, our thermochronologic data (Lee et al., in review a) show a northward decrease in mica ages, suggesting either that the north flank of the dome resided at slightly deeper structural levels before exhumation, consistent with the geothermobarometric data, or that there was a greater degree of reheating on this flank. In addition, micas ages increased with depth indicating refrigeration by underthrusting cold Tethyan sediments. In the Mabja Dome, was cooling radially symmetric, suggesting a diapiric origin, or asymmetric, suggesting a tectonic origin? If cooling was asymmetric, do ages young in the direction opposite to the shear sense, suggesting exhumation during contraction? And do ages increase with depth suggesting refrigeration by underthrusting of cold Tethyan sediments? We propose to address these problems by undertaking detailed 40 Ar/ 39 Ar analyses. The Mabja Dome is well suited to 40 Ar/ 39 Ar studies because the orthogneissic rocks, metasedimentary rocks, and leucogranites contain abundant mineral phases suitable for such studies. We will use conventional resistance-furnace step-heating experiments to obtain 40 Ar/ 39 Ar age spectra on hornblende, muscovite and biotite, and multiple diffusion domain analyses of K- feldspar Arrhenius data and 40 Ar/ 39 Ar age spectra. Careful selection of samples, based on our geologic mapping, structural and kinematic studies, will provide data essential to assess variations in cooling history across this dome, with depth, and with proximity to plutons. Well-behaved, conventional step-heating 40 Ar/ 39 Ar experiments on hornblende, muscovite and biotite will provide a t-t point for each sample (argon closure temperatures for hornblende, muscovite and biotite depend on composition, grain-size, and cooling rate, among other factors, but are roughly 525 C, 400 C and 300 C, respectively) (Harrison et al., 1985; Harrison and McDougall, 1980; Snee et al., 1988; Hodges, 1991). K-feldspars, on the other hand, may be characterized by a discrete distribution of diffusive length scales (Fitz Gerald and Harrison, 1993; Harrison et al., 1991; Lovera et al., 1993; Lovera et al., 1989; Lovera et al., 1991). As such, analyses of Arrhenius and 40 Ar/ 39 Ar age spectrum data from a single sample provide a segment of its thermal history. This technique provides more detail on cooling histories, over a range in temperatures from as high as ~400 C to as low as ~150 C, than can be provided by conventional 40 Ar/ 39 Ar age spectra (e.g. Heizler et al., 1988; Lovera et al., 1989). Assuming well-behaved samples, we will be able to calculate a well-constrained cooling history for each dome from as high as the argon closure temperature of hornblende, 525 C, to as low as argon closure temperatures of ~150 C for the smallest K-feldspar domains. Furthermore, if we discover and successfully date a synextensional granite, this cooling history may be extended to as high as ~800 C. This is a temperature range over which both crystal-plastic processes (above ~300 C or so) and brittle processes (below ~300 C) are the dominate deformation mechanisms in continental rocks. Therefore, younger ages in cover rocks and rapid cooling rates that young in the direction opposite to the sense of shear will largely reflect motion along contractional shear zones and/or thrust faults, whereas older ages in cover rocks and rapid cooling rates in gneissic core rocks that young in the direction of shear will largely reflect motion along normal sense shear zones and/or normal faults. In contrast, symmetrically distributed cooling rates that decrease asymptotically from rapid (on the order of C/Ma) to slow ( 5 C/Ma) are indicative of cooling following pluton emplacement. However, a number of mechanisms such as fluid convection and lateral heat flow can cause crustal rocks to rapidly cool. Our proposed geologic mapping and structural studies will also allow us to assess the importance of these complications as well as eliminate or minimize their effects when sampling. It is our experience, based upon our thermochronologic studies in the Kangmar Dome, that six K-feldspar samples, four hornblende, and 10 mica samples should be analyzed to obtain sufficient detail on the three-dimensional variation in cooling history. C-13

14 4.5 Implications for the Evolution of the Tibetan Plateau The regional spatial and temporal setting of the North Himalayan Gneiss Domes hint at different interpretations. For example, the North Himalayan Gneiss Domes are exposed within the core of the North Himalayan antiform, suggesting that the domes formed as a result of contraction. INDEPTH reflections have been interpreted to show an antiformal duplex in the hangingwall of the Main Himalayan Thrust (MHT) that extends to a depth of ~35 km beneath the Kangmar Dome (Hauck et al., 1998), also implying a contractional origin. On the other hand, the domes are also exposed within a belt of Miocene leucogranites, which have been suggested to be anatectic melts that diapirically rose to the middle crust (Harrison et al., 1997). The results of detailed, integrated geologic investigations of the Kangmar Dome provide plausible evidence that this dome formed as a result of contraction, then extension, and finally doming by thrusting upward and southward over a north-dipping thrust ramp. Middle to late Miocene thrusting in the Kangmar Dome region was contemporaneous with normal slip along the STDS and thrust motion along the RZT, suggesting a kinematic link among these structures. This exciting new kinematic picture for the evolution of the Himalaya and southern Tibet needs to be confirmed. For example, limited geologic mapping and structural studies within the Mabja Dome suggest that it exposes a similar deformational and metamorphic history as in the Kangmar Dome. However, the Mabja Dome exposes significant differences that hint at buoyancy-driven exhumation. What factors controlled the spatial and temporal development of these gneiss domes? Was it solely structural, or did magmatism play a critical role? Was the formation of these domes linked temporally and structurally to the STDS? Determining the processes that lead to gneiss dome development in southern Tibet, therefore, are key to gaining a better understanding of the development of overthickened crust and perhaps the timing that high elevations were achieved on the Tibetan plateau. For example, we may be able to address such important issues as: If these domes are the result of crustal contraction, then documenting the geometry and quantifying the magnitude of contraction involved in bringing them to the surface will yield better constraints on the degree of strain partitioning between crustal contraction and lateral extrusion, both of which accommodate convergence between Asia and India (e.g. Tapponnier et al., 1982). In addition, if contraction occurred coevally with normal slip along the STDS, this will confirm our working hypothesis that the hangingwall of the STDS was shortening during normal slip along the STDS. If these domes are the result of diapirism, then documenting the geometry and timing of diapirism will lead to a better assessment of the role of deformational heating along the basal decollement and subsequent anatexis (Harrison et al., 1997) in the evolution of the Tibetan plateau. 4.6 Role of Personnel and Work Plan For Jeff Lee, Brad Hacker, Bill McClelland, and Wang Yu, this project is a natural extension of their work completed in the Kangmar Dome and started in the Mabja Dome. Lee, McClelland, a UCSB graduate student, and Wang will complete the field mapping, structural studies, and collection of samples for structural, kinematic, metamorphic petrology, geochronology, and thermochronology investigations. Two months of field work focusing in the areas shown in Figure 2a will be conducted during Spring, These studies will provide the structural and geometric framework for the samples collected for kinematic, metamorphic petrology, geochronology and thermochronology studies. Dr. Wang will be in charge of organizing logistics for the field work. C-14