The Origin of Himalayan Anatexis and Inverted Metamorphism: Models and Constraints

Journal of Asian Earth Sciences 17 (1999) 755±772

The origin of Himalayan anatexis and inverted metamorphism: Models and constraints

T. Mark Harrison*, Marty Grove, Oscar M. Lovera, E.J. Catlos, Jessica D'Andrea

Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA

Abstract

The key to comprehending the tectonic evolution of the Himalaya is to understand the relationships between large-scale faulting, anatexis, and inverted metamorphism. The great number and variety of mechanisms that have been proposed to explain some or all of these features re¯ects the fact that fundamental constraints on such models have been slow in coming. Recent developments, most notably in geophysical imaging and geochronology, have been key to coalescing the results of varied Himalayan investigations into constraints with which to test proposed evolutionary models. These models fall into four general types: (1) the inverted metamorphic sequences within the footwall of the Himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting; (2) thrusting results from anatexis; (3) anatexis results from normal faulting; and (4) apparent inverted metamorphism in the footwall of the Himalayan thrust is produced by underplating of right-way-up metamorphic sequences. We review a number of models and ®nd that many are inconsistent with available constraints, most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related. The generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that, due to speci®c petrological and tectonic controls, episodic magmatism and out-of-sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks. This coincidental juxtaposition has proven to be something of a red herring, unduly in¯uencing attention toward ®nding a causal relationship between anatexis and inverted metamorphism. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction (Figs. 1 and 2) was to assume that recrystallization adjacent to the MCT and anatexis were temporally re- The extent to which we understand any evolutionary lated. The development of in situ dating methods that process is governed by how well we know the timing can overcome certain limitations of conventional of events. Although fundamental age relationships for methods (e.g., Harrison et al., 1995a, 1997a) produced the most signi®cant igneous and metamorphic activity results that challenged this widely held assumption of in the Himalaya were to a large extent unknown until Himalayan tectonics and suggest to us a rather dier- recently, a remarkable number and variety of contrast- ent history for the region (e.g., Harrison et al., 1998b). ing tectonic models were proposed in the absence of In this paper, we summarize the petrological, geo- such constraints to explain important tectonic features chronological, and structural constraints on the evol- of the mountain belt. A common point of departure ution of the Himalayan range and describe recent for many models seeking to account for the juxtaposi- developments which indicate that the inverted meta- tion of inverted metamorphic sequences in the footwall of the Main Central Thrust (MCT) beneath higher morphic sequences formed during Late Miocene slip grade rocks that host a belt of Tertiary leucogranites along the MCT and is thus unrelated to Early Miocene recrystallization and anatexis above the thrust. This recognition leads to our favored model * Corresponding author. (i.e., Harrison et al., 1998b), which we describe in this

Fig. 1. Geological map of the Himalaya and Southern Tibet. Ages for granites identi®ed by numeral are obtained from U±Th±Pb monazite dates. Sources of data are: (1) Gangotri: 22.420.5 Ma, Harrison et al. (1997b); (2) Shivling: 21.920.5 Ma, Harrison et al. (1997b); (3) Manaslu: 22.920.6 Ma (Larkya La phase), 19.320.3 Ma (Bimtang phase), Harrison et al. (1999); (4) Makalu: 2321 Ma; SchaÈ rer (1984); (5) Shisha Pangma: 20.220.2 Ma and 17.320.3 Ma, Searle et al. (1997); (6) Nyalam: 17.220.9 Ma; SchaÈ rer et al. (1986); (7) Gonto-La: 12.520.5 Ma, Edwards and Harrison (1997); (8) Dolpo: 17.620.3 Ma, Harrison et al. (1999); (9) Lhagoi Kangri: 15.1205 Ma, SchaÈ rer et al. (1986); (10) Maja: 9.520.5, SchaÈ rer et al. (1986); (11) Zanskar (not shown): 20.020.5 Ma, Noble and Searle (1995). Location of thermobarometry transects detailed in Fig. 3 are indicated by letters A through I. paper in some detail, that appears to reconcile the new models to evaluate the extent to which they are con- timing constraints with the large pool of knowledge sistent with current geologic evidence. In general, we gained from earlier geophysical measurements and the ®nd that many of these hypotheses are either incom- rock record. We then return to previously proposed pletely supported by, or inconsistent with, the recently

Fig. 2. Generalized cross section through the central Himalaya illustrating the juxtaposition of the major lithostratigraphic units across the major Himalayan faults, inverted metamorphism, and plutonic belts (modi®ed from Schelling and Arita (1991) and Zhao et al. (1993)). See Fig. 1 for key to abbreviations for faults. T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 757 derived timing constraints. The picture that emerges Himalayan Formations against unmetamorphosed from this review is of continuous Himalayan conver- Miocene±Pleistocene molasse (Siwalik Group), and the gence being manifested as episodic phenomena (e.g., MFT is presently active within Quaternary sediments. out-of-sequence thrusting, intermittent magmatism) Estimates of the amount of slip along the MHT based while creating geological relationships with a high po- on balanced cross section reconstruction's (Schelling, tential for imparting misleading clues (e.g., juxtaposi- 1992; Srivastava and Mitra, 1994; DeCelles et al., tion of anatectic rocks against apparent inverted 1998) are consistent with a displacement of about metamorphic sequences). 500 km. The Greater Himalayan Crystallines are juxtaposed against lower-grade Tethyan shelf deposits by the 2. Review of Himalayan geology South Tibetan Detachment System (STDS) (see Figs. 1 and 2). Although it is widely assumed that slip on the 2.1. Indo-Asian collision STDS and MCT occurred at least in part simultaneously (Burch®el et al., 1992; Hodges et al., 1992; Because of the initially irregular continental margins Searle et al., 1997), no evidence has yet been documen- of India and southern Asia, the period between their ted requiring such a relationship (cf. Vance et al., ®rst contact and ®nal suturing was undoubtedly pro- 1998). Constraints on the timing of thrusting along the tracted. Sedimentological evidence indicates that the MCT are that its hanging wall was deforming at 022 northwest tip of India had collided with Asia at 050 Ma (Hodges et al., 1996; Coleman, 1998) and a broad Ma (e.g., Le Fort, 1996; Rowley, 1996) and both con- shear zone below the GHC was active between about tinents appear to have met along the full length of a 8±4 Ma (Harrison et al., 1997a). Brittle faulting within 03000 km long suture by about 40 Ma (Dewey et al., the MCT hanging wall at ca 3 Ma has also been 1988). Paleomagnetic evidence indicates that the Indo- reported (e.g., Macfarlane, 1993). It is generally Australian plate has moved northward by assumed, but not certain, that the MCT was inactive 26002900 km relative to the Eurasian plate since this during the Middle Miocene development of the MBT time (Dewey et al., 1989; Le Pichon et al., 1992). In (Burbank et al., 1996). Whether or not slip along the that same interval, southern Tibet moved north with MCT overlapped with displacement on the STDS is respect to Eurasia by 20002600 km (Besse and uncertain. The clearest constraint on the timing of dis- Courtillot, 1988) suggesting something less than placement along the STDS is that the exposed detach- 1000 km underthrusting of India beneath Asia. ment fault near Gonto-La (Fig. 1) was active at Immediately prior to the onset of the Indo-Asian 12.420.4 Ma (Edwards and Harrison, 1997). collision, the northern boundary of the Indian shield Although extensional structures cut by the Manaslu was likely a thinned margin on which Proterozoic clas- intrusive complex (Guillot, 1993) must be older than tic sediments and the Cambrian±Eocene Tethyan shelf 23±19 Ma (Harrison et al., 1999), the relationship of sequence were deposited (Le Fort, 1996). South-di- the STDS to this body remains uncertain. Reports that rected thrusts within the Himalaya, including the the STDS north of Mt. Everest is cross-cut by an MCT, Main Boundary Thrust (MBT), and the Main Early Miocene granite (Hodges et al., 1992; Harrison Frontal Thrust (MFT) (Gansser, 1964; Bouchez and et al., 1995a) have proven to be in error (Murphy and PeÃcher, 1981; Arita, 1983; Le Fort, 1986; Burbank et Harrison, 1999), probably due to a sampling mishap. al., 1996) appear to sole into a common decollement, The ages of ductiley deformed granite sills in this area the Main Himalayan Thrust (MHT) (Zhao et al., are consistent with Middle Miocene slip on the STDS 1993; Nelson et al., 1996; Brown et al., 1996). In gen- (SchaÈ rer et al., 1986; Hodges et al., 1998; Murphy and eral, the MCT places high grade gneisses of Indian ori- Harrison, 1999). gin, the Greater Himalayan Crystallines (GHC), on top of the Lesser Himalayan Formations (LHF), com- 2.2. Inverted metamorphism prised largely of intermediate grade schists and phyl- lites. The protoliths of the Lesser Himalayan The juxtaposition of the Greater Himalayan Formations and Greater Himalayan Crystallines are Crystallines and Lesser Himalayan Formations across interpreted, respectively, to be Middle and Late the MCT is associated at most locations in the Proterozoic clastic rocks (Parrish and Hodges, 1996). Himalaya with an increase in metamorphic grade with Dating studies (e.g., Parrish and Hodges, 1996; Vance higher structural position (i.e., shallower depth) (Figs. and Harris, 1999) suggest that high grade metamorph- 2 and 3). The Greater Himalayan Crystallines vary ism ®rst aected the protolith of the Greater substantially in thickness across the Himalaya. For Himalayan Crystallines during an Early Tertiary, or example, hanging wall thickness increases from about Eohimalayan, phase of crustal thickening (Le Fort, 2 km in the Kali Gandaki to 20 km in central Bhutan, 1996). The MBT juxtaposes schists of the Lesser probably due to (1) variable initial thickness, (2) the 758 T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772

Fig. 3. Distribution of pressures and temperatures calculated from mineral equilibria versus structural distance from the MCT for samples col- lected along transects indicated on Fig. 1. Sources for the data shown are as follows: A, Kali Gandaki River (Hodges et al., 1996); B, Modi Khola (Kaneko, 1995); C, Darondi Khola, grey (Catlos et al., 1997), black (Hodges et al., 1988); D, Burhi Gandaki (Hodges et al., 1988), E, Langtang, grey (Inger and Harris, 1992), black (Macfarlane, 1995); F, Nyalam (Hodges et al., 1993); G, Dudh Kosi (Hubbard, 1989); H, Everest (Hubbard, 1989); I, Arun, black (Pognante and Benna, 1993), grey (Brunel and Kienast, 1986). In general, pressures within the Greater Himalayan Crystallines decrease from 6±8 kbar at the MCT to values of 3±4 kbars close to the STDS. Temperatures within both the Greater Himalayan Crystallines and Lesser Himalayan Formations tend to be 0600±7008C at the MCT but vary widely elsewhere.

MCT cutting up section at certain locations, and (3) the-south shear sense, referred to as the `MCT Zone' imbrication within the MCT hanging wall. In the cen- (Fig. 2). Note that this de®nition is distinct from the tral Himalaya, where the structural thickness of the question of whether an inverted metamorphic ®eld gra- Greater Himalayan Crystallines is typically 8±12 km, dient exists within the MCT hanging wall at certain lo- the inverted metamorphic section reaches sillimanite cations (e.g., Davidson et al., 1997). grade (Fig. 2). Thermobarometric studies of the Greater Himalayan Crystallines indicate a general 2.3. Himalayan leucogranites decrease in pressure with increasing distance above the MCT (Fig. 3). Typically, pressures of 7±8 kbar were An apparently unique feature of the Himalayan achieved adjacent to the MCT (kyanite grade), range is the presence of two roughly parallel granite whereas peak pressures at the structurally highest belts, the High Himalayan leucogranites (HHL) and levels were only about 3±4 kbar (sillimanite grade). the North Himalayan granites (NHG) (Fig. 1). The Metamorphism within the Lesser Himalayan High Himalayan leucogranites form a discontinuous Formations (Fig. 2) increases from zeolite to kyanite chain of sills exposed adjacent to the STDS (Fig. 1). grade over a north±south distance of 020 km. The Magmatic temperatures have been estimated at ca region approximately bounded by the garnet isograd 700±7508C (Montel, 1993). The North Himalayan in the Lesser Himalayan Formations and the hanging granite belt runs parallel to, and 080 km to the north wall gneisses of the Greater Himalayan Crystallines is of, the High Himalaya and is composed of about one typically characterized by a highly sheared, 4±8 km and a half dozen generally elliptical-shaped plutons. thick zone of distributed deformation with a top-to- Exposed plutons of the northern belt appear in general T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 759

Fig. 4. Schematic illustrations of the four dierent models proposed to explain the relationship of inverted metamorphism and/or anatexis to large-scale faulting within the Himalaya. (a) Type 1: Inverted metamorphism developed within the footwall of the MCT and anatexis in the hanging wall are spatially and temporally related by thrusting. This illustration, modi®ed from Huerta et al. (1996), shows an inverted metamorphic geotherm forming as a result of the transfer of radiogenic crust from the footwall of a subduction zone to the hanging wall. The direc- tion of material transport is shown by the line with the arrowhead, and the shaded region corresponds to crust accreted to the hanging wall. Isotherms are shown in 8C. (b) Type 2: Thrusting results from anatexis. The illustration shows the interpretation of Nelson et al. (1996) of a partially molten Tibetan middle crust that is being actively extruded southward. In this interpretation, the Greater Himalayan Crystallines is an earlier extruded equivalent. (c) Type 3: Anatexis results from normal faulting. This ®gure illustrates the change in the depth±temperature conditions in the footwall of a normal fault. The initial depth, ZA, is associated with temperature TA. Following slip on the normal fault, the depth is substantially reduced to ZB, but the eect of heat advection results in only minor cooling to temperature TB.If(ZA,TA) is immediately above the solidus of a vapor absent equilibria (e.g., muscovite dehydration melting), the passage to (ZB,TB) induces partial melting (see inset). (d) Type 4: Apparent inverted metamorphism in the footwall of the MCT is produced by deformation of two right-way-up metamorphic sequences. This illustration, modi®ed from Hubbard (1996), shows the eect of deforming a normal metamorphic sequence across a broad shear zone. Note that for the topography shown (solid curve), the bedrock exposure corresponds to a monotonic, left to right, increase in metamorphic grade. to have been intruded at relatively shallow depths into of Himalayan granite or their contact aureoles yield the Tethyan cover rocks and well above the STDS (Le either ca 15±19 Ma cooling ages or older dates which Fort, 1986). They dier from the HHL in their empla- re¯ect contamination by excess 40Ar (e.g., Copeland et cement style (Figs. 1 and 2) and possibly higher melt- al., 1990; Guillot et al., 1994; Harrison and Mahon, ing temperatures (>7508C), suggested by non-eutectic 1995). U±Pb dating of zircon and monazite are com- compositions and high light rare earth contents plicated by the low solubilities of these phases in leuco- coupled with low monazite inheritance (Debon et al., granite magmas (Harrison and Watson, 1983; Montel, 1986; SchaÈ rer et al., 1986; Montel, 1993; Harrison et 1993), the likelihood of their containing an inherited al., 1997b). component (Copeland et al., 1988), and the possibly of Assessment of the crystallization age of Himalayan U loss from monazite (R. Parish, personal communi- granites is complicated by the minimum melt character cation, 1999). Igneous monazites commonly incorpor- of these magmas. Heterogeneous 87Sr/86Sr and ate signi®cant 230Th during crystallization resulting in 143Nd/144Nd initial ratios (e.g., Le Fort et al., 1987) the production of unsupported 206PbÃ. However, have, with rare exception (Deniel et al., 1987), pre- 208Pb/232Th ion microprobe dating of monazite does cluded whole rock isochron dating. 40Ar/39Ar dating not suer from problems of disequilibrium PbÃ and 760 T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 the high spatial resolution of the measurement method 1973). A third class of models also makes no predic- generally permits the question of homogeneity of cal- tions regarding the development of inverted meta- culated ages to be explicitly examined. What we con- morphism but proposes that anatexis is due to sider to be the reliable crystallization ages of decompression resulting from slip on the STDS rather Himalayan leucogranites, many of which are derived than to thrusting (e.g., Harris and Massey, 1994). The from recent ion microprobe Th±Pb monazite dates, are fourth model type is primarily concerned with the enumerated in the caption of Fig. 1. Plutons from the inverted metamorphism and assumes no causal re- High Himalayan leucogranite belt vary in age from lationship between anatexis and faulting. These models 24.0 to 17.2 Ma, but most the large granite bodies infer that the distribution of metamorphic assemblages comprising the majority of the leucogranite were resulted from subsequent deformation (e.g., Hubbard, emplaced during two pulses at 2321 Ma and 1921 1996). We return to these models later to assess their Ma (Harrison et al., 1998b). Crystallization ages for compatibility with recently derived constraints granites we assign to the North Himalayan belt range described below. from 17.6 to 9.5 Ma. Note, however, that classifying Himalayan granites into two discrete belts may be misleading in that temporally contiguous, northward pro- 3. Recent developments pagating melting (Harrison et al., 1997b) could produce spatially discontinuous patterns due to vari- 3.1. Inverted metamorphism able exposure. Despite the common assumption of a causal link 2.4. Models relating inverted metamorphism, anatexis, between anatexis and inverted metamorphism across and faulting the MCT, the timing of recrystallization of the Lesser Himalayan Formation rocks in the MCT footwall had A variety of models have been proposed to explain not, until very recently, been directly dated. Harrison the relationship of inverted metamorphism and/or ana- et al. (1997a) utilized the observation that detrital texis to large-scale faulting within the Himalaya (Fig. monazite is generally destabilized in pelitic rocks 4). Models that assume that anatexis and inverted during diagenesis but reappears under lower amphibo- metamorphism are spatially and temporally related to lite grade conditions (e.g., Kingsbury et al., 1993) to each other and result from slip on the Himalayan establish the timing of garnet-grade metamorphism thrust system include melting induced by thermal aecting the Lesser Himalayan Formations in the cen- relaxation following nappe emplacement accompanied tral Himalaya. Allanite is the principal host of LREE by ¯uid in¯ux from the subducting footwall (e.g., Le in the Lesser Himalayan Formations schists at chlorite Fort, 1975), frictional heating during thrusting (e.g., and biotite grades, but reacts to form monazite near Arita, 1983; Molnar and England, 1990; England et the garnet isograd (05008C). Using the 208Pb/232Th ion al., 1992, England and Molnar, 1993), radioactive microprobe method (Harrison et al., 1995a), monazite heating alone or combined with other sources under grains, including one encompassed by a mm-sized gar- prolonged deep crustal residence (e.g., Molnar et al., net, were dated at 5.620.2 Ma. Thermobarometry in- 1983), and accretion of highly radioactive crust to the dicates that garnet growth occurred at a P±T of 6±8 MCT hanging wall coupled with high denudation (e.g., kbar and 5502308C. As the garnet and monazite iso- Royden, 1993; Huerta et al., 1996). The origin of the grads are nearly coincident in temperature (i.e., North Himalayan granites has not been so inextricably 05008C; Kingsbury et al., 1993), Harrison et al. tied to the development of Himalayan faulting, but (1997a) concluded that garnet growth in the presently their relative youth has been ascribed to a low rate of exposed rocks occurred at a depth of 025 km at 06 ¯uid in®ltration across the MCT (Le Fort, 1986) and Ma. Additional Th±Pb monazite dating in the Lesser heat focusing by thermal refraction o low thermal Himalayan Formations from the central and Garhwal conductivity Tethyan metasediments (Pinet and Himalaya (Catlos et al., 1997, 1999) all yield Late Jaupart, 1987). The second type of model proposes Miocene ages, indicating that this recrystallization that thrusting within the Himalaya is caused by melt- event was widespread across the Himalaya. K±Ar mica ing rather than vice versa. In this view, regions of the ages from N±S transects across these regions decrease crust thermally weakened by melting are the locus of from 20±16 Ma in the upper MCT hanging wall to 6± deformation the leads to large-scale faulting (e.g., Bird, 3 Ma within and adjacent to the MCT Zone (Hubbard 1978; Nelson et al., 1996; Davidson et al., 1997). and Harrison, 1989; Copeland et al., 1991; Metcalfe, While these models generally do not propose mechan- 1993). isms for the origin of inverted metamorphism, Bird Assuming that slip along the MCT had terminated (1978) did allow for the possibility that preexisting iso- during the Early Miocene, Harrison et al. (1997a) grads had been recumbently folded (e.g., Frank et al., explained the pattern of geochronological results as T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 761

Fig. 5. Schematic illustration of tectonic development of the Himalayan thrust system (vertical exaggeration 6:1). Active faults are shown as bold black lines while abandoned faults are indicated with bold gray lines. Symbols shown correspond to sample positions described in detail in Harrison et al. (1997a, 1998b). Open and closed symbols indicate initial and ®nal positions for the time interval shown. (a) Possible 25 Ma distribution of the protoliths of Greater Himalayan Crystallines (GHC) and Lesser Himalayan Formations (LHF) with respect to Indian cratonic margin after Eohimalayan thickening from ca 50±25 Ma. Future site of the High Himalayan leucogranite (HHL) source region is shown at 25 Ma. (b) Thrusting along the Main Himalayan Thrust (MHT) ¯at and Main Central Thrust (MCT) decollement from 25±15 Ma. Note that this fault system forms immediately above refractory rocks of the Indian craton. (c) Thrusting along MHT ¯at and MBT ramp from 15±8 Ma. Abandonment of the MCT ramp at 15 Ma causes accretion of upper LHF rocks to the hanging wall. (d) Out-of-sequence thrusting in the high Himalaya from 8±6 Ma involving upper LHF (approximately equivalent to reactivated MCT thrust ramp). (e) Activation of MCT-I and further development of MCT Zone (6±2 Ma) leads to accretion of lower LHF rocks to hanging wall. (f) Abandonment of the MCT zone at 2 Ma. Southward transfer of displacement to MFT ramp/MHT decollement. Present predicted positions of HHL and North Himalayan Granite (NHG) source regions are shown by regions shaded white. 762 T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 due to thrust reactivation during the Late Miocene. A sembled Fig. 5a, and that during the Eohimalayan break back from the MBT to the MCT, presumed to stage, the Greater Himalayan Crystallines protolith have steepened as a result of post-Early Miocene de- underwent high grade recrystallization and anatexis formation (Fig. 5d), could re¯ect a stress increase in (see PeÃcher, 1989; Hodges et al., 1994, 1996; Parrish the hinterland resulting from an increase in elevation and Hodges, 1996; Edwards and Harrison, 1997; of the Tibetan Plateau during the Late Miocene Coleman, 1998; Vance and Harris, 1999). As a conse- (Harrison et al., 1995b) or readjustment of the thrust quence, metamorphism and anatexis in the Greater wedge following signi®cant erosional and/or tectonic Himalayan Crystallines protolith would produce a denudation of the hanging wall (Dahlen, 1984). Early strati®ed paragenetic sequence in which dehydration Miocene slip along the MCT emplaced gneisses of the and partial melting reactions caused grade to increase Greater Himalayan Crystallines over the Lesser regularly with depth (Harrison et al., 1997b). Himalayan Formations along an initially well-de®ned The 2-D thermal evolution models of Harrison et al. fault resulting in burial metamorphism of the latter. (1997b) demonstrated that a shear stress of 30 MPa After a period of either inactivity or slip at relatively along a shallow dipping fault that cuts through a low rates during the Middle Miocene, renewed defor- metamorphically strati®ed crust is sucient to produce mation resulted in development of the broad under- two horizontally separated granite belts. Assuming lying MCT shear zone during the Late Miocene, and that the present-day Himalayan convergence rate of ultimately accretion of metamorphosed rocks of the 20 mm/yr (Bilham et al., 1997) is appropriate to the Lesser Himalayan Formations to the hanging wall period 24±10 Ma, the model predicted that a phase of during Late Miocene±Early Pliocene time. In this anatexis would begin almost immediately, provided view, the inverted metamorphism within this portion that the thermal structure developed during the of the Himalaya largely re¯ects tectonic juxtaposition Eohimalayan phase had not signi®cantly decayed. of two temporally unrelated, right-way-up meta- Refrigeration of the shear zone near the fault ramp, morphic sequences. Results of numerical modeling coupled with the propagation of the melting front (Harrison et al., 1997a) demonstrated that isotopic and toward the ramp, causes the ®rst predicted phase of petrologic constraints are met by assuming that the melting to cease at 020 Ma. A second phase of melting MCT fault reactivated at 8 Ma with a slip rate of begins when temperature exceeds the higher tempera- about 20 mm/yr, followed by activation of the MCT ture melting reaction at 18 Ma and continues until 12 Zone and accretion of garnet grade Lesser Himalayan Ma. Model predictions regarding horizontal separation Formations rocks between 6±4 Ma. While the thermal of the belts, their age contrast and emplacement styles evolution of the MCT zone predicted by this model appear to broadly accord with observation. satis®es both the Th±Pb age results from monazite in- clusions preserved within garnet and the observed dis- 3.3. A uni®ed model for Himalayan metamorphism and tribution of K±Ar mineral ages (Harrison et al., anatexis 1997a), it remains possible that reactivation of the MCT ramp initiated somewhat earlier than ca 8 Ma, The generally good match between geologic obser- or that it remained active (e.g., Henry and Copeland, vation and predictions of the models proposed for the 1999) at reduced rates throughout the Middle origin of the inverted metamorphic sequences Miocene. (Harrison et al., 1997a) and the two granite belts (Harrison et al., 1997b) demonstrated the physical 3.2. Origin of the two Himalayan leucogranite belts plausibility of these mechanisms, but stops well short of constituting conclusive evidence of their validity. In One of the principal implications of the recognition fact, evaluating each of these models in isolation pro- that recrystallization of the MCT footwall is a Late vides an unjusti®able degree of freedom Ð i.e., the Miocene phenomenon, and thus not temporally related temperature distribution and structural development to production of the Early Miocene High Himalayan predicted by the two granite model at ca 8 Ma should leucogranites, is that anatexis of the Greater be consistent with initial conditions of the model Himalayan Crystallines need not be restricted to the which successfully predicts the pattern and age of MCT ramp (cf. England et al., 1992). Harrison et al. inverted metamorphism. This problem can be further (1997b) explored an alternative model that ascribed constrained by incorporating estimates of the total the spatial and temporal variations of granite emplace- amount of slip along the MHT over the past 25 Ma. ment to continuous slip on a shallow dipping decolle- This requirement, and the recognition that the earlier ment that cuts through crust previously models had incorporated an overly simplistic melting metamorphosed during the Eohimalayan phase (Le model (Harrison et al., 1997b) and geothermal struc- Fort, 1996) of collision. They assumed that, immedi- ture (Harrison et al., 1997a), led Harrison et al. ately prior to collision, the northern Indian margin re- (1998b) to link the model for generation of the paired T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 763 granite belts with the development of the inverted muscovite-rich (relatively fertile) and plagioclase-poor metamorphic sequences. They adopted the assumption (relatively infertile) lithologies (e.g., Gardien et al., of Harrison et al. (1997b) that footwall rocks under- 1995). lying the basal decollement consisted of refractory The assumption that the onset of partial melting Indian basement that was not susceptible to partial does not appreciably lower rock shear strength is fun- melting at the temperatures <7608C, which has the damental to the Harrison et al. (1998b) model. This eect of limiting anatexis to hanging wall rocks over- behavior is indicated when dislocation creep is the lying the basal decollement. dominant mechanism and strain rate is low relative to Harrison et al. (1998b) were able to take advantage the rate of melt migration. Studies of texturally equili- of several recent advances in understanding of the brated materials indicate that plastic deformation can structure of the collision zone to further constrain squeeze excess melt from the rock leaving the defor- boundary conditions, model variables, and aspects of mation rate essentially unaected (e.g., Dell'Angelo the geometric evolution of the Himalaya. The general and Tullis, 1988; Rushmer, 1996). At strain rates structural framework was based on results of seismic appropriate to Himalayan convergence across a re¯ection pro®ling in southern Tibet which show the narrow shear zone (e.g., 1011 to 1012/s) and tem- Himalayan thrusts soling into a shallow (098) dipping peratures of 07508C, shear stress values of approxi- decollement (Zhao et al., 1993; Nelson et al., 1996; mately 10 MPa are predicted from empirical ¯ow laws Brown et al., 1996). The magnitude of shortening for both partially molten (e.g. Rutter, 1997) and solid within the Greater Himalayan Crystallines, Lesser quartzo-feldspathic materials (e.g., StuÈ we and Himalayan Formations, and Sub-Himalaya have Sandiford, 1994). recently been estimated from balanced cross sections The restricted time interval over which low-tempera- (e.g., Schelling, 1992; Srivastava and Mitra, 1994) that ture melting occurs in the Harrison et al. (1998b) suggest that about half of the ca 1000 km of shorten- model agrees well with observed span of HHL ages. ing between the Indian Shield and southern Tibet Although HHL ages vary from 24±17 Ma, the ma- (Chen et al., 1993; Patzelt et al., 1996) has been jority of the melt was produced at 2321Ma absorbed within the Himalaya. Geodetic measurements (Harrison et al., 1998b). Moreover, the agreement of indicate that the present convergence rate across the model predictions with the known crystallization ages Himalaya is 2021 mm/yr (Bilham et al., 1997). The of North Himalayan granites (17±10 Ma) is equally timing of activation of the MBT and MFT are broadly good. The apparent success in matching the apparent known (Schelling and Arita, 1991; Burbank et al., separation distance between the HHL and NHG of ap- 1996) and the age of both granite belts and recrystalli- proximately 80 km (Fig. 1) is the direct result of an zation of the MCT footwall are reasonably well arbitrary assumption regarding the position of the known (Harrison et al., 1997a,b). ramp/¯at and the prior history of Eohimalayan melt- The model of Harrison et al. (1998b) also employs a ing. The volume of magma calculated by the model is ramp-¯at geometry to simulate crustal thickening and broadly consistent with the 03% exposure of leucogra- assumes that the initiation of the Himalayan thrust nites in the Himalaya (Le Fort et al., 1987) and the system was localized at the boundary between the physical attributes of the two magmatic belts agree Greater Himalayan Crystallines and the Indian craton. reasonably well with the melting model of Harrison et The slip history they utilized is schematically depicted al. (1998b). For example, the relatively viscous melts in Fig. 5. Thrusting begins after the crust has been produced in the lower temperature melting reaction (ca thickened in response to 025 m.y. of Eohimalayan 7408C) are consistent with the emplacement of the crustal shortening (Fig. 5a) and proceeds at a constant HHL as syntectonic sills and dikes rather than diapirs. slip rate of 20 mm/yr. The resulting 250 km of short- The magmas formed by the higher temperature reac- ening in the hanging wall is accommodated as follows: tions (>7608C) predicted for the NHG are reasonably slip occurs on the MHT/MCT ramp between 25±15 expected to be suciently buoyant and thermally ener- Ma (Fig. 5b), on the MHT/MBT ramp between 15±8 getic to ascend as diapirs into the middle crust, con- Ma (Fig. 5c), and on the MHT and various fault sistent with the character of the NHG belt. The ramp- ramps de®ning the MCT zone between 8±2 Ma (Fig. ¯at model is consistent with the observation that the 5d±f). Greater Himalayan Crystallines immediately above the Using a shear stress of 30 MPa, Harrison et al. present exposure of the MCT (Fig. 5f) did not experi- (1998b) calculated the amount of heat produced by ence temperatures high enough to cause widespread dissipation and partitioned this thermal energy melting (Barbey et al., 1996). Migmatization higher up between heating the rocks and melting. They described section may re¯ect imbrication of the Greater melt production by an experimentally-determined re- Himalayan Crystallines (e.g., Davidson et al., 1997), lationship for melt fraction as a function of tempera- the advection of magmatic heat associated with leuco- ture appropriate for an intercalated sequence of granite emplacement into shallower crustal levels, or a 764 T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 nearly isothermal middle crust (e.g., Hodges et al., instead requires a complex, time-dependent transfer 1988) in which the geotherm is steeper than the dP/dT among several of these processes, often with multiple slope of the melting reaction. With respect to this lat- mechanisms operating simultaneously (Kong et al., ter point, the available thermobarometric data (Fig. 3) 1997; Harrison et al., 1998a). The tremendous amount and transition from kyanite to sillimanite with increas- of attention that has been directed at understanding ing structural position appear inconsistent with the the petrologic evolution of the Himalaya belies the fact existence of inverted metamorphism in the Greater that the two mechanisms most often called upon to Himalayan Crystallines, at least within the central produce crustal melting, footwall anatexis following Himalaya. overthrusting (e.g., England and Thompson, 1984) and Harrison et al. (1998b) found that varying certain underplating of ma®c magma (e.g., Huppert and model parameters could dramatically eect predicted Sparks, 1988), have rarely been invoked to explain melting histories. For example, small dierences in in- Himalayan anatexis (cf. Bird, 1978). Furthermore, sig- itial temperature distribution (10's of 8C) signi®cantly nature geologic features of the Himalaya appear to be altered calculated melt volumes, and reducing the ¯ow unique to that range. For example, we are unaware of stress within the shear zone below 10 MPa essentially any other mountain range that contains parallel belts eliminated melting due to biotite breakdown and fore- of leucogranites of contrasting age. Although our goal stalled initial melting at locations appropriate to the in understanding the tectonic development of the HHL for several million years. In contrast, the eect Himalaya cannot be to develop a general model for of increasing the dimensions of the shear zone in the direct export to other mountain belts, recovering model permits the melting of increasingly larger details of the geologic evolution should permit us to volumes of rock. better understand what factors control changes in tec- Temperature±time histories and peak pressures and tonic regime. temperatures predicted by the Harrison et al. (1998b) The models that have been advanced to explain the model as a function of structural distance from the relationship of inverted metamorphism and/or anatexis MCT generally agree reasonably well with measured to large-scale faulting within the Himalaya are charac- values. In addition to producing kinematic and ther- terized by their originality and ingenuity. However, mal histories consistent with the available thermochro- several can clearly be seen to fail tests of their predic- nology and geothermometry for the MCT zone, the tions in light of recent observations. In this section, we fault±bend±fold geometry employed in the Harrison et critically examine the four general groups of models of al. (1998b) model also accounts for the observed vari- Himalayan petrogenesis: (1) inverted metamorphism ations of metamorphic conditions within the MCT developed within the LHF and anatexis of the GHC hanging wall (e.g., Hodges et al., 1988). are spatially and temporally related by thrusting; (2) Thermobarometric results from the MCT hanging wall thrusting results from GHC anatexis; (3) GHC ana- indicate that shallower crustal levels are exposed to the texis results from normal faulting; and (4) apparent north. Such paleobarometric variation is consistent inverted metamorphism in the LHF is produced by de- with rotation produced by the fault±bend±fold geome- formation of two right-way-up metamorphic try (Ruppel and Hodges, 1994; Harrison et al., 1997a) sequences. and indicates an apparent lithostatic gradient of 00.27 kbar/km within the hanging wall (Hodges et al., 1988). 4.1. Type 1: Anatexis and inverted metamorphism are spatially and temporally related by thrusting

4. Evaluation of evolutionary models linking Type 1 models assume a causal relationship between deformation, magmatism, and metamorphism anatexis within the Greater Himalayan Crystallines and the inverted metamorphic sequence (Fig. 4a). The The spectacular nature of the mountain belt contain- general requirement of these models is an extraordi- ing the highest peaks on our planet has drawn a tre- nary source of heat to maintain high temperatures in mendous amount of interest to Himalayan geology. the GHC to permit melting while Indian underthrust- Despite this attention, we caution that what is learned ing cools the hanging wall from below. Although most from the Himalayan range may possess limited value type 1 models thus share a common ¯aw (i.e., the as a general model for orogeny. For example, the pat- underlying assumption that anatexis and the exposed tern of crustal displacements in response to the Indo- inverted metamorphic sequences are temporally re- Asian collision is inconsistent with simple permu- lated), aspects of the processes they invoke may be ap- tations of end-member tectonic mechanisms (e.g., plicable to originally contiguous, though now widely Indian or Asian underthrusting, continental injection, spaced, footwall and hanging wall rocks of the mantle delamination, delayed underplating, orogenic Himalayan thrust system. collapse, lateral extrusion, intra-arc thickening), but The ®rst modern type 1 model was Le Fort's (1975) T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 765 proposal that thermal relaxation following thrusting MPa) be maintained in hydrated crustal rocks at liqui- along the MCT heated the footwall suciently to dus temperatures (e.g., Bird, 1978; Molnar et al., induce dehydration reactions. When these ¯uids were 1983), but who nonetheless accepted a causal link introduced into the still hot hanging wall, they ¯uxed between melting and inverted metamorphism, sought the gneisses producing the leucogranite melts. other possible sources of heat to explain Himalayan Anticipating that this mechanism alone was not petrogenesis. One possible heat source, suggested by capable of creating a lateral thermal gradient high the relatively high U and Th concentrations of the enough to create the apparent inverted metamorphism Himalayan gneisses, is radioactivity. Molnar et al. while maintaining hanging wall temperatures sucient (1983) explored this avenue and found that, coupled for anatexis to occur, Le Fort (1975) introduced shear with the generally high heat ¯ow inferred for northern heating to this role. As noted above, the fact that India (e.g., Rao et al., 1976; cf. Gupta, 1993), radio- GHC melting and LHF recrystallization in the pre- active heat generation of several mW/m3 could create sently exposed section are not temporally related obvi- conditions conducive to melting after several ten's of ates the need for high hanging wall temperatures millions of years of continental subduction. Royden during thrusting along the ramp. Furthermore, the (1993) examined the steady-state eect of accreting necessity of introducing ¯uids to produce conditions highly radioactive material from an accretionary prism appropriate for melting has been challenged on at least to the hanging wall of a subduction zone under con- two grounds. Harris et al. (1993) noted that the high ditions of rapid erosion and concluded that this scen- Rb/Sr ratios of Himalayan leucogranites relative to ario could explain Himalayan anatexis and the their assumed GHC source rocks precludes water-satu- apparent inverted geothermal structure. Huerta et al. rated melting and instead favors muscovite dehy- (1996) further investigated the time-dependent eects dration melting (e.g., Thompson, 1982). Speci®cally, of erosion and accretion of heat producing elements the comparatively small proportion of Rb-rich mica on the thermal structure of orogenic systems. They relative to Sr-rich feldspar consumed in water-satu- directly compared the results of the model following rated melting could not produce elevated Rb/Sr ratios 32 m.y. of evolution to the MCT thrust zone and con- whereas modally more signi®cant breakdown of mus- cluded that this scenario may have exerted a ®rst order covite during dehydration melting has the potential to control on the thermal and metamorphic evolution of do so. Moreover, recent experimental results show that the Himalayan orogen. Henry et al. (1997) adopted the water-saturated melting of Himalayan source rocks main elements of the Royden (1993) model with an under likely crustal conditions yield melts of trondhje- emphasis on understanding the eect of the thermal mitic composition (PatinÄ o Douce and Harris, 1998) structure on Himalayan topography. Limitations of that are unlike the leucogranite compositions observed. these continuous accretion±erosion models, such as the The basic elements of the Le Fort (1975) model (i.e., predication of widespread exposure of rocks from synchroneity of anatexis and inverted metamorphism, depths far greater than recognized, can be circum- ¯uid-present melting, shear heating) were subsequently vented by addition of a thrust ¯at in the model (e.g., adopted by numerous workers, but perhaps no more Henry and Copeland, 1999). However, in addition to enthusiastically than by Phillip England and his co- the unsupported assumption that a high (ca 58C/km) workers who developed this problem quantitatively in horizontal thermal gradient was maintained on the a series of papers extending over 17 years (e.g., MCT ramp during the Early Miocene, implications of Graham and England, 1976; Molnar and England, this variant of the type 1 model (i.e., Royden, 1993; 1990; England et al., 1992, England and Molnar, Huerta et al., 1996, 1998; Henry et al., 1997) appear 1993). The argument stressed in these papers is that inconsistent with two fundamental characteristics of the observed thermal structure requires shear stresses Himalayan anatexis; the synchronous appearance of along the MCT in the range 100±1100 MPa (England the voluminous HHL magmas and their isotopic char- and Molnar, 1993). These values, however, appear to acter. far exceed an upper bound for ductile shearing of a The large plutons of the Garhwal, Manaslu, and few 10's of MPa inferred from paleopiezometry (e.g., Makalu regions, which comprise the majority of leuco- Engelder, 1993), laboratory deformation experiments granite exposed along the western and central High (e.g., Rutter, 1997), or tectonic modeling (e.g., Kong Himalaya (Le Fort et al., 1987), appear to have been and Bird, 1996). Moreover, relative to the wet melting emplaced at 2321 Ma (SchaÈ rer, 1984; Harrison et al., conditions they assumed, even higher shear stresses 1997b). No older plutons of comparable size have than those advocated by England and co-workers been documented. Given that ten's of millions of years would be required to create conditions suitable for are required by the radiogenic heating/rapid erosion vapor absent melting of metapelitic source rocks. mechanism to create conditions suitable for melting, Other workers who were critical of the requirement this model would appear not to predict the synchro- that extraordinarily high shear stresses (i.e., e100 nous appearance of melting across the collision front. 766 T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772

Such behavior is more consistent with a mechanism On the basis of seismic re¯ection pro®ling along a that produces localized thermal anomalies, such as southern Tibetan graben, Nelson et al. (1996) advo- shear heating. cated a reversal of roles for thrusting and anatexis. The second potential weakness of the radiogenic They interpreted bright spot anomalies beneath the heating/rapid erosion model (e.g., Royden 1993; Yadong±Gulu rift as indicating that the Tibetan Huerta et al., 1996) is the requirement that signi®cant middle (15±20 km) crust is partially molten, and specu- material transfer occur from the footwall of the lated that the region between the MCT and STDS is Himalayan thrust to the hanging wall. As described the earlier extruded equivalent (Fig. 4b). The variation earlier, the MCT marks the boundary between the in age among the Himalayan granites is interpreted to Middle Proterozoic Lesser Himalayan Formations and re¯ect a semi-continuous record of this partially mol- Late Proterozoic Greater Himalayan Crystallines. ten, mid-crustal layer. The Nelson et al. (1996) model These two rock types can be clearly distinguished on requires temperatures sucient for muscovite dehy- the basis of detrital zircon ages (Parrish and Hodges, dration melting (i.e., 07508C; Harris and Inger, 1992) 1996) and isotopic composition (Deniel et al., 1987; at a depth of 15 km, but, because the seismogenic Derry and France-Lanord, 1996). For example, the mantle beneath southern Tibet is constrained to be Lesser Himalayan Formations are characterized by an 07508C (Ruppel and McNamara, 1997), the crust average 87Sr/86Sr 1 1.1 whereas the Greater would have to be essentially isothermal. The generally Himalayan Crystallines range only from 0.73±0.78 high heat ¯ow (Francheteau et al., 1984) and upper (France-Lanord and Le Fort, 1988; Guillot, 1993). crustal residence of the Curie isotherm (Nelson et al., Had lower plate rocks continuously accreted to the 1999) in southern Tibet are both consistent with elev- MCT hanging wall, the distinctive isotopic character ated temperatures within the middle crust. The exist- of the LHF and GHC, as well as the granites derived ence of anomalously warm conditions beneath from the GHC, would not be expected to persist to the southern Tibet throughout the Miocene and persisting present. While this speci®c objection precludes the until present-day appears to be explicable by the pre- large-scale addition of material to the hanging wall viously described continuous accretion±erosion models. from the LHF, the general principle of the model may For example, the nearly isothermal structure required still be valid (i.e., accretion coupled with rapid erosion by the Nelson et al. (1996) model from 15 km depth to is restricted to within the GHC). the base of the crust is similar to that predicted at 20 Although radioactive heat generation has also been Ma for the seemingly reasonable case of a convergence suggested as a contributing cause for the melting of rate of 15 mm/yr, an erosion rate of 1 mm/yr, and a the North Himalayan granites (e.g., Pinet and Jaupart, radioactive heat production of 2.5 mW/m3 (Henry et 1987), this proposal is not subject to the timing con- al., 1997). However, this model requires uniformly straints imposed by the 2321 Ma age of the HHL rapid erosion from a region equivalent to the 0200- nor does it involve accretion of LHF material. km-wide zone between the trace of the Himalayan thrust system and the Indus Tsangpo suture. 4.2. Type 2: Himalayan thrusting is caused by melting Moreover, the model geometry predicts increasingly deep levels of exposure to the north. In fact, the level In contrast to type 1, this class of model proposes of exposure throughout the vast majority of this region that thrusting within the Himalaya is caused by heat- is greenschist grade with only minor amounts ing/melting rather than being a consequence of it. Bird (<15 km) of post-Oligocene exhumation indicated for (1978) argued that mantle lithosphere delamination much of this area (e.g., Ratschbacher et al., 1994). The beneath the Himalaya was a plausible consequence of deep crustal exposures are largely restricted to the continental collision. One eect of delamination would GHC, which forms a rather narrower (i.e., 5±100 km be to uplift the crust immediately above the region wide) aperture than required by the Henry et al. (1997) where the mantle lithosphere had been removed. This model and occurs at the opposite side of the wedge in turn would create a large strain at the edge of the (i.e., to the south) than predicted. The fact that the delaminated region making it a likely site for for- Tethyan sediments have not been completely removed mation of a large-scale thrust. As asthenospheric heat from atop the heat producing element enriched Indian diuses upward, the thrust propagates into the newly supracrustal section represents a severe shortcoming of weakened crust. Although Bird (1978) showed quanti- this model. tatively that this model could produce sucient heat The model of Nelson et al. (1996) has been chal- to produce synchronous anatexis and inverted meta- lenged on several fronts. Makovsky and Klemperer morphism over a period of 5±10 Ma, it is now clear (1999) concluded that the bright spots do not represent that crustal melting and metamorphism in the exposed melts, but rather regions containing on the order of MCT footwall occurred at widely separated times 10% aqueous ¯uids. Owens and Zandt (1997) found (Harrison et al., 1997a). that the seismic velocity structure and magnitude of T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 767

Poisson's ratio beneath southern Tibet indicate a gen- Le Fort, 1995; Searle et al., 1997; Davidson et al., erally cold crust and explained this as due to the 1997; Harrison et al., 1998b). However, the correlative underthrusting of the Indian craton beneath southern proposal of Harris and co-workers that muscovite de- Tibet. The broadly coherent stratigraphy within the hydration melting was triggered in the Himalaya by GHC (Le Fort, 1996) appears inconsistent with the tectonic decompression rather than by heating appears extrusion model of Nelson et al. (1996). Basement problematic to us for the following reasons: (1) the rocks uplifted on the ¯ank of the Yadong±Gulu rift extremely rapid and large magnitude denudation that are in essence a window into the Tibetan middle crust. are required by the minor eect that decompression While this terrane contains leucogranites that were has on melting likely source rock compositions; (2) the intruded immediately prior to the initiation of detach- diculty of producing multiple anatectic phases via ment faulting at 08 Ma, reconnaissance-scale obser- decompression; and (3) the lack of de®nitive timing vations are not suggestive of widespread, in situ partial constraints linking slip on the STDS with anatexis. melting at that time (Pan and Kidd, 1992; Harrison et The depth dependence of muscovite dehydration al., 1995b; D'Andrea et al., 1999). Isotopic tracers melting in the lower crust of 03.78C/km (Huang and from these Middle Miocene granitoids indicate the pre- Wyllie, 1973) is small relative to the geotherm expected sence of a ca 2 Ga component (D'Andrea et al., 1999), at these depths (015±208C/km). Biotite dehydration is in contrast to the protolith of crustal melts from else- even less of a factor since dP/dT slopes of relevant where in the Lhasa Block which are characterized by breakdown equilibria are even steeper. Consequently, ca 1 Ga neodymium model ages (Harris et al., 1988). decompression melting is a relatively inecient process Lincoln Hollister and his co-workers (e.g., Swapp when compared to anatexis on a thrust ¯at driven by and Hollister, 1991; Hollister, 1993; Davidson et al., modest dissipative heating (Harrison et al., 1998b). 1997) have emphasized the role of anatexis in localiz- For example, to produce a melt fraction of rock by ing strain to approximately doubly thicken the GHC decompression melting comparable to that resulting in the Bhutan Himalaya. Davidson et al. (1997) con- from 1 m.y. of subhorizontal 20 mm/a slip (Bilham et cluded that the advection of heat by thrusting and syn- al., 1997) at 30 MPa shear stress requires 030 km of tectonic migration of leucogranites from the base of denudation over 1 m.y (Harrison et al., 1999). The the GHC contributed to the formation of an inverted eect produced by slower denudation is negligible. For metamorphic ®eld gradient within the MCT hanging example, Harrison et al. (1998b) found that incorpor- wall. While possibly valid for the Bhutan region, struc- ating a denudation rate characteristic of that documen- tural elements unique to that region (e.g., the ted for the Himalaya (01 mm/a; e.g., Henry et al., Kakhtang thrust, >20 km thickness of the GHC) 1997) into a model evaluating the conditions required (Gansser, 1964) may restrict their model from being for melting along an MHT-type thrust ramp (see Zhao generalized to much of the rest of the Himalaya. et al., 1993) did not perceptibly increase the melt fraction. 4.3. Type 3: Himalayan anatexis results from Episodic melting by decompression is particularly decompression melting during slip along the STDS problematic. Recently, several studies have documented cases of protracted anatexis at certain locations in A third class of model (Fig. 4c) links anatexis the Himalaya (e.g., Searle et al., 1997; Harrison et al., directly to decompression melting related to slip on the 1999). Harrison et al. (1999) assessed the feasibility of STDS rather than to thrusting (e.g., Harris et al., decompression melting in producing the two pulses of 1993), although the development of the STDS may ul- magmatism at 2321 and 1921 Ma documented to timately be a response to thickening via the various have built the Manaslu intrusive complex. Using a nu- splays of the MHT (e.g., Burg et al., 1984). This merical thermal model, they calculated P±T±t paths in model is super®cially attractive because of the positive the footwall source region at horizontal distances of 5, dP/dT of vapor absent equilibria, particularly for reac- 10, and 20 km from a normal fault dipping at 308. tions involving muscovite, coupled with the large verti- After introducing a pressure dependence to Harrison cal displacement inferred across the STDS (Burch®el et et al.'s (1998b) melting model, they found that while a al., 1992). 10% initial melt could be obtained for source regions Nigel Harris and his co-workers (e.g., Harris and located >10 km from the fault given a slip rate of Inger, 1992; Harris et al., 1993; Harris and Massey, 20 mm/yr (i.e., a 10 mm/yr denudation rate), lower 1994; PatinÄ o Douce and Harris, 1998) used a variety slip rates or positions closer to the normal fault did of approaches, including trace element modeling and not result in conditions appropriate to melting. For lo- experimental petrology, to establish that most cations far from the fault, it was found that an initial, Himalayan anatexis occurred by ¯uid absent reactions rapid decompression followed relatively shortly there- rather than via water-saturated melting. This view has after by a second, similarly rapid pulse of exhumation subsequently been widely accepted (e.g., Guillot and could create temporally distinct melting episodes. For 768 T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 example, a vertical component of slip of 10, 2.5 and restricted to the MCT ramp, has largely removed the 10 mm/yr for the time intervals 0±1, 1±3 and 3±4 m.y., need for exceptional heat sources as proposed by type respectively, produces two melting phases separated by 1 models. While essentially all models described above 4 m.y. However, the extreme (and improbable) denu- can explain the generation of leucogranite in the hang- dation rates required by the model to produce the two ing wall, none can account for hanging wall anatexis temporally distinct melting episodes would create wide- and footwall metamorphism being separated in time spread exposure of rocks originating from 10±12 kbar by more than 10±15 Ma (e.g., Harrison et al., 1997a; depths. Such high pressure assemblages are not recog- Catlos et al., 1997, 1999). As described in the previous nized within or adjacent to the Manaslu intrusive com- section, this requirement led us (Harrison et al., 1998b) plex (Fig. 3), in the nearby HHC gneisses, or, with to propose that inverted metamorphism within the minor exceptions, elsewhere in the Himalayan range. footwall was created and later juxtaposed against the In addition, timing constraints that de®nitively link granite source region in the hanging wall. slip on the STDS with anatexis are lacking. Presently, A rather dierent approach was taken by Jamieson there is little direct evidence that the STDS was active et al. (1996) who proposed a mechanism by which an between 24±19 Ma when the majority of the HHL apparent inverted metamorphic sequence could be pro- formed. Although earlier reports from north of Mt. duced in the absence of an inverted crustal isotherm. Everest indicated otherwise (e.g., Hodges et al., 1992; Their model assumes a highly speci®c geometry in Harrison et al., 1995a), the timing of ductile defor- which the orogen is characterized by a strain singular- mation related to the STDS is only constrained to ity at the point where the subducting mantle litho- have occurred in the interval 17±14 Ma (SchaÈ rer et al., sphere detaches from the thickening crust. This 1986; Hodges et al., 1998; Murphy and Harrison, con®guration produces two symmetrical, oppositely 1999). dipping, intra-crustal thrusts referred to as the pro- and retroshear zones. One consequence of crustal 4.4. Type 4: Inverted metamorphism is due to thickening in this geometry is the tectonic juxtaposi- transposition of right-way-up metamorphic sequences tion of widely diering initial positions that reach peak metamorphic grade at varying times and locations A few investigators have emphasized the role of ret- within the orogen. Jamieson et al. (1996) compared rograde transposition of a normally zoned meta- model predictions with P±T data from the MCT zone morphic sequence to create the inverted pattern of of central Nepal and concluded that an inverted crus- isograds developed beneath the MCT (Fig. 4d). tal thermal gradient was not required to explain the Models have been proposed that involve ductile fold- inverted metamorphic sequence which instead could ing of pre-existing isograds (Searle and Rex, 1989; result from dierential horizontal transport and exhu- Grujic et al., 1996), imbricate thrusting (Arita, 1983; mation of deep-seated metamorphic rocks. They did Brunel and Kienast, 1986), and ductile shearing of an not speci®cally address the origin of crustal melting or existing zone right-way-up metamorphic sequence the fate of the Indian lithosphere that previously (Hubbard, 1996). Although each of these mechanisms underlay the Himalaya. can explain the creation of an apparent inverted meta- Jamieson et al. (1996) acknowledge that their model morphic sequence, this category of models has gener- challenges conventional wisdom in positing the MCT ally been viewed as either inconsistent with geological as a retroshear of opposing vergence to that of the constraints (e.g., PeÃcher, 1989) or of limited import- subduction zone, usually assumed to be south-directed ance (e.g., England et al., 1992). Field relations alone as India thrusts beneath southern Asia. Rather, they have not been able to select between model types 1 suggest that the Asian mantle was subducted beneath and 4 and, until recently, no dating method had been India during the period in which they assumed the developed capable of establishing the timing of foot- MCT zone formed. Although their model is provoca- wall recrystallization. tive in showing how an inverted metamorphic sequence Although the above mentioned mechanisms stand could be dynamically created in the absence of an apart from type 1 models in rejecting the role of the inverted crustal geotherm, we have signi®cant reser- hanging wall as heat source in producing the inverted vations regarding their view of its application to the metamorphic sequences, they have also tended to Himalaya. First, we question whether present geologic assume that peak metamorphism recorded in the hang- and geophysical data are compatible with either sub- ing wall and footwall occurred simultaneously. The duction geometry. Seismic evidence appears to support recognition that recrystallization of the MCT footwall a north-dipping Moho beneath the Himalaya (e.g., is a Late Miocene phenomenon (Harrison et al., Nelson et al., 1996; Owens and Zandt, 1997; Henry et 1997a), and thus not temporally related to production al., 1997). Although the Jamieson et al. (1996) model of the Himalayan leucogranites, and the corollary that does predict diachronous peak metamorphism, the pat- Greater Himalayan Crystalline anatexis need not be tern is inconsistent with the bimodal timing constraints T.M. Harrison et al. / Journal of Asian Earth Sciences 17 (1999) 755±772 769

(Harrison et al., 1997a) that have been brought to Acknowledgements light subsequently. Although Jamieson et al. (1996) do not oer candidates for the proshear in a central This research was sponsored by grants from the Himalayan context, the Renbu±Zedong Thrust (Fig. 2) National Science Foundation. We thank Professors Le has the appropriate geometry. However, present timing Fort and Upreti for inviting this contribution, and constraints (Quidelleur et al., 1997) suggest that it was Nigel Harris, Leigh Royden and Peter Bird for helpful inactive between 24±18 Ma and 8±4 Ma when the reviews. MCT is known to have accommodated signi®cant displacement (Hodges et al., 1996; Coleman, 1998; Harrison et al., 1997a). References

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