E.J. Catlos, T. Mark Harrison, M.P. Searle, and M.S. Hubbard

The Main Central Thrust (MCT) which extends across the 2500 km length of the Himalayan orogen is the dominant structural feature of that mountain belt. The MCT thrusts Late Proterozoic gneisses (Greater Himalayan Crystallines) over Middle Proterozoic schists (Lesser Himalaya Formations) and accommodated several 100’s of km of displacement [DeCelles et al., 1998]. A 4-8 km thick shear zone, the MCT Zone, underlies the MCT fault. The lack of pronounced structural and metamorphic breaks across and within the MCT and MCT Zone makes it difficult to recognize surfaces that accommodated significant displacement. Over a N-S distance of ~20 km, metamorphism within the Lesser Himalaya Formations increases up section (i.e., is ‘inverted’) towards the fault from zeolite to kyanite grade [e.g., Colchen et al., 1986]. A common point of departure for many models seeking to account for the juxtaposition of inverted metamorphic sequences in the footwall of the MCT against a belt of Early Miocene leucogranites emplaced above the fault was to assume that their petrogeneses are temporally related.

Geochronological studies have established that the MCT hanging wall was deforming at ~22 Ma [e.g., Hodges et al., 1996]. Th-Pb ion microprobe analyses of monazites from the inverted metamorphic sequence led Harrison et al. [1997] to conclude that the MCT Zone was active at ~6 Ma, or 10-15 Ma later than suggested by the timing of deformation in the hanging wall. They interpreted their results as indicating that a large-scale reactivation of the MCT occurred during the Late Miocene. However, the generality of their conclusion awaits confirmation that this episode affected most other locations across the collision front.

To assess the lateral extent of Late Miocene reactivation, samples were obtained from transects perpendicular to the belt in the Marysandi River and Darondi Khola, central Nepal, the Bhagirathi River, Garhwal Himalaya, and Dudh Kosi, south of Mt. Everest [Figure 1]. In central Nepal, the samples are structurally within ~2 km of an important tectonic break within the Lesser Himalaya Formations, defined as the MCT-I by Arita [1983]. The Dudh Kosi sample is from graphitic schists of the upper Lesser Himalaya Formations [85H20g; see Hubbard, 1989]. The Garhwal sample is from upper structural levels of the MCT zone [GM74; see Metcalfe, 1993]. Using the UCLA CAMECA ims 1270 ion microprobe [Harrison et al., 1995], we dated both monazite inclusions in garnets and monazites within the rock matrix from samples collected in central Nepal and Everest and only matrix monazites from the Garhwal sample. We believe these ages represent monazite growth during recrystallization associated with reactivation. These results indicate that post-Early Miocene reactivation of the MCT footwall occurred over an along strike distance of at least ~800 km [Figure 1].

Monazites situated in the upper Lesser Himalaya Formations (i.e., graphitic schists) typically yield Middle Miocene ages. Catlos et al. [1998] report 13-11 and 15.8 Ma monazites ages from samples collected along the Darondi Khola and here we report 20-13 Ma ages from samples collected along the Dudh Kosi-Everest transect, and 30-6 Ma and 39-11 Ma ages from samples collected along Marysandi River. These older ages, probably reflective of diffusive Pb loss, are interpreted to indicate that the MCT hanging wall was maintained at high temperature during the Early Miocene and cooled as a consequence of Late Miocene MCT reactivation. Evidence for this suggestion comes from depth profiling measurements made on monazite grains separated from a pegmatite immediately above the MCT which revealed the existence of a Pb diffusion profile within 2 mm of grain surfaces [Catlos et al., 1998].

Late Miocene monazite ages from rocks collected along transects spaced ~20 km apart in central Nepal and over 400 km west in the Garhwal Himalaya reveal evidence of widespread reactivation of the MCT and development of the MCT Zone. These ages places important constraints on models proposed to describe the evolution of the Himalaya. Some of the earliest models include Le Fort’s [1975] thrusting of the relatively cold Lesser Himalaya Formations under the hotter Greater Himalayan Crystallines in the presence of dissipative heating to create the inverted signature. Fluids released from the footwall during thrusting migrated to the MCT hanging wall to form the granites. Subsequently, shear stresses of ³ 100 MPa [e.g., England et al., 1992] were thought to be necessary to explain anatexis and inverted metamorphism. The widespread occurrence of Late Miocene recrystallization in the inverted metamorphic sequences strongly suggest that the presently exposed inverted metamorphic sequences and Early Miocene crustal melts are not temporally related and thus there is little justification in seeking a causal relationship between the two.

The inverted metamorphic sequences beneath the MCT ramp appear to have formed during reactivation of the thrust following ~10 m.y. of inactivity. Measured monazite ages from the lower Lesser Himalayan Formations rocks are restricted to the interval 8-5 Ma that we interpret to reflect reactivation of the MCT at ca. 8 Ma and activation of the MCT Zone at ~6 Ma. The contrast between these ages and those of the graphitic schists confirm the presence of an important structural break within the lower Lesser Himalayan Formations, as proposed by Arita [1983] (i.e., the MCT-I). Harrison et al. [1997] propose that the inverted metamorphism is the result of the accretion of successive tectonic slivers of the Lesser Himalaya Formations to the hanging wall and attributes anatexis to much earlier shear heating on a continuously active thrust flat that had previously experienced partial melting.

Figure 1: Generalized Geological Map of Nepal with geological units, faults, and late Miocene reactivation ages indicated.

Arita, K., 1983, Origin of the inverted metamorphism of the Lower Himalaya, central Nepal, Tectonophysics, 95, p. 43-60.

Catlos, E.J., T.M. Harrison, M. Grove, O.M. Lovera, A. Yin, M.J. Kohn, F.J. Ryerson, P. Le Fort, and B.N. Upreti, 1997, Further evidence for Late Miocene reactivation of the Main Central Thrust (Nepal Himalaya) and the significance of the MCT-I, EOS Transactions, AGU 78, Fall Meeting Supplement, p. F651.

Colchen, M., P.LeFort, and A. Pecher, 1986, Annapurna-Manaslu-Ganesh Himal, Paris, Centre National de la Recherches Scientifiques, pp.136.

DeCelles, P.G., G.E. Gehrels, J.Quade, T.P.Ojha, P.A. Kapp, and B.N. Upreti, 1998, Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal, GSA Bull., 110, p. 2-21.

England, P., P. Le Fort, P. Molnar, and A. Pecher, 1992, Heat sources for Tertiary metamorphism and anatexis in the Annapurna-Manaslu region, Central Nepal, J. Geophys. Res., 97, p. 2107-2128.

Harrison, T.M., F.J. Ryerson, P. Le Fort, A.Yin, O.M. Lovera, and E.J. Catlos, 1997, A late Miocene-Pliocene origin for Central Himalayan inverted metamorphism, Earth and Planetary Science Letters, 146, p. E1-E8.

Harrison, T.M., K.D. McKeegan, and P. Le Fort, 1995, Detection of inherited monazite in the Manaslu leucogranite by ²⁰⁸Pb/²³²Th ion microprobe dating: crystallization age and tectonic implications, Earth and Planetary Science Letters 133, 271-282.

Hodges, K.V., R.R. Parrish, and M.P. Searle, 1996, Tectonic evolution of the central; Annapurna Range, Nepalese Himalayas, Tectonics, 15, p. 1264-1291.

Hubbard, M.S., 1989, Thermobarometric constraints on the thermal history of the Main Central Thrust zone and Tibetan slab, eastern Nepal Himalaya, J. Metamorphic Geol., 7:19-30.

Le Fort, P., 1975, Himalayas, the collided range. Present knowledge of the continental arc, Am. J. Sci., 275A, p. 1-44.

Metcalfe, R.P, 1993, Pressure, temperature, and time constraints on metamorphism across the Main Central Thrust zone and High Himalayan Slab in the Garwhal Himalaya, In Himalayan Tectonics, edited by MP Searle and PJ Treolar, Geological Society Special Publication, 74, pp. 485-509.