Crustal Melting, Ductile Flow, and Deformation in Moun- Tain Belts

RESEARCH FOCUS

Crustal melting, ductile ﬂ ow, and deformation in mountain belts: Cause and effect relationships

Mike Searle DEPT. EARTH SCIENCES, OXFORD UNIVERSITY, SOUTH PARKS ROAD, OXFORD OX1 3AN, UK

ABSTRACT

Exhumed sections of migmatites are beautifully exposed in the middle crust of old orogens such as the Proterozoic Wet Mountains of Colo- rado and young Tertiary–active orogens such as the Himalaya and Karakoram. Migmatites and leucogranites occur both on a regional scale (e.g., Greater Himalayan Sequence) and along more restricted shear zones and strike-slip faults (e.g., Karakoram, Jiale, and Red River faults). Melting and deformation are clearly diachronous across orogenic belts over space and time, yet in general, deformation must precede regional metamorphism and melting in order to thicken the crust and increase pressure and temperature. Some deformation can be synchronous with partial melting and there is almost always post-melting deformation along shear zones or faults. The distinction between pre-, syn-, and post-kinematic granites in three dimensions, in pressure-temperature space, and in time becomes critical. In particular, mapping of macro- and micro-structures combined with precise U-Th-Pb dating of migmatitic leucosomes and granitic rocks in deformation zones can be used to constrain the relative timing of metamorphism, melting, ductile shearing and brittle faulting. In the Himalaya, multiple sill intrusions emanating from a regional migmatite terrane fed growth of leucogranite bodies over a time span of ~30 m.y. and channel ﬂ ow, the southward extrusion of a partially melted section of the mid-crust, occurred along the Himalaya during the Miocene from ca. 24–15 Ma. The Karakoram batholith formed by pre- to post-collisional metamorphism, migmatisation and magmatism over a period lasting at least 65 m.y. Comparisons of the Himalaya and Karakoram migmatite-granite belts with the Proterozoic Wet Mountains in Colorado provide strong evidence for weak middle crust capable of aseismic ﬂ ow, leading to question models of lithospheric rheology that call on strong middle crust.

LITHOSPHERE; v. 5; no. 6; p. 547–554 | Published online 30 September 2013 doi: 10.1130/RF.L006.1

INTRODUCTION seismometers (Hetényi et al., 2007; Nábelek et al., 2009) have imaged the Moho deepening beneath the Himalaya to a depth of ~75 km beneath the Exhumed sections of middle crustal rocks that underwent partial Tibetan plateau with a prominent mid-crustal layer between ~15–30 km melting and are composed of regional migmatites are known from sev- depth, interpreted as a zone of partial melting. Magnetotelluric data and eral mountain ranges including the Proterozoic Wet Mountains of central seismic “bright spots” at depths of only 15–18 km beneath southern Tibet Colorado (Levine et al., 2013) and the Tertiary–ongoing Himalaya-Kara- (Unsworth et al., 2005) are pockets of liquid interpreted as anatectic gran- koram ranges (Fig. 1; Searle et al., 2003, 2006, 2010a, b, 2011; Law et ites forming today at similar depths and pressures as the Miocene leuco- al., 2004, 2011; Jessup et al., 2006, 2008). Comparing old Precambrian granites along the Greater Himalaya most of which formed at 20–17 Ma orogens to the geologically young India-Asia collision along the Hima- (Searle et al., 2003, 2006; Gaillard et al., 2004). The major refl ectors laya-Karakoram-Tibet (HKT) orogen has proven extremely useful (e.g., bounding the partially molten middle crust can be traced southwards into St-Onge et al., 2006). Precambrian orogens tell us what the lower crust alignment with the two north-dipping crustal-scale shear zones that bound looks like, whereas the HKT orogen gives a broad indication of the struc- the Greater Himalayan Sequence (GHS)—the Main Central Thrust (MCT) ture and composition of an active and ongoing mountain range formed by along the base, and the South Tibetan Detachment (STD) low-angle nor- continental collision. Geological mapping combined with thermobarom- mal fault along the top (Fig. 2). Between these two synchronously active etry, metamorphic modeling and U-Pb geochronology along the Himala- Miocene ductile shear zones a mid-crustal layer of migmatites and leu- yan (Indian plate) and Karakoram (Asian plate) ranges has resulted in a cogranite sheets makes up most of the GHS. More than ~50–100 km of better understanding of the structural and thermal evolution of continental southward extrusion of partially molten mid-crust during the Miocene has crust in a classic continent-continent collision belt. been interpreted from the synchronous timing of granitic melts (ca. 24–15 One of the most exciting recent discoveries is that regions of thickened Ma U-Pb ages) and similar offsets along the MCT and STD (Searle and continental crust can host enormous volumes of partially molten rocks, Szulc, 2005; Searle et al., 2003, 2006, 2010b; Cottle et al., 2007; Law et usually in the middle crust. Key questions under debate here are; (1) Do al., 2004, 2011). Inverted metamorphic isograds from sillimanite to bio- shear zones control the generation and ascent of magmas or do magmas tite above the MCT have been mapped, folded around the GHS to join trigger nucleation of shear zones? (2) Are granites in shear zones pre- the right way-up isograds beneath the STD (Searle and Rex, 1989; Searle syn- or post-kinematic? (3) Do U-Pb ages of these granites date timing of et al., 2008) demonstrating that the mid-crustal GHS has moved south shearing along the fault? (4) Does regional scale partial melting support relative to the structurally lower Lesser Himalaya and the upper crustal mid-crustal channel fl ow models or a lower crustal fl ow model? Tethyan Himalaya above. What is the evidence for mid-crustal melt? The INDEPTH deep crustal Whereas mid-crustal zones of partial melting are well exposed in seismic profi les across Tibet (Nelson et al., 1996; Hauck et al., 1998; Til- mountain belts like the Wet Mountains, Colorado and the Himalaya, lower mann et al., 2003) and the HiCLIMB receiver function array of broadband crustal fl ow is more diffi cult to determine. Where lower crustal rocks are

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78° E Everest, Rongbuk Kangmar Dome Pamir Dzakaa Chu Karakoram fault section Hunza Mabja Dome Karakoram Baltoro Bhutan 150 km

Hindu Kush Kohistan Ladakh Zanskar

P Ladakh STDS K DS ZSZ Tangste ST MCT

MBT

Pakistan Everest S Sutlej Valley Indus Suture Zone Manaslu Makalu Sikkim Bhutan 30° N MCT

STD S 75° E MBT MCT Transhimalayan batholith STDS STDS Kohistan - Ladakh arc India Nepal Indus Tsangpo suture zone MBT MCT Tethyan zone MBT South Tibetan Detachment System High Himalaya STDS Main Central thrust Kathmandu Darjeeling Lesser Himalaya MCT Main Boundary thrust MBT

84° E 90° E

0 500 km Figure 1. Geological map of the Himalaya. Inset shows the North Himalayan domes in south Tibet, the northern extension of the Greater Himalayan sequence. ZSZ—Zanskar shear zone, equivalent to the South Tibetan Detachment. P—Peshawar basin; K—Kashmir basin; S—Sutlej basin.

exposed, generally in old Precambrian cratonic regions, they show exten- named Wet Mountains and the Himalaya. I then discuss how melts migrate sive dry granulite-facies metamorphism with little or no evidence for par- from their source to make granites of batholithic proportions using the tial melting. It has been proposed that the thickened crust of the Tibetan ~35 km of structural depth exposed in the Himalaya and Karakoram. plateau is presently undergoing lower crustal fl ow to the east toward the The relationships between partial melting, granite movement along shear thinner crust of SE Asia (Royden et al., 1997; Clark and Royden, 2000). zones and deformation temperatures and fabrics, as shown in ancient and There is, however, no exposure of the lower crust anywhere in Tibet or the modern examples have a key bearing on interpreting the large-scale rheol- area adjacent to the eastern margin, and the eastern margin of Tibet shows ogy of the lithosphere, whether the “jelly sandwich” model (strong upper no evidence of extrusion of middle or lower crust rocks like the Himalaya. crust, weak lower crust, strong upper mantle; e.g., Burov and Watts, 2006) Instead, all the metamorphism and most granites have older Indosinian is more applicable or whether the “crème brûlée” model (strong upper (Triassic-Jurassic) ages (Weller et al., 2013). Only a very few cases of crust, weak lower crust, weak upper mantle; e.g., Jackson, 2002; Jackson post-collision (<50 Ma) granitic melts are known from the Tibetan Plateau, et al., 2008) is more appropriate. although there is evidence of widespread shoshonitic (mantle-derived) and adakitic (thickened lower crust-derived, requiring a garnet-bearing METAMORPHISM—MELTING ALONG THE HIMALAYA AND amphibolite or eclogite source) intrusions all across the plateau (Chung KARAKORAM et al., 2005; Searle et al., 2011). Although present-day global position systems (GPS) does show eastward motion of the surface of North and East The India-Asia collision resulted in crustal thickening and shortening, Tibet (Gan et al., 2007), there is no geological evidence to support lower metamorphism and partial melting along the 2200-km long Himalayan crust fl ow in eastern Tibet, whereas the evidence for mid-crustal channel range. The Himalaya shows a relatively simple evolution from early deep fl ow along the Himalaya during the Miocene is overwhelming (Grujic et crustal subduction along the northern margin at ultra-high-pressure coesite al., 2002; Searle et al., 2003, 2006, 2010b; Law et al., 2004, 2006, 2011; eclogite grade (late Palaeocene–early Eocene) through kyanite (late Searle and Szulc, 2005; Godin et al., 2006). Eocene–Oligocene) and sillimanite + muscovite, sillimanite + K-feldspar This paper fi rstly reviews the metamorphism and melting processes and sillimanite + cordierite grade, culminating with widespread partial along the Himalaya (Indian plate) and Karakoram (Asian plate) and the melting in the middle crust and generation of anatectic leucogranites granite wet melting reactions that form partial melts in the appropriately (Early Miocene). In the core of the Greater Himalaya widespread in situ

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S Everest N ZSZ

PreC-Camb-Ord sediments

grt STD st ky sill migmatite GHS leucogranite

S Greater Himalaya Tibetan Plateau N Figure 2. The Himalayan Channel Flow model after folded metamorphic GREATER HIMALAYAN SEQUENCE Searle et al. (2003, 2008, isograds Middle crust 2010b), Law et al., (2004, Kfs + pl + bt + ms + sil + crd ± grt 2011) and Godin et al., MAIN SOUTH (2006) showing the south- CENTRAL TIBETAN Tur + ms + bt ± sil ± crd THRUST DETACHMENT leucogranites ward extrusion of a duc- km tile partially molten layer of middle crust during the 0 20 Ma Early Miocene. Inset dia- INDIAN SHIELD ARCHAEAN Seismogenic upper crust grams show the condensed GRANULITES 15 Ma right-way isograds beneath 10 Ma Leucogranites Moho the Zanskar Shear zone 50 5-0 Ma Partially molten middle crust (ZSZ) equivalent to the South Tibetan Detachment MANTLE High-P granulite lower crust (STD) and the condensed inverted isograds above 100 the Main Central Thrust deep earthquakes (~60-80 km) High-P Granulites (Dry) (MCT). Photos show the at base of crust Cpx + qrt + pl [± opx] STD as exposed on Everest Grt + opx + pl + Kfs + bt + ky + qtz (top), and the 70 km to the Crd + sil + opx + qtz north of Everest at Dzacha chu. GHS—Greater Himala- yan Sequence. N S Greater Himalayan Sequence Dzakaa chu

migmatite

sill M.C.T. st ky ductile grt shear Lesser zone Himalaya brittle fault

partial melting in sillimanite + K-feldspar gneisses resulted in formation relaxation due to crustal thickening and from high internal heat production of migmatites and Ms + Bt + Grt + Tur ± Crd ± Sil ± And leucogran- rates within the Proterozoic source rocks in the middle crust. Himalayan ites, mainly by muscovite dehydration melting (Harris et al., 1995; Harris granites have highly radiogenic Pb isotopes and extremely high uranium and Massey, 1994; Visona and Lombardo, 2002) and in the latter stages concentrations. Little or no heat was derived either from the mantle or by biotite dehydration melting (Streule et al., 2010; Groppo et al., 2012; from shear heating along thrust faults. Visona et al., 2012). Initial melting occurred in the kyanite stability fi eld Mid-crustal melting triggered southward ductile extrusion (channel (~10–12 kbars) forming Late Eocene–Oligocene kyanite-bearing leuco- fl ow) of a mid-crustal layer bounded by a crustal-scale ductile shear somes and leucogranites (e.g., Zanskar, Langtang), but the bulk of melting zone - thrust fault (Main Central Thrust) along the base, and a low-angle occurred at shallow depths (4–6 kbar; 15–20 km) in the middle crust, dur- ductile shear zone and normal fault (South Tibetan Detachment or Zan- ing the Miocene resulting in sillimanite, andalusite or cordierite-bearing skar shear zone) along the top (Fig. 2). Multi-system thermochronology leucogranites (e.g., Everest, Makalu, Langtang). 87Sr/86Sr ratios of the (U-Pb, Sm-Nd, 40Ar-39Ar, and fi ssion track dating) show that partial melt- leucogranites are very high (0.74–0.79) and heterogeneous, indicating a ing triggered mid-crustal fl ow between the simultaneously active shear 100% crustal protolith. Melts were sourced from fertile muscovite-bearing zones of the MCT and STD. Melts were channeled laterally along giant pelites and quartzo-feldspathic gneisses of the Neoproterozoic Haimanta- sill complexes via hydraulic fracturing into sheeted sill complexes from Cheka formations. Melting was induced through a combination of thermal the migmatitic source region, and intruded for up to 100 km parallel

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to the foliation in the host sillimanite gneisses (e.g., Everest–Rongbuk ness (up to 85–90 km) active granulite or eclogite facies metamorphism profi le). Crystallization of leucogranites was immediately followed by occurs today at depth beneath the Karakoram. rapid exhumation, cooling, and enhanced erosion during the Early- Middle Miocene. The channel fl ow hypothesis for the Greater Hima- PARTIAL MELTING IN CONTINENTAL CRUST layan Sequence fi ts all known geological (Searle et al., 2003, 2006, 2010b; Law et al., 2004, 2006, 2011) and geophysical (Nelson et al., Through careful fi eld observations and mapping of melt microstructures, 1996; Hauck et al., 1998; Tilmann et al., 2003) parameters. The thermal- shear zones, and melt reactions at all scales, Levine et al. (2013, see “Rela- mechanical model of Beaumont et al. (2001) using all the Himalayan- tionship between syn-deformational partial melting and crustal-scale mag- specifi c geological and geophysical parameters shows that Miocene matism and tectonism across the Wet Mountains, central Colorado,” Litho- mid-crustal channel fl ow along the Himalaya is a viable process. The sphere, v. 5, no. 5, p. 456–476) document widespread partial melting in the upper, middle, and lower Himalayan crust are decoupled and each show middle crust of the Wet Mountains, Colorado, that appear to have many different rheology, fl uid composition, seismic properties, and structural similarities to the Tertiary and active Himalaya-Karakoram orogenic belt in evolution. Although southward-directed channel fl ow must have been Asia. During progressive Barrovian metamorphism volumetrically signifi - operative during the Miocene along the Himalaya, it is possible that the cant anatexis fi rst occurs as a result of the muscovite dehydration reaction: same process could still be active beneath southern Tibet where an active mid-crust layer of partial melt has been imaged. In the Nanga Parbat Qtz++= Ms Pl Als (Sil) ++ Kfs Melt ±± Grt Tur. (1) Western Himalayan syntaxis region of the GHS very young (Pliocene- Pleistocene) sillimanite + cordierite grade metamorphism and anatexis This fl uid-absent reaction occurs over a temperature range of ~710–790 °C may provide insight into active thermal structure of the deep crust today (Petö, 1976; Patiño-Douce and Harris, 1998) and produces the fi rst in situ (Crowley et al., 2009). melts in migmatites. In standard pelitic compositions, the muscovite dehy- The Asian-plate Karakoram terrane in North Pakistan shows a much dration reaction could occur at temperatures as low as 670–690 °C. Melts more complex and long-lasting structural and thermal evolution. During are preferentially localized along anisotropic planes of weakness, foliation and following accretion of the Kohistan Arc to the southern margin of Asia planes, forming layered or stromatic migmatites (Fig. 3A). In the Hima- and collision with the Indian plate, crustal thickening along the Karakoram laya, these muscovite dehydration melts are typically peraluminous garnet resulted in polyphase deformation, metamorphism and melting. Abundant + muscovite + tourmaline leucogranites, although the leucogranites are Middle Jurassic and Cretaceous (170–90 Ma) Andean-type subduction- almost certainly not pure melts, but mixtures of melt plus crystals inher- related granite intrusion is recorded as well as widespread kyanite and sil- ited from the restite. The fi rst crustal melts formed along the Himalaya are limanite grade regional metamorphism spanning at least 65 m.y. (Searle et kyanite-bearing migmatitic leucosomes indicating that the earliest melts al., 2010a). U-Pb monazite ages from the Baltoro granites range between were the deepest (~10–12 kbar), with progressively later, shallower sil- 25 and 13 Ma, very similar to most of the Himalayan leucogranites along limanite and then cordierite-bearing partial melts (~6–3 kbar). the Indian plate. Metamorphism along the Karakoram is diachronous in The second slightly higher temperature reaction is the biotite dehydra- space and time and it is quite likely that, given the extreme crustal thick- tion melting reaction:

Figure 3. (A) South face of Lingtren (6749 m), west of Everest showing horizontal sheeted sill complex with leucogranites intruding sillimanite gneisses, with a massive sill above containing several large xenoliths. (B) Early, layered leucogranite sills (set 1) with fabrics intruded into gneisses cut by later granite melts with sills (set 2), Manaslu leucogranite. (C) Syn-kinematic in situ melting in migmatite, cut by later post-kinematic lec- uogranite dykes, Manaslu leucogranite. (D) Thick leucogranite sills intruding sillimanite gneisses, with dykes bent toward the north beneath the South Tibetan Detachment (STD) low-angle normal fault, North face of Ama Dablam (6828 m) south of Everest, Khumbu Himalaya.

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Bt+++ Qtz Pl Als (Sil) =+±+ Kfs Grt Crd Melt, (2) Armorican Shear Zone in Brittany (Brown and Dallmeyer, 1996), the Karakoram Fault in Ladakh (Searle et al., 1998; Phillips et al., 2004, that occurs at temperatures in the range 760–850 °C (LeBreton and 2013; Lacassin et al., 2004) and the Red River shear zone/fault in Yun- Thompson, 1988; Spear et al., 1999). At lower pressures (<5 kbar) cordi- nan and North Vietnam (Leloup et al., 1995, 2001; Searle, 2006; Searle et erite is the stable peritectic phase whereas at pressures above 5 kbar, peri- al., 2010c, 2011). Partial melting leads to a dramatic decrease in strength tectic garnet is stable. In the Himalaya, these crustal melts typically form when the melt fraction is relatively small (Rosenberg and Handy, 2005). cordierite-bearing leucogranites (Groppo et al., 2012; Streule et al., 2010) Strain partitioning into the rheologically weaker areas can be preferen- or andalusite-bearing leucogranites (Visona et al., 2012). Andalusite can tially localized into early melt channels (Brown, 2006, 2007; Brown and be residual and/or peritectic. Solar, 1998; Rosenberg and Handy, 2005) and deformation can also pro- On a broad scale, deformation and partial melting are usually synchro- mote partial melt reactions through enhanced diffusion and reaction rates nous leading to a positive feedback. Deformation enhances melting by (Brodie and Rutter, 1985) by the positive feedbacks noted earlier. aiding diffusion and reaction rates, and strain is frequently partitioned into In the Wet Mountains of Colorado, Levine et al. (2013) document rheologically weaker partial melt zones. On a more local, outcrop scale, strain differences where the upper crust has mainly vertical foliations with batch melting throughout an actively deforming migmatite region results localized discrete granite bodies, and the middle crust has widespread in intrusion of multiple sills and dykes showing varying levels of strain migmatites with horizontal foliations and abundant mid-crust melt chan- through time and space (Fig. 3B, and 3C). nels. This is very similar to the Karakoram region of Ladakh and North Pakistan where transpression along the dextral strike-slip Karakoram fault MELT MIGRATION AND BATHOLITH GENERATION has exhumed Cretaceous and early Tertiary pre-collisional I-type diorites and granodiorites, Cretaceous amphibolite facies metamorphic rocks, mid- Partial melting promotes fl ow in rocks since viscous material has no crustal migmatites, foliation-parallel leucogranite bodies, and Miocene yield strength and will fl ow given any deviatoric stress. Along the Hima- sills and dyke swarms in between two strands of the fault (Phillips et al., laya migmatites up to 10 km in structural thickness occur in the middle 2004, 2013; Phillips and Searle, 2007; Searle and Phillips, 2007; Streule crust above the MCT zone with its inverted and condensed metamorphic et al., 2009). Levine et al. (2013) suggest that deformation is concentrated sequence, and beneath the STD low-angle normal fault with its right way- along areas where melt-producing reactions occur, and inversely that up and condensed metamorphic sequence (Fig. 2). These migmatites are melts are concentrated along shear zones, suggesting a strong correlation dominantly stromatic or layer-parallel migmatites with abundant gar- between partial melting and deformation. The obvious question that arises net, tourmaline, and muscovite in the melt phase. The melts percolated is which is the chicken and which is the egg? Do shear zones control the along the schistosity planes and can be traced both along and across strike generation and ascent of magmas (Brown and Solar, 1998; Brown, 2006, (down-dip) for great distances. At higher structural levels the melts merge 2007) or do magmas trigger nucleation of shear zones? Are crustal melt into layer-parallel sills intruding the high-grade gneisses. Individual sills migmatites and granites formed as a result of shearing and fed upward become progressively thicker at higher levels eventually forming sills up to through ductile shear zones to the surface (Leloup et al., 1995; Lacassin 3 km thick in the Everest, Makalu, and Kangchenjunga regions for example et al., 2004; Weinberg and Mark, 2008; Weinberg et al., 2009) or are the (e.g., Searle et al., 2003, 2010b; Streule et al., 2010). Along the Rongbuk migmatites and granites produced by regional Barrovian metamorphism valley north of Everest the foliation-parallel sills die out along the duc- unrelated to the faults (Searle, 1996; Phillips et al., 2004, 2013; Phillips tile strand of the STD, the Lhotse detachment (Searle et al., 2003, 2006). and Searle, 2007; Streule et al., 2009). The key to answering these vexing Leucogranite dykes emanating from the top of the Everest-Nuptse granite questions lies in the interpretation of partial melt textures in granites and are defl ected to the north beneath the Lhotse detachment (Fig. 3D), which migmatites and the deformation temperatures recorded in the rock fabrics. merges toward the north with the upper brittle Qomolangma detachment (Cottle et al., 2007). Himalayan crustal melting has occurred by multiple DEFORMATION TEMPERATURES WITH STRUCTURAL DEPTH batch melting over a time span of ~10 m.y. (U-Pb ages of all granites rang- ing 24–15 Ma; Cottle et al., 2009). Melts migrated laterally along giant The brittle-ductile transition marks the change from localized defor- foliation-parallel sill complexes with dykes feeding magma to higher mation along faults in the seismogenic upper crust (up to 17 km) to distrib- sills, and even later cross-cutting sets of dykes feeding the highest level, uted ductile deformation in the lower crust (Rutter, 1986; Kohlstedt et al., younger biotite-dehydration granites that contain cordierite and andalusite. 1995). Even in deep viscous-deforming crust, strain can still localize into Whereas Himalayan crustal melting is exposed in the form of stromatic shear zones. There are many factors that will induce strain localization: migmatites and giant sill complexes, crustal melting along the Karakoram stress fi eld, temperature, effective confi ning pressure, melt fraction, etc. (Asian side of the collision) has produced far greater volumes of melt that The relationships between granitic melts and shear zones are examined in have amalgamated to form a large batholith (~700 × 20 km dimension). two case examples here, the South Tibetan Detachment low-angle normal The Karakoram batholith includes pre-collision subduction-related I-type fault along the Himalaya and the Karakoram dextral strike-slip fault in granites (diorites, tonalites, and amphibolites) as well as post-collision Ladakh and Western Tibet. S-type granites (garnet, muscovite, tourmaline monzogranites, and leucogranites). The latter have also built up from giant sill complexes intruding South Tibetan Detachment, Himalaya a high-grade metamorphic (kyanite- and sillimanite-zone) metamorphic terrane to the south (Searle et al., 1992, 2010a). In a petrological and microstructural study of fabrics along the South Tibetan Detachment along the top of the Greater Himalayan Sequence, RELATIONSHIP OF PARTIAL MELTING TO SHEAR ZONES Law et al. (2004, 2011) determined deformation temperatures from quartz c-axis fabric opening angles. They showed that deformation temperatures The close spatial association between high-grade metamorphic rocks, increased linearly with depth beneath the detachment from ~480–680 °C migmatites and granitic melts and crustal-scale shear zones and faults over >400 m along with metamorphic grade. Deformation temperatures has been noted along many orogenic belts. Prime examples are the South within 10 m of the detachment were estimated at 490–480 °C steadily

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increasing to 625–680 °C at depths of 420 m below the detachment. Leu- collision processes from Proterozoic (e.g., Wet Mountains, Colorado) to cogranite sills increase down-section from the STD with large-scale sills present (Himalaya) times. The volume of crustal melting is controlled by making up the 2–3 km thick Makalu, Everest and Nuptse granites, located the fertility of the dominantly pelitic protolith, the internal radiogenic heat 1.5–2.5 km structurally beneath the STD (Searle, 2007). Deformation fl ow, and the rates of crustal shortening and thickening. temperatures in thin sills are similar to those in the surrounding silliman- 2. Deformation and melting are diachronous across mountain chains. ite gneiss, and temperatures of fabrics in the larger sills suggest they had In general, metamorphism may mimic structures with propagation from cooled to ambient temperatures (~550–525 °C) when plastic deformation hinterland toward foreland with time. Outcrop-scale fi eld constraints (e.g., of quartz ceased and fabrics were “locked in” (Law et al., 2011). In the undeformed dykes cross-cutting regional metamorphic fabric) must be case of the STD, the granites were already intruded and cooled to ~550 °C extrapolated to a wider area with caution, based on careful fi eld mapping. when the fabrics were superimposed on them. Therefore, U-Pb zircon and 3. Batch melting produces numerous “phases” of melting and sill-dyke monazite ages on migmatitic melts and leucogranites beneath the STD injection. In the Himalaya, precise U-Pb dating of individual sills/dykes will give maximum ages of ductile shearing in each locality. The tight constrains batch melt phases over time scales of 1–20 m.y. Timing of fi eld, structural mapping, microstructural and U-Pb geochronological data deformation fabrics can also be constrained by using U-Pb ages of cross- from all along the STD in Zanskar, Nepal and Tibet shows that meta- cutting granitic dykes. morphism and melting was a regional GHS-wide event related to regional 4. Himalayan migmatites and leucogranites have remained in their crustal thickening and not related to shear heating along the STD. mid-crust depth of formation (in sillimanite + muscovite or sillimanite + K-feldspar ± cordierite grade) and migrated laterally along giant sill com- Karakoram Strike-Slip Fault, Ladakh and Tibet plexes during channel fl ow. They have been exhumed by underplating of Lesser Himalayan and Indian shield rocks beneath the MCT from south to The NW-SE oriented right-lateral Karakoram Fault cuts obliquely across north, passively uplifting the overlying GHS, combined with up to 20–25 the Pamir, Karakoram, and Trans-Himalayan (Ladakh) ranges of western- km of erosion since the Miocene. There are few, if any, vertical diapiric most Tibet for over 800 km (Searle, 1996). In Ladakh and SW Tibet a linear intrusions along the Himalaya. belt of high-grade metamorphic rocks and granites are exposed between 5. Migmatites and granites exposed along strike-slip shear zones (e.g., two strands of the fault. Mylonites along both strands of the fault zone were Karakoram, Red River, Jiale faults of Tibet and SE Asia) were not formed derived from a variety of earlier metamorphic rocks, including amphibo- as a result of shear heating along the faults. Barrovian metamorphism and lites, orthogneisses, marbles, psammites, and pelites. Whereas diorites, most granites were formed during regional crustal thickening and partial amphibolites, and I-type granites of the Pangong Metamorphic complex melting processes prior to, and unrelated to shearing along the fault, but immediately adjacent to the fault zone are older (mainly Cretaceous–Early were exhumed either by transpression (e.g., Karakoram fault) or by linear Tertiary), most leucogranites formed between 22 and 17 Ma, prior to ductile core complex processes beneath low-angle normal faults (e.g., Ailao Shan, shearing along the fault (Phillips et al., 2004, 2013). Several studies simply Red River fault; Searle, 2006; Searle et al., 2010c). assumed that metamorphism and leucogranite intrusion was syn-kinematic 6. The highest temperature deformation fabrics seen in granite melts with dextral shearing along the fault (Lacassin et al., 2004; Valli et al., 2007, (~600–650 °C) are lower than muscovite and biotite dehydration melting 2008; Weinberg and Mark, 2008; Weinberg et al., 2009). However, U-Pb temperatures (710–850 °C), and therefore U-Pb zircon or monazite ages dating of metamorphic rocks immediately adjacent to the fault showed that date timing of melt crystallization can only give a maximum age for that metamorphism was Cretaceous in age (ca. 108–105 Ma: Streule et deformation fabrics in the granite. U-Pb ages of granite melts rarely date al., 2009). A range of garnet, two-mica leucogranites includes early sills timing of shearing along major faults, but can be used to constrain maxi- emplaced parallel to the mylonite foliation and that contain a shear fabric, mum and minimum ages of shearing if the fi eld relationships are inter- and uncommon late undeformed dykes that cross-cut the mylonite fabric. preted correctly. Like the deformation temperatures recorded in the GHS rocks beneath 7. The mid-crustal partial melting seen in both the Wet Mountains, the STD, the dextral strike-slip shear fabrics in leucogranites along the Colorado (Levine et al., 2013) and along the Himalaya (Searle et al., 2003, Karakoram fault were imposed at high temperatures (550–600 °C) after 2006, 2010b; Law et al. 2011) do not support either the “jelly sandwich” initial crystallization of the granite. A few late, relatively undeformed, leu- (strong upper crust, weak lower crust, strong upper mantle) or the “crème cogranite dykes cross-cut the ductile fabrics and have U-Pb ages between brûlée” (strong crust, weak aseismic mantle) models for rheology of the 15.8 and 12.7 Ma, indicating that ductile shearing along the Tangtse fault lithosphere. In most cases the brittle upper crust (~0–17 km) is seismo- strand was over by then (Phillips et al., 2004, 2013; Wang et al., 2012). All genic; the dry, granulite facies lower crust can be seismogenic, but only if leucogranites and metamorphic rocks are cut by brittle fault motion along the temperatures are <600 °C (Priestley et al., 2008); the middle crust is the Karakoram Fault system. In summary, these data do not support mod- hot, weak, aseismic, partially molten and capable of fl ow (channel fl ow). els involving shear heating, syn-kinematic metamorphism, or melt forma- 8. Granulitic lower crust is strong, generally devoid of hydrous phases tion and large geological offsets, despite spectacular high-strain mylonite and lacks partial melts, and therefore lower crustal fl ow is unlikely. Conti- zones. Instead although the Karakoram Fault is one of the largest and most nental lithosphere retains its strength and relative rigidity through its dry, prominent strike-slip faults in Tibet, it has only minor offset, possibly 52 strong granulite facies lowermost crust. km (Wang et al., 2012) or as little as 17–25 km in the Ladakh-Pakistan sector (Searle et al., 2011), it cuts through all the earlier-formed metamor- ACKNOWLEDGMENTS phic rocks and granites, and it bears little relationship to melt formation. Thanks to Rick Law, Marc St-Onge, Brendan Dyck, and editor John Goodge for reviews of the manuscript. I am grateful to them and also to Dave Waters, Tony Watts, Richard Phillips, Ernie Rutter, Micah Jessup, John Cottle, Mike Streule, Richard Palin, and Owen Weller for MAJOR CONCLUSIONS discussions.

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