Bachelor-colloqium TU Bergakademie Freiberg 4 th /5 th April, 2008

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1 Bachelor-colloqium TU Bergakademie Freiberg 4 th /5 th April, 2008 Structural models of North Himalayan Gneiss Dome evolution Institute for Geology, Bernhard-v.-Cotta Str. 2, Freiberg, Germany Abstract: Gneiss domes are common thermo-tectonic structures, documented from all over the world and throughout the geologic past. In the past decade extensive research work was carried out on the North Himalayan Gneiss Domes (NHGDs) because of their assumed role in mid-crustal processes also responsible for Tibetan Plateau formation. I discuss the following models for NHGD formation: (1) channel flow-extrusion (Hodges 2006); (2) intrusion triggered extension (Aoya et al. 2005); and (3) thrusting over a ramp (Lee et al. 2004, 2006). Field, geochronologic and thermochronologic data from various authors give the possibility to test these models to a certain extent while especially geophysical data are missing at the moment. 1. Definition of a gneiss dome There is no generally accepted definition for gneiss domes, but a review of common features is given below. Gneiss domes are three-dimensional structures that consist of pre-kinematic high-grade rocks. In the North Himalayan Gneiss Domes (NHGDs) these high-grade cores can be differentiated into basal orthogneisses and high-grade metasediments. In most cases younger, synkinematic granitoids intruded the high-grade core during Gneiss Dome formation. The metamorphic core is rimmed by a tectonic contact. This is a shear zone or fault. Both thrust and normal movements may occur. The tectonic contact separates the high-grade core from low-grade volcano-sedimentary rocks, mostly schists and phyllites. These low-grade metasediments are not necessarily preserved. In map-view gneiss domes appear as elliptical features. Syn-kinematic fabrics dip away from the centre of the dome (e.g., Yin 2004, Whitney et al. 2004, Whittington 2004). 2. North Himalayan Gneiss Domes 2.1 Geological Setting The NHGDs lie within the Tethys Himalayan series, which comprises low-grade and unmetamorphosed sediments of Ordovician to Quaternary age. The South Tibetan Detachment system separates the Tethys Himalayan sequence from the underlying Greater Himalayan sequence (GHS), a crystalline basement nappe of high-grade metamorphic rocks, thrusted along the Main Central Thrust on top of

2 2 the Lesser Himalayan sequence (LHS). Tectonic windows within the GHS expose low-grade sediments of the LHS. (see fig. 1) Figure 1: Regional tectonic map of the central Himalaya orogen after Burchfiel et al. (1992) and Burg et al. (1984) showing location of the Gneiss Domes. Abbreviations: STDS, South Tibetan Detachment System; MCT, Main Central Thrust; MBT, Main Boundary Thrust System; YCS, Yadong crossstructure; ITSZ, Indus-Tsangpo suture zone, GKT Gyrong-Kangmar thrust system. From Lee et al. (2006). 2.2 Model of Hodges (2006) - Climate controlled channel-flow extrusion leads to tectonic extrusion of gneiss domes. Hodges (2006) proposes a model that explains the interplay of the three important sets of processes in the Himalayan - Tibetan orogenic system: (i) those related to plate convergence, (ii) those related to a supposed channel flow of a fluid-like mid crust, (iii) and those related to the erosion processes at the southern flank of the orogenic plateau (the Himalayas). He reconstructed three main phases in the evolution of the Himalayan-Tibetan orogenic system (fig. 2): Phase I During the first phase (Early-Middle Miocene) plate convergence and subduction lead to accretion of Indian crust into the lower crustal sections of the Himalaya. Partial melting in the mid-lower crust results in formation of a low viscosity channel and accounts for formation of part of the southern Tibetan plateau. While

Structural models of North Himalayan Gneiss Dome Evolution 3 the metasedimentary rocks deformed and partially melted in the channel, the orthogneisses (proposed to form parts of the NHGDs later on)

3 Structural models of North Himalayan Gneiss Dome Evolution 3 the metasedimentary rocks deformed and partially melted in the channel, the orthogneisses (proposed to form parts of the NHGDs later on) are more competent and are in general weakly deformed. Figure 2: Conceptual cross-sections illustrating the three phases of channel extrusion at the Himalayan front. Dark-grey shading designates the down going Indian plate. Fields with light-grey shading, random-dash patterning, and no shading are Indian crust that has been accreted to the overriding plate. Unpatterned material corresponds to unmetamorphosed to weakly metamorphosed Tibetan sedimentary series. Material with random dash patterning includes high-grade metamorphic rocks of extruded channels. The actively extruding material in each frame has a light-red overlay pattern. Previously extruded material has no overlay shading. Note the development of a ductile shear zone (dashed heavy line) at the base of the actively extruding material in the frame for Phase II. The dark-red shading in Phase III frame indicates partially molten material as imaged in the INDEPTH seismic reflection experiment (Nelson et al. 1996). Circled A and B in Phase I indicate divergence point of the down going slab and the tunnelling channel and the proposed lower crust duplex. Blue bars represent the zone of orographic rainfall, colour intensity indicates intensity of rainfall. Note that the average rainfall is much lower during Phase II, a predication based on sedimentological evidence. Green bar indicates a zone of extension over the North Himalayan gneiss domes, with the gradient representing intensity of extensional strain in upper crustal material. Abbreviations as in Figure 1. From Hodges (2006). Intensive erosion along the southern flank of the Himalaya causes channel extrusion (Greater Himalayan Sequence). The upper boundary of this channel is formed by the normal-slip Southern Tibetan Detachment System (STF), the lower

4 4 boundary is formed by the Main Central Thrust (MCT). A steady-state condition is suggested, with equilibrium between erosion, channel-flow extrusion, and the supply of material through plate convergence. Phase II In the Middle-Late Miocene the erosion rates decreased at the Southern Himalayan Front and dramatically slowed the channel flow-extrusion. This could be related to a climate change that stopped a wet monsoon, possibly active during Phase I. The uneroded and cooled material acted as a plug in the channel. Because channel flow-extrusion was an important factor conducting hot material and stress/shortening and because that processed had now ceased, the thrust systems were transferred to the foreland and the Main Boundary Thrust (MBT) overtook the former role of the channel flow-extrusion in relaxation of the orogen. While convergence and supply of material into the channel continued, it searched for a new pathway to extrude. This pathway was possibly created by extensional denudation caused by orogenic collapse between the Indus-Tsangpo Suture zone and the South Tibetan fault system. This normal faulting with a detachment reaching to the mid crust triggered a new extrusion channel for the mid crust as shown in figures 3(a) and 3(b). The ascending material, consisting of high-grade metamorphosed Indian metasediments and orthogneisses formed a duplex (thrust over the mid crustal layer), while the hanging walls evaded in northward and southward direction, creating normal faults as in figures 3(c) and 3(d). Figure 3: Conceptual model of the development by upper crustal extension. Darker-grey shading indicates upper crust. Random dashed pattern indicates channel material; light-grey overlay shading designates less active parts of the channel. Half-arrows indicate slip on individual faults. Thin dashed lines represent mylonite zones. Large freeform arrows indicate large-scale kinematics and brightness indicates kinematic activity. From Hodges (2006).

5 Structural models of North Himalayan Gneiss Dome Evolution 5 Phase III In the late Pliocene the climate changed again and, as in Phase I, erosion and exhumation rates were high at the Southern Himalayan Front. Channel flowextrusion was reactivated in the south and the extrusion in the Northern Himalaya stopped. Extrusion at the South Tibetan Front (along the Himalayas) may continue until today. Implications of Hodges model: 1. The orthogneisses in the NHGDs should show pre-himalayan crystallisation ages. 2. The NHGDs high-grade core should be bound by normal sense shear zones showing mainly top-to-the-south movement in the south and top-to-the-north movement in the north. 3. The orthogneisses that are seen as Indian basement should be in tectonic contact to the surrounding high-grade metasediments. 4. The NHGDs should have cooled upwards in the north and also downwards in the south depending on the displacement (fig. 3). Discussion of implications: 1. Zircon cores from the orthogneisses are dated by conventional U-Pb as Ma (Schärer 1986 and Lee et al. 2000). Their rims show ages of 18.6 Ma (Aoya et al. 2005). A crystallisation age of Ma could be interpreted as Indian (Cadomian) crust, while the 18.6 Ma age suggests a syn-himalayan intrusion. 2. At the northern flank of Mabja Dome the high-grade/low-grade contact shows top to the north movement along a mylonitic foliation (Lee et al. 2004). No data is available for the southern flank. At the northern flank of Kangmar dome the high-grade/low-grade contact is developed in a top-to-the-north shear zone and a top-to-the-south dominated shear zone in the south. At the southern flank some top-to-the north indicators do appear (Lee et al 2000). At the Malashan Dome both, at the southern and northern flank, top-to-thenorth shear sense is dominating, while there are two top-to-the-south indicators in the south (Aoya et al. 2006). In summary, the implication of a top-to-the-north in the north and a top-tothe-south in the south evasion is not completely fulfilled. This aberration of model and reality could be explained by a non symmetric evasion of the hanging wall. 3. The referenced work comprises contradicting statements about the contact of the high-grade metasediments to the basal orthogneiss inside the core. Lee et al. (2000) suggested a tectonic contact and a pre-himalayan intrusion age for the Kangmar orthogneiss of about 508 Ma. Lee et al. (2004) observed an instrusive contact between the high-grade metasediments and the Mabja basal orthogneiss. Aoya et al. (2005) also suggested an intrusive contact of the basal

6 6 orthogneiss at the Malashan dome. The question of the basal orthogneisses will be discussed in Model of Aoya et al and The dominating cooling direction is strongly based on the timing relations (especially the occurring young post-kinematic intrusion) and the magnitude of displacement at the thrust in the mid crustal layer (fig. 2 and 3). Because cooling ages were mainly produced by Lee et al. (2000) and (2006), they will be discussed in section 2.4. Generally, they do not confirm the implications of the model of Hodges 2006 and ask for a more complex mechanism. 2.3 Modell of Aoya et al. (2005) and (2006) - Basal orthogneisses in North Himalayan Gneiss Domes are syntectonic intrusions during the Himalayan orogeny that triggered north-south extension. Aoya et al. (2005) presented new geochronologic, thermochronologic, and structural data, which challenge the model of Lee et al. (2004) and Hodges (2006) that the basal orthogneisses are derived from Indian basement. The authors propose a modified model based on their new data: (i) The rims of the zircons dated by Aoya et al. (2005) have an age of 18.5 to 17.2 Ma suggesting that this was the intrusion/crystallisation age. (ii) This correlates with 40 Ar- 39 Ar cooling ages of muscovite and biotite around 15.7 Ma for the Malashan granite. [leave out: cooling ages tell little about intrusion] Microstructural analyses of K-feldspars in the Malashan granite support these suggestions. The K-feldspar porphyroclasts show top-to-the-south contractional shear and an asymmetric growth in that direction, which implies that they crystallized syntectonic to a D1 event. These microstructures are overprinted by an extensional top-to-the-north D2 event, which caused a steep foliation at a high angle to s1 (D1). The younger Cuobu and Paiku granites show only D2 suggesting that the Malashan granite formation stopped the contractional (D1) and triggered top-tothe-north (extension) in the area. This cause and effect identification is speculative. Aoya et al. (2006) confirmed the former idea of the syntectonic intrusion in a comparison with the Kangmar dome, which shows many similarities to the Kangmar Dome (Aoya et al. (2006)).

7 Structural models of North Himalayan Gneiss Dome Evolution 7 Figure 4: Time relations between granitoids and deformation at the Malashan Dome. In their model a contractional D1 takes place during the Himalayan orogeny and switches to an extensional D2 environment due to the emplacement of the first granitoids (fig. 4). The thermal input leads to a positive feedback with further extension and emplacement of Granites (Cuobu granite, Paiku granite). Discussion of the model by Aoya et al. (2005) and (2006) A contact metamorphism around the centre of the NHGDs is proven. But the source of this thermal overprint can not only be explained with the the basal orthogneises, but with a by Lee et al. (2006) proposed intrusion below. The question appears, if the data delivered by Aoya et al. (2005, 2006), especially the microstructures, cannot also be achieved due to partial melting (migmatization) and a re-crystallization during the deformation? Without availible Th/U ratios it is not possible to proof that the to 17.2 Ma zircon rims indicate a crystallisation age. Rim growth could occur during a deformational event too. The question which percentage of the malashan granite must have been molten during intrusion to explain the data of Aoya et al. (2005, 2006) arises? 2.4 Model of Lee et al. (2004 and 2006) - crustal flow driven lifting over midcrustal antiform and thrusting over GKT ramp for exhumation of an antiform. The model of Lee comprises different ideas concerning the evolution of the Mabja Dome. It is strongly bound to the hypothesis of channel flow. Figure 5 illustrates the chronology of events. Phase I: Channel flow was already active in the Early Miocene and transported high-grade orthogneisses toward the south as explained in section 2.2. These Indian block were metamorphosed already before the Himalayan orogeny and/or in the

8 accretion prism at depth (D1). While caught within the shear zone of the STDS they were subhorizontally lengthened (4 to 10 times) in a top-to-the-north sense (D2).

8 8 accretion prism at depth (D1). While caught within the shear zone of the STDS they were subhorizontally lengthened (4 to 10 times) in a top-to-the-north sense (D2). The increasing strength of D2 towards the basal orthogneiss suggests that it was situated in the central high strain part of the shear zone, while the hanging high-grade rocks were not. Migmatized basal orthogneisses interfingered with the upper high-grade schists. Figure 5: Modell of the Evolution of the Northern Himalayan Gneiss Domes,. Abbreviations as in fig. 1. From Lee et al. (2006). The emplacement of a leucocratic dike swarm (23.1±0.8 Ma) at the Mabja Dome suggests (partial) melting of lower structural sections. This dike swarm is pre- /syntectonic to D2.

9 Structural models of North Himalayan Gneiss Dome Evolution 9 Phase II: In the Early Miocene, the Gyrong-Kangmar thrust (GKT) formed as an additional compensator of N-S-shortening because of slowed channel-flow extrusion. Two ideas could explain this slowdown: (i) It may be an effect of the development of a ramp at the Main Himalayan Thrust (MHT) that formed an anti-formal thrust duplex. This duplex may have plugged the channel and slowed down channel flow-extrusion to the south. (ii) An increased friction along the MHT due to a change in erosion rates, as proposed by Hodges (2006), slowed down the channel flow-extrusion. Both processes would lead to a plugging of the mid crustal channel and caused an effort for bypassing the plug. This emerging bypass was triggered by thrusting along the GKT. The GKT developed from the former STDS and transported parts of the highstrain shear zone rocks upward with the hanging wall. Because the decompression was fast and nearly isotherrmal (in the inner part of the dome) granitoids were able to form at least at the Mabja Dome in the Late Miocene. The post D2 Paiku and Kuobu granite show ages of arround 14.0 to 14.6 Ma. These ages impose a minimum age of D2. The thrusting along the GKT lead to underthrusting of cold crust. 40 Ar/ 39 Ar cooling ages suggest that downward cooling dominated in the Mabja Dome at least up to a temperature of about 370 C at 13 (upper part) to 17 Ma (middle part). At the Kangmar Dome, the cooling ages range from 11 to 16 Ma, also showing a cooling downward. The decrease of ages in the depth could be explained by hypothetical granites at depth. These decreasing ages lack at the Kangmar Dome which probably exposes higher structural levels. Figure 6: 40 Ar/ 39 Ar (~370 C) cooling ages of Mabja Dome show an increase from the top to the middle part and then a decreasing age. From Lee et al Phase III: During further thrusting the domal structure developed. Problematic is the mechanism which formed the domal structure itself. Final Doming

10 10 The 40 Ar/ 39 Ar (~370 C at ~13.0 Ma) cooling isochrones are sub-parallel to the metamorphic isolines and S2, and are folded. The low-temperature step potassium feldspar 40 Ar/ 39 Ar ages ~200 C arround 11Ma and the apatite fission-track ages ~115 C at ~11.0 Ma are not folded. That means that the doming must have occured between 13.0 Ma and 11 Ma at temperatures C (probably above 300 C because of ductile deformation of quartz). Lee et al. (2004) picks up two possibilities for the late doming: (i) Buoyancydriven diapirism that could be explained with the proposed intrusions in the depth or (ii) thrusting over a ramp along the GKT. Discussion of Lee et al. Lee et al. (2006) explained the exhumation of the mid-crustal rocks very detailed by using structural, geochronologic and thermochronologic data. There are no conflicts with the data of Lee et al. (2000, 2004) and Aoya et al. (2005 and 2006), except the documented tectonic contact of basal orthogneiss and high-grade metasediments at Kangmar dome in Lee et al. (2000). The late doming could be explained with the ramp along the GKT (fig. 7). Figure 7: Proposed mechanism for creation of a domal shape in a cold (370 C to 200 C) stockpile. Doming through thrusting over a ramp. ramppropagation-fold. Based on Lee et al Conclusion: The model of Lee et al. (2006) fits the best to publicated explains more [??] to the model of Hodges and fits together with the suggestions of Aoya et al. (2005 and 2006). In contrast to Hodges Lee et al. can explain all documented details and does not need an extensional setting during the Himalayan orogeny in a stable collision zone. The origin of the basal orthogneisses remains an open question. Further investigation should proove if the youger rim ages reflect a youger crystallisation or metamorphic event. U/Th ratios are missing in the work of Aoya et al Maybe the basal orthogneisses contain a hint on how much partial melt the mid crustal channel contains. This poses the question how a partial melt as a crystall pulp and a completely molten rock could be differentiated in use of geological methods.

11 Structural models of North Himalayan Gneiss Dome Evolution 11 Doming must have occurred at a temperature within between of 370 C and 200 C (Lee et al. 2006). Assuming that ductile rock deformation ceases at about 300 C, the spectrum gets even smaller. Also the time window within 0.5 Ma (from 13.0 Ma to 12.5) is quite small (Lee et al. 2006). Why should diapirism appear in this phase? If a significant density difference existed it must have lead to diapirism before and afterwards. Diapirism is a very slow process and should not abruptly play an important role for a 0.5 Ma time span. This leads to the conclusion that diapirism cannot be a significant factor in the NHGD formation. The most equitable so far proposed reason for the doming seems to be the ramp along the GKT. The formation of this ramp could have different reasons. The thrusting over a ramp could even explain the tilt of about 5 of the Kangmar dome that was not related to this process in Lee et al The assumption that the ramp could be an effect of the mid crustal antiformal duplex that spikes in the upper crust would bring it into consensus with the rest of the Lee et al. model. The question how a anti-formal duplex in the mid crust could have formed remains highly speculative: Its formation could be related to a slab break off around 12 Ma. In dependece on data for the Himalayan orogeny (e.g. Hou et al. 2004, Chemenada 1999 and DeCelles et al. 2002) that predict such a slab break-off I suppose that a back flip in the lower crustal sections (tectonic underplating) of the orogen could have resulted in a not homogenous indention from below. This work has benefited from discussions and constructive reviewing by Konstanze Stübner and Lothar Ratschbacher. Thank you. References cited: AOYA MUTSUKI, WALLIS SIMON R., TEREDA KENTARO, LEE JEFFREY, KAWAKAMI TETSUO, WANG YU, HEIZLER MATT, 2005, North-south extension in the Tibetan crust triggered by granite emplacement. Geology, 33 (11), AOYA MUTSUKI, WALLIS SIMON R., KAWAKAMI TETSUO, LEE JEFFREY WANG Y. AND MAEDA H., 2006, The Malashan gneiss dome in south Tibet: comparative study with the Kangmar dome with special reference to kinematics of deformation and origin of associated granites. Geological Society, London, Special Publications 268, CHEMENDA ALEXANDER I., BURG JEAN-PIERRE, MATTAUERC MAURICE, 2000 Evolutionary model of the Himalaya Tibet system: geopoem based on new modelling, geological and geophysical data. Earth and Planetary Science Letters 174, DECELLES PETER G., ROBINSON DELORES M., ZANDT GEORGE, 2002 Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau. TECTONICS, 21 (6), 1062.

12 12 HODGES KIP V., 2006, A synthesis of the Channel Flow - Extrusion hypothesis as developed for the Himalayan-Tibetan orogenic system Geological Society Special Publications, 268, HOU Z.-Q., GAO Y.-F., QU. X.-M., RUI Z.-Y., MOC X.-X., 2004, Origin of adakitic intrusives generated during mid-miocene east-west extension in southern Tibet. Earth and Planetary Science Letters, 220, LEE JEFFREY, HACKER BRADLEY R., DINKLAGE WILLIAM S., WANG YU, GANS PHILLIP, CALVERT ANDREW, JINGLIN WAN, CHEN WENJI, BLYTHE ANN E. AND MCCLELLAND WILLIAM, 2000, Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic and thermochronologic restraints. Tectonis, October 19 (5) LEE JEFFREY, HACKER BRADLEY AND WANG YU, 2004, Evolution of North Himalayan gneiss domes: structural metamorphic studies in Mabja Dome, southern Tibet. Journal of Structural Geology 26, LEE JEFFREY, MCCLELLAN W., WANG Y., BLYTHE A. AND MCWILLIAMS M., 2006, Oligocene-Miocene middle crustal flow in southern Tibet: geochronology of Mabja Dome. Geological Society, London, Special Publications 268, WHITNEY DONNA L., TEYSSIER CHRISTIAN, VANDERHAEGHE OLIVIER, 2004, Gneiss domes and crustal flow. Geological Society of America, Special Paper 380. WHITTINGTON ALAN G., 2004, The exhumation of gneiss domes in bivergent wedges Geometrical concepts and examples from the Himalayan syntaxes. Geological Society of America, Special Paper 380. YIN AN, 2004, Gneiss domes and gneiss dome systems. Geological Society of America, Special Paper 380.

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

CONTRACTION AND EXTENSION IN CONVERGENT OROGENS NORTH HIMALAYAN GNEISS DOMES: STRUCTURAL, PETROLOGIC, AND GEOCHRONOLOGIC ANALYSES OF MABJA DOME PROJECT DESCRIPTION 1. Introduction The Himalayan orogeny