Inverted Metamorphism Research Articles

Silurian inverted Barrovian-type metamorphism in the Western Sierras Pampeanas (Argentina): a case of top to bottom heating?

AbstractThis paper focuses on one orogenic belt that formed during the Rinconada phase on the final stage of the Famatinian orogeny, between 445 and 410 Ma, which is well exposed at Sierra de Ramaditas and neighbouring ranges in western Argentina. The Ramaditas Complex is formed by metasedimentary and meta-ultrabasic rocks and amphibolites. This complex forms the upper nappe of a thrust stack resulting from westward thrusting. Deformation consists of an early high-temperature S1 foliation (stromatic migmatites), coeval with thrusting and metamorphism. Metamorphism attained peak P–T conditions of 6.0–6.9 kbar and 795–810 °C, at c. 440 Ma, i.e. coincident with the Rinconada orogenic phase. The lower unit and intermediate nappes crop out in the nearby sierras of Maz and Espinal and underwent low- to medium-grade Silurian metamorphism, respectively, together with the upper nappe, defining an inverted Barrovian-type metamorphism with T decreasing and P increasing downwards across the thrust stack (i.e. westward). We argue that the Rinconada orogenic phase developed near the continental margin of SW Gondwana, during a magmatic lull following accretion of the Precordillera terrane to the continental margin at c. 470 Ma. The active margin jumped to the west after accretion, and flat-slab subduction resumed in the early Silurian, provoking thrusting and imbrication of nappe stack under the still hot root (800–900 °C) of the older Famatinian magmatic arc. This ‘hot-iron’ process explains both the inverted Barrovian-type metamorphism and the missing overburden of 21 to 24 km implied by the P estimate.

Metamorphism of Jhyallaphay-Barpak-Bhachchek area of Gorkha District, Lesser Himalaya and Higher Himalaya, Central Nepal

The study is focused on geological mapping, petrography and metamorphism of Jhyallaphat–Barpak–Bhachchek area, a part of Gorkha District, Central Nepal using a base of 1:25000 scale covering an area of 139.80 sq. km. The rocks of the study area can be broadly divided into two tectonic zones; the Lesser Himalaya consisting of fie lithological units, and the Higher Himalaya consisting of Formation I of the Tibetan Slab. Three metamorphic zones can be distinguished in the study area; biotite zone, garnet zone and kyanite zone. The biotite zone of the mineral assemblage in pelitic rocks consists of biotite+muscovite+chlorite+quartz, in psammitic rocks comprises of biotite+muscovite+chlorite+feldspar+quartz and in carbonate rocks comprises of biotite+muscovite+calcite/dolomite+feldspar+quartz, respectively. These mineral assemblages show that the area belongs to the greenschist facies. The mineral assemblage of the garnet zone in pelitic rocks constitutes garnet+biotite+muscovite+chlorite+quartz, and in psammitic rocks constitutes of garnet+biotite+muscovite+feldspar+quartz. The minerals assemblages found within the biotite and garnet zones represent the well-known inverted metamorphism in the Lesser Himalaya. Mineral assemblage of the kyanite zones constitutes of kyanite+garnet+biotite+muscovite+feldspar+quartz. The mineral assemblages of the both garnet and kyanite zones show that the area belongs to the epidote amphibolite facies.
 The bedding and foliation planes are almost parallel, showing that isograds also cut across the foliation. Therefore, the main metamorphic event should have followed development of foliation in the area. The rocks of the area show at least two metamorphic events: syntectonic prograde and post-tectonic retrograde. Syn-tectonic prograde metamorphism (M1), which has grown during a single phase of deformation and most frequently encountered garnet prophyroblast. Metamorphic deformation is represented by the presence of metamorphic foliation, stretching lineation, and S-C fabric. Post-tectonic retrograde metamorphism (M2), which is followed by retrograde mineral formation changing its P-T condition from high to low grade minerals, such as the formation of the biotite and chlorite minerals around the rims of the garnet porphyroblasts.

Major role of shear heating in intracontinental inverted metamorphism: Inference from a thermo-kinematic parametric study

Inverted metamorphism corresponds to the stacking of high-temperature metamorphic units structurally on top of lower-temperature units and is commonly observed along main thrusts in major orogens. Yet, in spite of many existing models, the origin and preservation of the metamorphic inversion in intracontinental collision belts are still debated. In this study, we use a crustal-scale 2D thermo-kinematic model in order to investigate the key parameters controlling the inversion of the geothermal gradient at crustal scale. Our results confirm that the kinematic framework strongly impacts the thermal evolution around the thrust. Erosion velocity and thermal conductivity of rocks are two parameters that control the spatial location of the thermal perturbation and the intensity of inversion, respectively. However, even in extreme kinematic configurations, i.e., convergence velocities > 3 cm ∙ yr− 1 and relatively high thrust dip angles ~ 30°, the thermal inversion is fleeting and thrust temperatures cannot reach the high temperature peak values characteristic of natural occurrences (> 600 °C) if shear heating is not taken into account. Conversion of mechanical energy into heat represents a main contribution to the thermal budget along main crustal shear zones. It leads to high temperature conditions in the thrust zone and our results attest that it is the only process that allows the preservation through time of an intense thermal inversion. Our quantification shows that shear heating is much more efficient than other processes such as accretion and surface denudation and is compatible with the observations of inverted metamorphism in the Himalayan or Variscan belts, for example. This comparison with natural occurrences suggests that the formation and preservation of intracontinental inverted metamorphism require shear zone viscosity values of the order of 1020–1021 Pa ∙ s for convergence velocities between 1 and 3 cm ∙ yr− 1.

Metamorphic evolution, partial melting and rapid exhumation above an ancient flat slab: insights from the San Emigdio Schist, southern California

The San Emigdio and related Pelona, Orocopia, Rand and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent southern California batholith along a shallow segment of the subducting Farallon plate in Late Cretaceous to early Tertiary time. These subduction accretion assemblages represent a regional, deeply exhumed, shallowly dipping domain from an ancient slab segmentation system and record the complete life cycle of the segmentation process from initial flattening and compression to final extensional collapse. An important unresolved question regarding shallow subduction zones concerns how the thermal structure evolves during the slab flattening process. New field relationships, thermobarometry, thermodynamic modelling and garnet diffusion modelling are presented that speak to this issue and elucidate the tectonics of underplating and exhumation of the San Emigdio Schist. We document an upsection increase in peak temperature (i.e. inverted metamorphism), from 590 to 700 °C, peak pressures ranging from 8.5 to 11.1 kbar, limited partial melting, microstructural evidence for large seismic events, rapid cooling (825–380 °C Myr^−1) from peak conditions and an ‘out and back’P–T path. While inverted metamorphism is a characteristic feature of southern California schists, the presence of partial melt and high temperatures (>650 °C) are restricted to exposures with maximum depositional ages between 80 and 90 Ma. Progressive cooling and tectonic underplating beneath an initially hot upper plate following the onset of shallow subduction provide a working hypothesis explaining high temperatures and partial melting in San Emigdio and Sierra de Salinas schists, inverted metamorphism in the schist as a whole, and the observed P–T trajectory calculated from the San Emigdio body. Lower temperatures in Pelona, Orocopia and Rand schists are likewise explained in the context of this overarching model. These results are consistent with an inferred tectonic evolution from shallow subduction beneath the then recently active Late Cretaceous arc to exhumation by rapid trench-directed channelized extrusion in the subducted schist.

Geothermobarometry of the Dudatoli-Almora Crystallines, Garhwal, Kumaun Lesser Himalaya

The Garhwal and Kumaun regions of the Himalaya in Uttaranchal State of India are critical areas for studying the typical characteristics of the Himalayan fold-and-thrust belt, in contrast to the areas in close proximity of the northeast and northwest Himalayan syntaxes, where complications arise due to strike-slip faulting. In this region, the Lesser Himalaya is characterized by the occurrence of many crystalline bodies of varying dimensions, of which synformally disposed DudatoliAlmora Crystalline Zone (ACZ) is the largest. Dudatoli-Almora Crystallines (also called Almora Klippen) constitutes one of the most important tectonic units in the Garhwal-Kumaun Lesser Himalaya and it is the remnant of a large thrust sheet nappe that moved southward from the Higher Himalayan Crystalline Zone, to rest over the Lesser Himalayan Volcano-Sedimentary belt. The Almora klippen consists of amphibolite grade metapelites, quartzites, augen gneisses and granites belonging to the Precambrian Almora Group. At places, the greenschist grade metapelites of the Precambrian Ramgarh Group (Chails) occur underneath the Almora Group, forming a series of lower klippen. The North Almora Thrust (NAT) marks the northern boundary of the Almora klippen, its southern margin is bound by the South Almora Thrust (SAT). The metamorphic sequence of the Almora Klippen exhibits regional metamorphism of Barrovian type, which increases progressively upward in the sequence from peripheral part to the central part of the synform. In Champawat and Almora areas the metamorphism is largely restricted to the chlorite, biotite and garnet zones; however, in the Dudatoli-Bungidhar regions, the metamorphic grade reaches upto staurolite-kyanite and biotite-sillimanite zones. The EPMA data on garnet, biotite, muscovite and plagioclase were obtained on twenty samples collected from Chamoli, Pauri and Almora districts of Garhwal and Kumaun Himalaya by utilizing JEOL-8600 M super microprobe. In the rim of garnet, (X Mg /X Fe ) varies from 0.053 to 0.297, X Almandine varies from 0.488 to 0.780 and (Ca+Mn)/(Ca+Mn+Fe+Mg) varies from 0.065 to 0.423. In the core of garnet, (XMg/XFe) varies from 0.030 to 0.279, XAlmandine varies from 0.487 to 0.753 and (Ca+Mn)/ (Ca+Mn+Fe+Mg) varies from 0.118 to 0.461. Ti in garnets is significantly low, in the rim it varies from 0.004 to 0.012 and in cores 0.008 to 0.115. In the rim of biotites, (X Mg /X Fe ) varies from 0.513 to 2.245, (Al+Ti)/(Al+Ti+Fe+Mg) varies from 0.142 to 0.204 and XPhlogopite from 0.270 to 0.579. In the core of biotites, (XMg/XFe) varies from 0.547 to 2.198, (Al VI+Ti)/(AlVI+Ti+Fe+Mg) varies from 0.125 to 0.216 and XPhlogopite from 0.277 to 0.581. In all the samples from Dudatoli-Almora crystallines (Ca+Mn)/ (Ca+Mn+Fe+Mg) in garnets exceeds value of 0.2 or (AlVI+Ti)/ (Al+Ti+Fe+Mg) in biotite exceeds value of 0.15. P-T estimates have been done on the Dudatoli-Almora Crystallines in order to place quantitative constraints on the conditions attained during the regional metamorphism. P-T calculations were carried out using computer programs BGT (Jowhar 1999), GPT (Reche and Martinez 1996), TWQ (Berman 1991), WEBINVEQ (Gordon 1998). GPT computer program (Reche and Martinez, 1996) was used for simultaneous solution of P and T. For P estimation garnetplagioclase-muscovite-biotite-quartz geobarometer and for T estimation various models of garnet-biotite geothermometer were utilized. It is interpreted that in Dudatoli-Almora Crystallines T varies from 500 to 650 oC and P from 6 to 8 kbar. Spatial distribution of data indicates inverted metamorphism.

Geology and tectonic history of the Lohit Valley, Eastern Arunachal Pradesh, India

The Lohit River section of eastern Arunachal Pradesh comprises four tectonic units. From SW to NE these are: the Lesser Himalayan rocks, the Mishmi Crystallines, the Tidding Suture Zone and the Lohit Plutonic Complex. The Mishmi Thrust underlies the basal Lesser Himalayan unit, while the Mishmi Crystallines are thrust over the Lesser Himalayan unit along the Main Central Thrust. The grade of metamorphism in the Mishmi Crystallines increases up the structural section from chlorite to staurolite–kyanite zones, exhibiting inverted metamorphism. The relationship between deformation and metamorphism shows that the metamorphic peak was syn- to post-tectonic in relation to the main ductile shearing event. Continued deformation, after the metamorphic peak, was accommodated along millimetre scale shear zones, developed throughout the sequence, parallel to the regional schistosity. Movement along these shear zones has resulted in inversion of the metamorphic zones. The rocks of the Tidding Suture represent an ophiolitic mélange, thrust over the Mishmi Crystallines, which in turn are overthrust by the Lohit Plutonic Complex along the Lohit Thrust. The Lohit Plutonic Complex is subdivided into western and eastern belts separated by the Walong Thrust. The western belt consists of deformed quartz-diorite, diorite, gabbro and trondhjemite, intruded by basic and acid dykes. The eastern belt comprises garnet–sillimanite gneiss, intercalated with crystalline marble bands, followed by a complex zone of leucogranites, aplites and pegmatites, which intrude the early foliated quartz-diorite, soda-rich granite and microdiorite. The rocks of the eastern belt are the northward continuation of the Mogok Gneissic Belt of central Burma. The occurrence of intrusive rocks in the eastern belt suggests that the magmatism related to subduction extended to the east, far from the subduction zone. The peraluminous leucogranites, aplites and pegmatites are the products of crustal melting, induced by crustal thickening related to the intracontinental Walong Thrust. Subsequent to metamorphism and shearing, the whole sequence was folded into an antiform, forming the Eastern Syntaxis, and this deformation steeply tilted the earlier low angle thrusts and foliations. Later compression partitioned into right-lateral strike-slip motion, producing a superimposed sub-horizontal lineation observed mostly in the Lohit Plutonic Complex.

Synthrusting metamorphism, cooling, and erosion of the Himalayan Kathmandu Complex, Nepal

In this paper we tackle some of the outstanding problems of the Himalaya, in particular the external zone in the Kathmandu Complex, using an integrated approach involving field mapping, microstructure, thermobarometry, and geochronology. The result is a new model showing the evolution of one major Main Central Thrust: therefore we refute suggestions that the Kathmandu Complex is a klippe or separate thrust sheet. Compared to the Main Central Thrust sheet in the High Himalaya, the Kathmandu Complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the High Himalaya, fairly near the leading edge. Unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the Main Central Thrust wedge was ∼40 km thick on the northern side of the Kathmandu Complex and <20 km thick on the southern margin. Initiation of the Main Central Thrust occurred at ∼22 Ma, possibly during the closing stages of peak amphibolite facies metamorphism; slip at elevated temperature (500°–600°C) continued until ∼14 Ma. This is a little longer than has previously been proposed. In marked contrast to the famous inverted metamorphism on the Main Central Thrust in the High Himalaya, the metamorphic zonal scheme in the Kathmandu Complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base. These mylonites postdate the high‐grade regional amphibolite metamorphism and give an illusion of inverted metamorphism. A likely reason for the contrast is that the Main Central Thrust cut up section toward the foreland and therefore at Kathmandu, carries high levels in the metamorphic structure. Our model involves reactivation of the Main Central Thrust at 7–8 Ma as inferred from published monazite and mica ages, but because the Kathmandu rocks show no evidence for high‐temperature reactivation at this time, we presume that the late reactivation involved only the internal High Himalaya zone while the Main Central Thrust was inactive in the external Kathmandu zone. We attempt to quantify rates of cooling, exhumation and thrusting during time period 22 Ma to the present.

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

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 reflects 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 find 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 specific 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 influencing attention toward finding a causal relationship between anatexis and inverted metamorphism.

An overview of the stratigraphy and tectonics of the Nepal Himalaya

Nepal can be divided into the following five east–west trending major tectonic zones. (i) The Terai Tectonic Zone which consists of over one km of Recent alluvium concealing the Churia Group (Siwalik equivalents) and underlying rocks of northern Peninsular India. Recently active southward-propagating thrusts and folds beneath the Terai have affected both the underlying Churia and the younger sediments. (ii) The Churia Zone, which consists of Neogene to Quaternary foreland basin deposits and forms the Himalayan mountain front. The Churia Zone represents the most tectonically active part of the Himalaya. Recent sedimentologic, geochronologic and paleomagnetic studies have yielded a much better understanding of the provenance, paleoenvironment of deposition and the ages of these sediments. The Churia Group was deposited between ∼14 Ma and ∼1 Ma. Sedimentary rocks of the Churia Group form an archive of the final drama of Himalayan uplift. Involvement of the underlying northern Peninsular Indian rocks in the active tectonics of the Churia Zone has also been recognised. Unmetamorphosed Phanerozoic rocks of Peninsular India underlying the Churia Zone that are involved in the Himalayan orogeny may represent a transitional environment between the Peninsula and the Tethyan margin of the continent. (iii) The Lesser Himalayan Zone, in which mainly Precambrian rocks are involved, consists of sedimentary rocks that were deposited on the Indian continental margin and represent the southernmost facies of the Tethyan sea. Panafrican diastrophism interrupted the sedimentation in the Lesser Himalayan Zone during terminal Precambrian time causing a widespread unconformity. That unconformity separates over 12 km of unfossiliferous sedimentary rocks in the Lesser Himalaya from overlying fossiliferous rocks which are >3 km thick and range in age from Permo-Carboniferous to Lower to Middle Eocene. The deposition of the Upper Oligocene–Lower Miocene fluvial Dumri Formation records the emergence of the Himalayan mountains from under the sea. The Dumri represents the earliest foreland basin deposit of the Himalayan orogen in Nepal. Lesser Himalayan rocks are less metamorphosed than the rocks of the overlying Bhimphedis nappes and the crystalline rocks of the Higher Himalayan Zone. A broad anticline in the north and a corresponding syncline in the south along the Mahabharat range, as well as a number of thrusts and faults are the major structures of the Lesser Himalayan Zone which is thrust over the Churia Group along the Main Boundary Thrust (MBT). (iv) The crystalline high-grade metamorphic rocks of the Higher Himalayan Zone form the backbone of the Himalaya and give rise to its formidable high ranges. The Main Central Thrust (MCT) marks the base of this zone. Understanding the origin, timing of movement and associated metamorphism along the MCT holds the key to many questions about the evolution of the Himalaya. For example: the question of whether there is only one or whether there are two MCTs has been a subject of prolonged discussion without any conclusion having been reached. The well-known inverted metamorphism of the Himalaya and the late orogenic magmatism are generally attributed to movement along the MCT that brought a hot slab of High Himalayan Zone rocks over the cold Lesser Himalayan sequence. Harrison and his co-workers, as described in a paper in this volume, have lately proposed a detailed model of how this process operated. The rocks of the Higher Himalayan Zone are generally considered to be Middle Cambrian to Late Proterozoic in age. (v) The Tibetan Tethys Zone is represented by Cambrian to Cretaceous-Eocene fossiliferous sedimentary rocks overlying the crystalline rocks of the Higher Himalaya along the Southern Tibetan Detachment Fault System (STDFS) which is a north dipping normal fault system. The fault has dragged down to the north a huge pile of the Tethyan sedimentary rocks forming some of the largest folds on the Earth. Those sediments are generally considered to have been deposited in a more distal part of the Tethys than were the Lesser Himalayan sediments. The present tectonic architecture of the Himalaya is dominated by three master thrusts: the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). The age of initiation of these thrusts becomes younger from north to south, with the MCT as the oldest and the MFT as the youngest. All these thrusts are considered to come together at depth in a flat-lying decollement called the Main Himalayan Thrust (MHT). The Mahabharat Thrust (MT), an intermediate thrust between the MCT and the MBT is interpreted as having brought the Bhimphedi Group out over the Lesser Himalayan rocks giving rise to Lesser Himalayan nappes containing crystalline rocks. The position of roots of these nappes is still debated. The Southern Tibetan Detachment Fault System (STDFS) has played an important role in unroofing the higher Himalayan crystalline rocks.

Neoproterozoic high-pressure metamorphism and tectonic constraint from the nappe system south of the São Francisco Craton, southeast Brazil

The Neoproterozoic giant nappe system exposed south of the São Francisco craton underwent a minimum of 150 km of near-horizontal northeastward displacement. This nappe system comprises an uppermost unit derived from a plutonic magmatic arc terrane with high-pressure, high-temperature intermediate to mafic granulites at its base (Socorro–Guaxupé nappe), underlain in sequence by a high-pressure kyanite granulite nappe, an amphibolite facies metapelite nappe, and finally by a quartzite nappe which forms the lowest part of the pile and rests on a parautochthonous unit related to the passive margin of the craton. The kyanite granulites crystallized ca 13 kbar, 750°C and have only at their top sillimanite related to T-increase overprinting ( T=890°C), a consequence of downward heat advection from the overlying Socorro–Guaxupé granulites equilibrated at 900°C. Pressures of 13–14 kbar are documented in the underlying metapelite nappe with temperatures of 640–670°C, in which lenses of eclogite indicate P max of ca 17.5 kbar. Phyllites of the parautochthonous unit are directly overlain by HP granulites in the north and record P=7 kbar and T=500°C. The distribution of metamorphic facies is reminiscent of inverted metamorphism with eclogites from subduction-related metamorphism and kyanite granulites from early extrusion of subducted continental crust. The metamorphic conditions of the parautochthon may represent a stage achieved during late metamorphic thinning of the nappe pile. The overall picture suggests that high-pressure units recrystallized in a subduction zone located to the WSW. The frontal thrusts of the nappe system show a late, north northeastward transported thin-skinned pattern.

Flow controlled inverted metamorphism in shear zones

Numerous processes and models have been developed in order to explain inverted metamorphism in shear zones. This article demonstrates, by means of continuum mechanics, that the orientation of a sample profile across a shear zone before and after deformation can be used to quantify the conditions under which inverted metamorphism is likely to have developed. A thrusting shear zone deforming under ideal simple shear will develop an inverted metamorphic zonation if the angle between the thrust and isotherms is greater than 5–10°, even at only moderate strains. If the flow is general shear, this angle is considerably larger (i.e. >60° for a kinematic vorticity number of W k=0.5). Additionally, the stretch of the sample profile across the shear zone, which is not influenced by the flow type, bears important implications for the interpretation of petrological, geochronological and structural data. Although this kinematic model is clearly an end-member model, neglecting dissipative processes, it can easily account for the structural and petrological data of the inverted metamorphism observed throughout the Higher Himalayan Crystalline wedge in the Sutlej Valley (India).

Deformation-induced inverted metamorphic field gradients: an example from the southeastern Canadian Cordillera

Exhumed middle crustal rocks of the hinterland of the southeastern Canadian Cordillera were deformed and metamorphosed during Mesozoic and Tertiary progressive crustal thickening. High structural levels were transported to the northeast relative to lower levels through a combination of thrusting and ductile non-coaxial flow. Progressive growth of the orogen and advance of hinterland rocks toward the foreland are revealed through analysis of diachronous metamorphism and associated deformation. At the deepest exposed level, allochthonous rocks of the orogen (Selkirk allochthon) structurally overlie Early Proterozoic basement and younger cover rocks (Monashee complex) that are correlated with autochthonous North American crust underlying the eastern Foreland Belt. The crustal zone, which marks the boundary between the Selkirk allochthon and Monashee complex, exhibits an inverted metamorphic field gradient. New data presented in this paper refute previous interpretations, which asserted that the metamorphic inversion is a result of the downward transfer of heat from the allochthon to the underlying Monashee complex. Rather, the inversion is attributed to synmetamorphic non-coaxial progressive deformation. A model is proposed in which substantial northeastward directed shear strain and attendant attenuation led to lateral transfer of rocks thereby preserving evidence of strongly diachronous deformation and an apparent inverted metamorphism.

A Late Miocene-Pliocene origin for the Central Himalayan inverted metamorphism

Perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the Himalayan Main Central Thrust (MCT), generally thought to have been active during the Early Miocene. However, in situ 208Pb/232Th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the MCT footwall occurred in this portion of the central Himalaya at only ca. 6 Ma. The apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the MCT which tectonically telescoped the young metamorphic sequence. This explanation may resolve some outstanding problems in Himalayan tectonics, such why the MCT and not the more recently initiated thrusts marks the break in slope of the present day mountain range. It also renders unnecessary the need for exceptional physical conditions (e.g., high shear stress) to explain the generation of the Himalayan leucogranites.

Inverted metamorphism in the intracontinental ductile shear zone during Himalayan collision tectonics

Research Article| May 01, 1993 Inverted metamorphism in the intracontinental ductile shear zone during Himalayan collision tectonics A. K. Jain; A. K. Jain 1Department of Earth Sciences, University of Roorkee, Roorkee 247667, India Search for other works by this author on: GSW Google Scholar R. M. Manickavasagam R. M. Manickavasagam 1Department of Earth Sciences, University of Roorkee, Roorkee 247667, India Search for other works by this author on: GSW Google Scholar Author and Article Information A. K. Jain 1Department of Earth Sciences, University of Roorkee, Roorkee 247667, India R. M. Manickavasagam 1Department of Earth Sciences, University of Roorkee, Roorkee 247667, India Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (5): 407–410. https://doi.org/10.1130/0091-7613(1993)021<0407:IMITID>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. K. Jain, R. M. Manickavasagam; Inverted metamorphism in the intracontinental ductile shear zone during Himalayan collision tectonics. Geology 1993;; 21 (5): 407–410. doi: https://doi.org/10.1130/0091-7613(1993)021<0407:IMITID>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Collision tectonics in the Himalaya ca. 50 Ma remobilized the Proterozoic basement and its cover of the Indian plate into the Higher Himalayan crystalline belt. This belt displays an inverted metamorphism progressing from a garnet to a sillimanite-K-feldspar zone toward higher topographic and structural levels. Because previously hypothesized mechanisms are not sufficient to explain inverted metamorphism, we propose that ductile shear displacements along ubiquitous, closely spaced S-C shear planes may have caused metamorphic inversion across a 15-20-km-thick, southwest-vergent, Higher Himalayan shear zone. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Deformation and metamorphism within the Main Central Thrust zone, Arun Tectonic Window, eastern Nepal

Abstract The Lesser Himalayan Arun Tectonic Window comprises the Upper Precambrian, polymetamorphosed metasediments of the Nawakot Complex. These are tectonically overlain by Precambrian gneisses of the Higher Himalayan Kathmandu Complex. The main phase metamorphism within the Higher Himalayan rocks predated thrusting along the Main Central Thrust. By contrast, the main metamorphism (M2) within the Nawakot Complex commenced synchronously with it being underthrust during D2 along the Main Central Thrust Zone beneath the amphibolite facies rocks of the Kathmandu Complex. During D2 rocks on both sides of the Main Central Thrust Zone underwent intensive mylonitization within a several hundred meter wide zone of non-coaxial shearing. Although the stratigraphic order within the Nawakot Complex is the right way up, rocks within the Arun Tectonic Window show a concentric inverted metamorphic zoning, ranging from lower greenschist facies in the core of the window, up to lower amphibolite facies at the outer edge of the window. This ‘inverted metamorphism’ (M2) of the Nawakot Complex originated as a result of syn- to post-D2 downward heat conduction from the ‘hot’ hanging wall of the Main Central Thrust onto the ‘cold’ footwall. This heat flow also caused the strong retrograde overprinting at the base of the Kathmandu Complex. After a static period, a late- to post-M2 NNE-shortening (D3) commenced along the Main Boundary Thrust. The Arun Tectonic Window is the result of doming of the Main Central Thrust during D4 followed by erosional exhumation.

The nappes of La Chataigneraie, southwest Massif Central, France

The well-known inverted metamorphism in the La Chataigneraie zone has previously been interpreted as a reverse limb of a recumbent fold with SW-vergence. Newly interpreted data indicates that it is the result of Early Carboniferous thrusting.

Inverted metamorphic zonation and large-scale thrusting in the Variscan Belt: an example in the French Massif Central

Summary Half of the northern part of the French Massif Central has been shown to be allochthonous. In the allochthonous rocks, preserved high-pressure granulites represent an early tectonometamorphic event which took place before thrusting. During the southward-directed overthrusting of this high-grade metamorphic slab a plurifacial inverted metamorphism was developed in the underlying sediments. Adjacent areas show different types of metamorphism in the autochthonous and parautochthonous pelites. The climax of the metamorphism is shown to have been reached by the end of the thrusting episode. This was followed by the retrogression of the high-grade rocks and the establishment of a low-pressure metamorphism in both the thrust unit and the autochthonous-parautochthonous terranes beneath. We describe this example of inverted metamorphic zonation associated with the large-scale thrusting of an already metamorphosed series.

Origin of the inverted metamorphism of the lower Himalayas, Central Nepal

The southern slope of the central Nepal Himalayas is divided geologically by the Main Central Thrust zone into the underlying Midland metasediment zone (Lower Himalayas) in the south and the overlying Himalayan gneiss zone (Higher Himalayas) in the north. The former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium, and the latter zone consists of high-grade crystalline rocks with a north-dipping homoclinal structure and is further overlain by the Tethys Himalayas, composed of non- or slightly metamorphosed Tethyan sediments. The Main Central Thrust zone shows a characteristic lithology such as calcareous schist, quartzite, phyllitic schist and augen gneiss, and is intensely mylonitized. The mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the Midland metasediment to the Main Central Thrust zones, that is, the metamorphic grade of the Midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies, and the greenschist-amphibolite transitional zone is attained in the thrust zone. The Himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone. On the basis of the compositional zoning of garnets and muscovite compositions, however, it is concluded that the Himalayan gneisses suffered a polymetamorphism. The younger metamorphism resulted in a retrograde metamorphism of the Himalayan gneisses together with the above-mentioned inverted metamorphism and an anatectic melting in depth. This was caused by shear heating due to the thrust movement along the Main Central Thrust zone during the Alpine tectonism. The older metamorphism, which seems to be Precambrian in age, was much higher in metamorphic grade than the younger one and formed the pyrope-rich cores of garnets in the Himalayan gneisses. The maximum metamorphic temperature of 550°C, estimated from the mineral paragenesis, in the thrust zone, is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5–10 cm/yr, maximum stress of 300 bars and thrusting distance of 250 km (Bird, 1978).