Peter J. Treloar

Cooling and uplift histories of the crystalline thrust stack of the Indian Plate internal zones west of Nanga Parbat, Pakistan Himalaya

Abstract Amphibole and mica K-Ar, Ar-Ar and Rb-Sr geochronology for the crystalline internal zones of the Indian Plate define both an extensive pre-Himalayan thermal history and a post-Himalayan metamorphism cooling history. South of the Main Mantle Thrust, near Besham, hornblende Ar-Ar ages from basement gneisses record an ca. 1850 Ma mid-Proterozoic thermal event. Hornblende, muscovite and biotite cooling ages from cover sequences metamorphosed during the Himalayan orogeny are 35 ± 4, 30 to 24, and 29 to 22 Ma respectively. The mica ages, together with those derived from zircon and apatite fission track data (Zeitler, 1985) demonstrate a rate of cooling, of about 30°C/Ma, during the late Oligocene to early Miocene that was greater than that either before or since. This rapid cooling was initiated during the post-metamorphic evolution of the Indian Plate south-verging crustal-scale thrust stack, during which cover sequences metamorphosed during the Himalayan orogeny were imbricated with basement rocks thermally unaffected during that event. Most of the cooling, which happened during the stripping of some 10 ± 2 km of overburden, reflects exhumation due to a combination of erosion, recorded in the Miocene molasse sediments of the foreland basin, and major crustal extension within the MMT zone. Both erosion and extension were the direct consequence of the evolution of the thrust stack.

Indian Plate motion and shape: constraints on the geometry of the Himalayan orogen

Abstract Sea-floor palaeomagnetic data that reflect variations in rate and vector of Indian Plate movement and rotation suggest that initial collision between India and Asia occurred at about 50–55 Ma ago. As the pre-collisional Indian Plate was diamond shaped, with the northern margin comprised of two oblique boundaries, collision was earliest where these boundaries meet, or in what is now the northwest Himalaya. Oblique convergence along each of these two boundaries would generate rotation of thrust sheets as they climb on to the Indian Plate. The oroclinal shape of the main Himalayan chain to the east of the northwest Himalayan syntaxes reflects a combination of the effects of oblique convergence, post-collisional anticlockwise rotation of the Indian Plate, and the pinning of the main thrusts at their northwestern terminations by crust thickened during the earliest collisional stage. The Indian Plate rotation enhances a strike-slip component of movement along the western oblique margin, with the transpressively sinistral Chaman fault zone now acting as a continental escape structure.

Metamorphism and crustal stacking in the North Indian Plate, North Pakistan

Abstract The northern part of the Indian Plate in North Pakistan is composed of a number of large-scale crustal nappes, each of which are stratigraphically distinct, and which were stacked late in the main phase of southeasterly directed thrusting associated with the Himalayan event. The major nappes recognised in the Swat to Kaghan area of North Pakistan are the Besham, Swat, Hazara, Banna, Lower Kaghan and Upper Kaghan nappes. Metamorphism was synchronous with early stages of deformation. Within each nappe the metamorphic grade increases upwards, an overall inversion that represents post-metamorphic imbrication within individual nappes, synchronous with the main phase of nappe stacking, rather than a “hot iron” type inversion as described under the MCT in Nepal and India. As a result of this “within-nappe” imbrication each thrust slice within any particular nappe contains rocks of a higher metamorphic grade than those in the slice below, with sharp metamorphic breaks across the imbricating thrusts as well as across the major shears that bound the crustal-scale nappes. Uplift along these imbricating thrusts initiated cooling of the stack. K-Ar mica and hornblende cooling ages imply that much of this uplift was completed by 30 Ma, or within 20 Ma of the collision.

Short Paper: Cadomian terrane tectonics and magmatism in the Armorican Massif

The North Armorican composite terrane, NW France, is a collage of displaced terranes which result from the amalgamation of Cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone. Early Cadomian arc activity occurred at c. 700-650 Ma, but terrane accretion did not occur until c. 540 Ma, and post-tectonic magmatism persisted well into the Palaeozoic. Cadomian events thus span a considerably greater period than previously supposed.

The geological evolution of the Magondi Mobile Belt, Zimbabwe

Abstract The Zimbabwe Archaean craton is flanked to the NW by the Magondi Mobile Belt. Within this belt, Magondi Supergroup volcanics and sediments were deposited during the early Proterozoic before being deformed and metamorphosed ∼ 1850 Ma ago. In the south, the belt is a typical thin-skinned thrust belt with Magondi Supergroup rocks thrust SE onto the Archaean craton. Northwards, the character of the belt changes with the structural style changing to a more thick-skinned type, metamorphic grade increasing from greenschist to granulite facies and with increasing amounts of early Proterozoic basement gneisses imbricated within the sedimentary sequence. Metamorphism in the northern part of the belt is explained by thermal relaxation after the overthrusting of hot early Proterozoic rocks from the west effectively sandwiching the Magondi rocks between Archaean basement below and allochthonous early Proterozoic basement above. Flat-lying isograds that date from this event were subsequently folded along S-plunging axes during continued orogenic squeezing. The rocks within the Magondian belt are essentially unaffected by subsequent Pan-African deformation and metamorphism. The evolution of the belt seems to fit the emerging pattern of cratonisation of the African plate, within which pattern the Magondian Mobile Belt may be correlated with other ‘Eburnian’ cycle belts in South Africa but not with any of the tectonothermal provinces to the north of the Pan-African Zambezi-Damara Belt.

Metamorphism and geochronology of granulites and migmatitic granulites from the Magondi Mobile Belt, Zimbabwe

The Magondi Mobile Belt, which crops out to the west of the Zimbabwe Archaean craton, is an early- to mid-Proterozoic feature within which sediments of the Magondi Supergroup were deformed and metamorphosed. Metamorphic grade increases northwards along the strike of the belt from greenschist facies in the south to granulite facies in the north. The granulite-facies rocks were partially melted during metamorphism at pressures of 5–7 kbar and temperatures of about 700–750°C, with fluids buffered to low values of Pfl. Late stages of partial melting were driven by vapour-poor dehydration melting reactions. An age for the metamorphism is indicated by Rb-Sr ages of 1890 ± 260 Ma from enderbitic granulites at the Rukomeshe River causeway, east of Makuti, and of 1780 ± 280 Ma from garnet-bearing and granitic granulites from the Nyaodza region just east of lake Kariba. These ages confirm an early Proterozoic age for the Magondi orogeny, which can be correlated with deformation in the Kheis Belt occupying a similar position on the western margin of the Kaapvaal craton. Later brittle deformation and cataclasis, distributed throughout the granulites but locally concentrated in N-S-striking shear zones, are the result of deformation in the footwall of the south-verging Pan-African thrust sheets.

Pressure-Temperature-Time Paths of Granulite Metamorphism and Uplift, Zambesi Belt, N.E. Zimbabwe

In the Rushinga district of NE Zimbabwe at the eastern end of the Zambesi Belt two allochthonous nappes containing high pressure granulites were thrust southwards onto the northern margin of the Zimbabwe Archaean craton during the Pan-African Zambesi orogeny. Granulites in the lower sheet underwent an early granulite metamorphism associated with mylonitisation and intense ductile deformation at 12 ± 2 kb and 725–800°C. Inclusions within coarse garnets suggest that this event was preceded by an earlier higher temperature and pressure event. Subsequent uplift, documented by a variety of decompression textures, culminated in a second period of granulite metamorphism at 7 ± 2 kb and 625–700°C. These events reflect a deep crustal metamorphism early in the Zambesi orogeny, followed by re-equilibration during the upward and southward emplacement of granulite rocks within allochthonous sheets during later stages of the orogeny. The decompression textures indicate that uplift, controlled by the southward thrusting of the allochthonous granulite sheets, was accompanied by extensive unroofing, characterised by rapid erosion and/or high level continental extension.