Tom Argles

Attenuation and excision of a crustal section during extensional exhumation: the Carratraca Massif, Betic Cordillera, southern Spain

Extensional dismemberment of the Betic Cordillera during Late Oligocene to Early Miocene time caused attenuation and excision of a crustal section overlying mantle peridotite. An integrated study of metapelites overlying the peridotite sheet in the Carratraca area shows truncation of a typical orogenic cycle of burial and heating by rapid exhumation and late- to post-tectonic, low-pressure metamorphism. Attenuation by a factor of five or more condensed a greenschist to granulite section to 4 km structural thickness. Initially coaxial attenuation of the sequence, involving formation of a flat-lying foliation and conjugate shear bands, was superseded by non-coaxial deformation concentrated along low-angle, top-to-the-NE, extensional fault zones. These excised parts of the metamorphic section; a 2 km loss was calculated across one fault zone. Thermobarometry throughout this section demonstrates that the structural attenuation was related to isothermal decompression to low pressures (2-4 kbar) even at the base of the crustal section. A late thermal event largely postdates the ductile stage of attenuation, particularly in the upper part of the section. Thermal modelling implies that this thermal event could not have been produced by the hot peridotite sheet alone. Rapid tectonic exhumation and asthenospheric upwelling are the probable causes, following detachment of material from the base of the lithosphere.

Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P–T–time array

The Ronda peridotite in the Betic Cordillera of southern Spain is a relic of the sub-orogenic lithospheric mantle that was exhumed during earliest Miocene time from about 66 km depth. Overlying crustal rocks show an apparently coherent metamorphic zonation from high-pressure granulite-facies rocks at the contact to unmetamorphosed rocks 5 km higher in the structural sequence, indicating drastic tectonic thinning of the whole orogenic crust during exhumation. P–T paths from the peridotite and its crustal envelope indicate decompression with rising temperature to shallow depths. U–Pb ion microprobe dating of zircon, Ar/Ar dating of hornblende, Ar/Ar laserprobe dating of muscovite and biotite, and fission-track analysis of zircon and apatite reveal that cooling was extremely rapid in the interval 21.2–20.4 Ma. One-dimensional thermal modelling of the array of P–T–time paths indicates that an asthenospheric heat source at an initial depth of about 67.5 km is required to explain heating during exhumation, and that the main period of exhumation lasted 5 Ma, starting at around 25 Ma. Exhumation must therefore have directly followed removal of most, but not all, of the lithospheric mantle beneath the Betic orogen, and was coeval with a period of late orogenic extension that profoundly modified the crustal structure and created the present-day Alboran Sea in the western Mediterranean.

Contribution of crustal anatexis to the tectonic evolution of Indian crust beneath southern Tibet

This geochemical, geochronological and structural study of intrusive rocks in the Sakya Dome of southern Tibet has identified two distinct suites of anatectic granites that carry contrasting implications for the tectonic evolution of the India-Asia collision zone. The northern margin of the dome core was intruded by anastomosing, equigranular two-mica garnet granites between 28.1 ± 0.4 Ma and 22.6 ± 0.4 Ma, coeval with top-to-the-south shear. Trace-element and isotopic (Sr-Nd) characteristics indicate an origin from partial melting of a biotite-bearing source in the Indian crust, under conditions of high fluid-phase activity. These granites thus provide evidence for the melt weakening required by some thermo-mechanical models that predict the southwards extrusion of a low-viscosity channel during the Oligocene. Evidence for subsequent shear-sense reversal may document initiation of this process. However, a younger suite of porphyritic two-mica granite plutons, emplaced between 14.5 ± 0.9 Ma and 8.81 ± 0.22 Ma, are derived from anatexis of muscovite-bearing metasediments of the High Himalayan Series under fluid-absent conditions. Ar-Ar cooling ages of 14.4 to 8.0 Ma from the Sakya dome postdate crystallisation of the Oligocene granite suite by ca. 10 Ma, but are coincident with mid-Miocene granite emplacement, suggesting uplift to depths of <10 km by the mid-Miocene. We propose that plate flexural response to Miocene slab steepening is a likely cause of dome uplift, and that this exhumation of mid-crustal rocks triggered decompression melting at 15-9 Ma and emplacement of discrete granite plutons into the upper crust under brittle conditions.

NEW GARNETS FOR OLD? CAUTIONARY TALES FROM YOUNG MOUNTAIN BELTS

Major and trace elements in garnet may be used to characterize P–T–t paths in orogenic belts, but they are also often used without age constraints to obtain P–T estimates alone. During investigations into the prograde phase of the Alpine–Himalayan Tertiary orogeny, garnets with anomalously old Sm–Nd ages were discovered in three different areas. They occurred in identical parageneses to garnets that grew during Tertiary prograde metamorphism. Ages >500 Ma were determined for homogenized garnets from the Garhwal Himalaya, while garnets with distinctive growth zonation from the Betic Cordillera (S. Spain) gave an age of 235.1±1.7 Ma. Although the Garhwal date coincides with a recognized magmatic and metamorphic event, the 235.1 Ma age from the Betics at first sight appears one of the more unlikely times for garnet growth. Duplicate analyses precisely date growth of the garnet cores, which X-ray maps reveal to be entirely distinct from the rims, implying a hiatus before resorption and rim development. This initial garnet growth may be linked to early crustal extension during Pangaea break-up. These results reaffirm the value of garnet in the integrated study of polymetamorphic orogenic belts. At the same time, the data show the potential for unravelling the history of these complex zones, and strike a cautionary note for thermobarometry.

Correlation of lithotectonic units across the eastern Himalaya, Bhutan

Clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed—provided their source regions are properly characterized. Isotopic data from the lithotectonic units of the eastern Himalaya (Bhutan) indicate clear isotopic differences directly comparable with the equivalent units from the central Himalaya. Zircons from metaquartzites of the High Himalayan Series that range in age from 980 to 1820 Ma and an orthogneiss intruded at 825 ± 9 Ma bracket deposition to between 816 and 980 Ma. Model ages derived from Nd isotopes of associated metapelites range from 1700 to 2200 Ma. In contrast, zircons from a metaquartzite from the Lesser Himalayan Series range from 1850 to 2550 Ma. A Paleoproterozoic deposition age (ca. 1750 Ma) is inferred from the age of a metarhyolite associated with the sediments. Model Nd ages of metapelites from the Lesser Himalayan Series range from 2500 to 2600 Ma. The Bhutanese Himalayan units can be correlated more than 1000 km westward along strike from catchments of the Brahmaputra to the headwaters of the Sutlej on the basis of their structural position, provenance, and isotopes. Because current discharge from the Brahmaputra carries the dominant sedimentary flux into the Bengal Fan, results from this work validate the interpretation of isotopic variations observed in offshore deposits, in terms of the unroofing history of the component lithologies of the Himalaya.

A short‐duration pulse of ductile normal shear on the outer South Tibetan detachment in Bhutan: Alternating channel flow and critical taper mechanics of the eastern Himalaya

In easternmost Bhutan the South Tibetan detachment (STD) is a ductile shear zone that juxtaposes the Radi (or Sakteng) klippe of the Tethyan Sedimentary Series from underlying high-grade Greater Himalayan rocks. In situ LA-ICPMS U-Th-Pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the STD within the shear zone at the base of the klippe, constrains north-vergent normal shear to between 25 and 20 Ma. Coeval thrusting on the Main Central Thrust during this time supports a phase of channel flow–viscous wedge model activity, lasting only ca. 3 Ma. Geochronologic data from the eastern Himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the Late Oligocene through to the Late Miocene, switching from channel flow–viscous wedge behavior to critical taper–frictional wedge behavior, each phase lasting approximately only 2 to 5 Ma. The tectonic evolution of the eastern Himalaya is comparable to central and western Himalayan tectonics during the Early Miocene, but during the Middle Miocene metamorphism and magmatism in the eastern Himalaya migrated towards the orogenic hinterland, a process not widely documented elsewhere in the Himalaya, thus highlighting the need for an orogenic model in three spatial dimensions.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵNd(0) signature of −18.3 to −12.1, and is intruded by c. 800 Ma and c. 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵNd(0) signature of −27.7 to −23.4, intruded by c. 1830 Ma granites. The data reveal a c. 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704.

Isotope studies reveal a complete Himalayan section in the Nanga Parbat syntaxis

Many models of orogenesis invoke simple anatomies for mountain belts, comprising a small number of major tectonic provinces separated by major faults. In the Himalayan arc, three main tectonic provinces (the Lesser Himalayan, High Himalayan and Tethyan Series) have been recognized over 2500 km along strike from Bhutan to Kashmir. However, their extension westward to the Nanga Parbat syntaxis remains equivocal. We have supplemented detailed field work in the area with isotopic analysis aimed at revealing distinct signatures for each of the three main provinces. Using Sr isotopic data to refine previous Nd-based discrimination, we demonstrate that the three main tectonic provinces of the central Himalaya also occur in the western syntaxis of northern Pakistan. These three units are thus continuous along the entire orogenic arc. However, their metamorphic grade is generally higher in the syntaxis than in the central Himalaya, challenging the validity of distinctions commonly drawn on this basis elsewhere in the mountain belt. The corollary is that these high-grade units probably continue beyond the syntaxis into northwestern Pakistan, which suggests that, although the precollisional materials may be identical, the western syntaxis marks a change in tectonic style from the main orogen. This conclusion in turn requires that the burial and exhumation history in the western Himalaya be radically different from that in the central Himalaya and thus necessitates a reexamination of models for India-Asia collision.

First field evidence of southward ductile flow of Asian crust beneath southern Tibet

There is lively debate on whether Asian plate material was involved in southward flow of mid-lower crust in a ductile channel beneath southern Tibet. One argument against such involvement is the apparent absence of material derived from Asian lithosphere within the High Himalayan Series (Indian plate) that could represent the putative channel. A north-south–trending mid-Miocene dike swarm that intrudes the Tethyan sedimentary cover of the Sakya gneiss dome (Indian plate) yields new Sr-Nd isotopic data (87Sr/86Sr = 0.7071–0.7079; ϵNd −4 to −6) indicating that these melts share the same source as Miocene dacitic dikes from north of the Indus-Tsangpo suture. Moreover, dikes on both sides of this suture represent crustal melts derived largely from mid-lower crust of the Asian plate, exposed today as the Nyainqentanglha gneisses that underlie the Gangdese batholith. We infer that melting of the Asian lithosphere extended south of the surface trace of the suture, requiring southward propagation of anatectic Asian middle crustal material during the Miocene. The emplacement ages of the southern dike swarm (12–9 Ma) thus delimit the timing of active southward ductile flow of Asian material.

Tectonic implications of Palaeoproterozoic anatexis and Late Miocene metamorphism in the Lesser Himalayan Sequence, Sutlej Valley, NW India

Unravelling the kinematic evolution of orogenic belts requires that the defining tectonostratigraphic units, and structural elements that bound them, are properly identified and characterized. In the Sutlej Valley (western Himalaya), the Munsiari and Vaikrita thrusts have both been correlated with the Main Central Thrust. The sequence of amphibolite-grade rocks (the Jutogh Group) bounded by these faults has been variously assigned to the Lesser Himalayan Sequence (based on provenance ages) and to the Greater Himalayan Sequence (from its metamorphic grade). Trace-element and geochronological data from leucogranites in the Jutogh Group (1) indicate crustal melting at c. 1810 Ma, before the deposition of the Greater Himalayan Sequence, thus correlating the Jutogh Group with the Lesser Himalayan Sequence, and (2) record Proterozoic metamorphism overprinted at 10.5 ± 1.1 Ma (established from U–Pb analysis of uraninite) during the Himalayan orogeny. Pressure–temperature–time data indicate that the Jutogh Group and Greater Himalayan Sequence represent distinct tectonic units of the metamorphic core that were decoupled during their extrusion. This precludes extrusion along a single, widening channel, and requires a southward shift of the locus of movement during the Late Miocene, coincident with present-day precipitation patterns.