Gweltaz Mahéo

Reconstructing the total shortening history of the NW Himalaya

The onset of India-Asia contact can be dated with both biostratigraphic analysis of syn-collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction-collision stages. These data, together with paleomagnetic data show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intra-oceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo-Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo-Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto-Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50-45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm.yr-1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening vs amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr −1 . We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effectoftheevolvingtopographycanexplainafractionofthescatterobservedinthedatabut not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumationarelikely.Ithasbeenarguedthatthesteepphysiographictransitionatthefootof the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet

On the South Karakorum margin, Neogene high-temperature–medium-pressure (HT–MP) gneisses define an east–west trending thermal anomaly. These rocks have been heated from 600 to 750°C during a slight pressure drop from 0.7 to 0.5 GPa. Their retrogressive path cross-cuts the relaxed geotherm of tectonically thickened crust. Such a P–T evolution occurs only if an advective source of heat is involved. Involvement of an advective heat source is also implied by the occurrence of Neogene granitoids and lamprophyres within the HT–MP gneiss area. These rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies. We interpret these geochemical characteristics to be the result of melting of metasomatized Asian lithospheric mantle. The Nd and Sr isotopic compositions of the South Karakorum Neogene magmatic rocks (ϵNd=−12 to −7 and 87Sr/86Sr=0.705–0.725) further suggest they could have originated from mixing between Asian variously metasomatized mantle and Precambrian crust. By contrast, the origin of the youngest magmatic rocks (<10 Myr), here exemplified by the Hemasil syenite and associated lamprophyres, requires involvement of a depleted mantle. The combined ϵHf–ϵNd signature of these rocks (ϵHf=+10.4–+11.5 and ϵNd=+3.4–+4.3) suggests that the source of the Hemasil syenite could have been depleted mantle contaminated by oceanic sediments, likely during the earlier subduction of the Tethyan ocean. Neogene magmatic rocks with the same geochemical characteristics and evolution as those of South Karakorum have previously been described in South Tibet. Based on their location and the geochemical evolution of their source region, we here propose that the Neogene magmatic and metamorphic evolution of the South Asian margin was controlled by slab breakoff of the subducting Indian continental margin starting at about 25 Ma. This model is supported by available geophysical data from South Karakorum and South Tibet.

Large-scale geometry, offset and kinematic evolution of the Karakorum fault, Tibet

The total offset, lifespan and slip rate of the Karakorum fault zone (KFZ) (western Tibet) are debated. Along the southern fault half, ongoing oblique slip has exhumed dextrally sheared gneisses intruded by synkinematic leucogranites, whose age (V23 Ma, U/Pb on zircon) indicates that right-lateral motion was already in progress in the late Oligocene. Ar/Ar K-feldspar thermochronology confirms that rapid cooling started around 12 Ma, likely at the onset of the present dextral normal slip regime. Correlation of suture zones across the fault requires a total offset greater than 250 km along the active ^ northern ^ fault branch. An average long-term slip rate of 1 ? 0.3 cm/yr is inferred assuming that this offset accrued in a time span of 23^34 Ma. Southwest of the Ladakh-Karakorum Range, the large-scale boudinage of ophiolitic units suggests that an offset of several hundreds of kilometers exists along another ^ southern ^ branch of the KFZ. Towards the southeast, in the Mount Kailas region, the fault zone does not end at Gurla Mandatha, but continues eastwards, as a transpressive flower structure, along the Indus^Tsangpo suture. Our new data thus suggest that the KFZ contributed to absorb hundreds of kilometers of India^Asia convergence.

The non-equilibrium landscape of the southern Sierra Nevada, California

The paleoelevation of the Sierra Nevada, California, is important to our understanding of the Cenozoic geodynamic evolution of the North America–Pacific plate boundary, and the current debate is fueled by data that argue for conflicting elevation histories. The non-equilibrium or transient landscape of the Sierra Nevada contains information about both past and present controls on the topography of the range. Using geomorphology and thermochronometry, two parts of the landscape of different geodynamic significance and age can be identified: (1) a long-lived, slowly eroding low-relief highland or relict landscape, which we relate to a period of lower relief and elevation from 80–32 Ma; and (2) younger, rapidly- incising river gorges created by at least two stages of elevation and relief increase since 32 Ma. Our data argue for moderate range elevation of ~1500 m at the cessation of arc magmatism in Late Cretaceous time, followed by two events at between 32 and 3.5 Ma and since 3.5 Ma that increased the range elevation to the 4000 m observed elevation today.

New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault

[1] Zircons and monazites from 6 samples of the North Ayilari dextral shear zone (NAsz), part of the Karakorum fault zone (KFZ), have been dated with the U-Th-Pb method, using both ID-TIMS and SIMS techniques. The ages reveal (1) inheritance from several events spanning a long period between the late Archean and the Jurassic; (2) an Eocene-Oligocene magmatic event (similar to 35-32 Ma); (3) an Oligo-Miocene magmatic event (similar to 25-22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (similar to 22-14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located similar to 375 km farther Southeast along the KFZ is dated at 21.1 +/- 0.3 Ma. Such occurrence of several Oligo-Miocene granites along the KFZ, some of which show evidence for synkinematic emplacement, suggests that the fault zone played an important role in the genesis and /or collection of crustal melts. We discuss several scenarios for the onset and propagation of the KFZ, and offset estimates based on the main sutures zones. Our preferred scenario is an Oligo-Miocene initiation of the fault close to the NA range, and propagation along most of its length prior to similar to 19 Ma. In its southern half, the averaged long-term fault-rate of the KFZ is greater than 8 to 10 mm/a, in good agreement with some shorter-term estimates based on the Indus river course, or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault but played a major role in the collision history.

Tectonic control on southern Sierra Nevada topography, California

In this study we integrate the apatite (U-Th)/He thermochronometric technique with geomorphic, structural, and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late Cenozoic faulting in the southern Sierra Nevada. The geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low-relief landscape, little affected by internal deformation, to a more complex landscape affected by numerous faults. Regionally, the relict landscape surface is readily resolved by age-elevation relationships of apatite He ages coupled to geomorphology. These relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting. On the basis of 63 new apatite He ages and stratigraphic data from proximal parts of the San Joaquin basin we resolve two sets of normal faults oriented approximately N–S and approximately NW. Quaternary west-side-up normal faulting along the N–S Breckenridge–Kern Canyon zone has resulted in a southwest step over from the Owens Valley system in the controlling structure on the regional west tilt of Sierran basement. This zone has also served as a transfer structure partitioning Neogene-Quaternary extension resulting from normal displacements on the NW fault set. This fault system for the most part nucleated along Late Cretaceous structures with late Cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region.

Step-over in the structure controlling the regional west tilt of the Sierra Nevada microplate: eastern escarpment system to Kern Canyon system

The Sierra Nevada and Great Valley are coupled, and behave as a semi-rigid microplate. The microplate formed as it was calved off the western edge of the Nevadaplano in the late Miocene, at which time westward regional tilting began. Tilting is controlled by west-side-up normal faulting primarily along the eastern Sierra escarpment system. Uplift and exhumation along the eastern Sierra are balanced by subsidence and sedimentation along the western Great Valley. The west tilt of the microplate is expressed by the west slope of a regional relict landscape surface that developed across much of the Sierra Nevada basement, and by the westward continuation of the surface as the basal Eocene nonconformity of the west-dipping Great Valley Tertiary section. The rigid behaviour of the microplate breaks down along its southern ∼100–150 km segment as expressed by seismicity, pervasive faulting and the development of a deep marine basin, the San Joaquin Basin (SJB), whose facies and palaeogeographic patterns diverge from regional patterns of the rest of the Great Valley. The disrupted state of the southern segment of the microplate resulted from its Late Cretaceous position above a regional lateral ramp in the underlying Franciscan-related subduction megathrust. The Kern Canyon fault system began its polyphase history as a complex oblique dextral shear zone above the megathrust lateral ramp. It was remobilized in the Neogene as an oblique transfer structure partitioning differential extension between the southern Sierra Nevada and the SJB. In Quaternary time, the Kern Canyon zone was again remobilized as a west-side-up normal fault system whose geomorphic and structural expressions are best developed south of ∼36.4° N. This normal fault system controls the west tilt of the relict landscape surface in the southern Sierra region, as well as the west dip pattern in the strata of the adjacent SJB. To the east of the Kern Canyon normal fault system, the relict landscape surface slopes continuously southwards from the high eastern Sierra into a low-lying, multiply extended terrane. Thus, from ∼36.4° N southwards, the west tilt along the western Sierra and the west dip of the adjacent Great Valley strata are controlled by the Kern Canyon system. Fresh normal scarps along the eastern Sierra escarpment system become more subdued and ultimately die out southwards from ∼36.4° N. Thus, currently, the controlling structure for the west tilt of the microplate steps westwards in the south from the eastern escarpment system over to the Kern Canyon system.

Exhumation of Neogene gneiss domes between oblique crustal boundaries in south Karakorum (northwest Himalaya, Pakistan)

In southeast Karakorum (northwest Himalaya, Pakistan), kilometric size migmatitic domes were exhumed in a context of north-south shortening during Neogene times. The domes are characterized by a conical shape, and ductile deformation criteria indicate both radial expansion and extrusion of the migmatitic core relative to the surrounding gneisses. Most of the domes are aligned along the dextral, strike-slip Shigar fault that is parallel to the N130°E Karakorum fault. Along the Shigar fault, exhumation of the domes is mainly vertical with a slight dextral component. We propose that the high temperature exhumation of the domes is due to diapiric ascent of the molten mid-crust helped by the compressive regime. The localization of the initial diapir was controlled by crustal-scale vertical structures parallel to the Karakorum fault. The later stage of exhumation in mid to low temperature conditions was related to the uplift and erosion of the whole southeastern Karakorum by crustal-scale east-west folding. In south Tibet, the westward prolongation of south Karakorum, Neogene crustal melting is also supported by geophysical data and volcanism, but mid-crustal rocks have not been exhumed. This difference between the amount of exhumation in south Karakorum and south Tibet could be related to the transpressive context of south Karakorum inducing a strain partitioning between the N130°E faults and east-west folding. Such partitioning produces heterogeneous uplift in this area. Moreover, zones of rapid uplift rate are associated with erosion due to the high incision rate of the large Shyok and Braldu rivers and the large Biafo-Hispar and Concordia glaciers in south Karakorum.

Témoins d'un arc immature téthysien dans les ophiolites du Sud Ladakh (NW Himalaya, Inde)

Abstract The ophiolites of the South Ladakh are evidence of the Neo-Tethys obduction onto the Indian continental margin, during Late Cretaceous times. The mafic rocks are cogenetic and were extracted from a N-MORB like depleted source slightly metasomatized above a subduction zone. Thermobarometry on ultramafic rocks confirms this geodynamic setting. Considering their position in the Ladakh-Zanskar area and their geochemical signatures, these ophiolites could correspond to an immature arc rather than a back-arc basin. Moreover, they are related to a subduction zone located south of the one related to the Dras arc and the Ladakh Batholith. The model deduced from this study brings new constraints on the thermo-mechanical evolution of the Indian margin.