David A. Foster

Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation

The active left-slip Altyn Tagh fault defines the northern edge of the Tibetan plateau. To determine its deformation history we conducted integrated research on Cenozoic stratigraphic sections in the southern part of the Tarim Basin. Fission-track ages of detrital apatites, existing biostratigraphic data, and magnetostratigraphic analysis were used to establish chronostratigraphy, whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region. Much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults, implying a temporal link between sedimentation and deformation. The results of our studies, together with existing stratigraphic data from the Qaidam Basin and the Hexi Corridor, suggest that crustal thickening in northern Tibet began prior to 46 Ma for the western Kunlun Shan thrust belt, at ca. 49 Ma for the Qimen Tagh and North Qaidam thrust systems bounding the north and south margins of the Qaidam Basin, and prior to ca. 33 Ma for the Nan Shan thrust belt. These ages suggest that deformation front reached northern Tibet only ∼10 ± 5 m.y. after the initial collision of India with Asia at 65–55 Ma. Because the aforementioned thrust systems are either termination structures or branching faults of the Altyn Tagh left-slip system, the Altyn Tagh fault must have been active since ca. 49 Ma. The Altyn Tagh Range between the Tarim Basin and the Altyn Tagh fault has been a long-lived topographic high since at least the early Oligocene or possibly late Eocene. This range has shed sediments into both the Tarim and Qaidam Basins while being offset by the Altyn Tagh fault. Its continuous motion has made the range act as a sliding door, which eventually closed the outlets of westward-flowing drainages in the Qaidam Basin. This process has caused large amounts of Oligocene–Miocene sediments to be trapped in the Qaidam Basin. The estimated total slip of 470 ± 70 km and the initiation age of 49 Ma yield an average slip rate along the Altyn Tagh fault of 9 ± 2 mm/yr, remarkably similar to the rates determined by GPS (Global Positioning System) surveys. This result implies that geologic deformation rates are steady state over millions of years during continental collision.

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.

Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau

Abstract Thermochronological data from the Songpan-Ganze˛Fold Belt and Longmen Mountains Thrust-Nappe Belt, on the eastern margin of the Tibetan Plateau in central China, reveal several phases of differential cooling across major listric thrust faults since Early Cretaceous times. Differential cooling, indicated by distinct breaks in age data across discrete compressional structures, was superimposed upon a regional cooling pattern following the Late Triassic Indosinian Orogeny. 40Ar/39Ar data from muscovite from the central and southern Longmen Mountains Thrust-Nappe Belt suggest a phase of differential cooling across the Wenchuan-Maouwen Shear Zone during the Early Cretaceous. The zircon fission track data also indicate differential cooling across a zone of brittle re-activation on the eastern margin of the Wenchuan-Maouwen Shear Zone during the mid-Tertiary, between ∼38 and 10 Ma. Apatite fission track data from the central and southern Longmen Mountains Thrust-Nappe Belt reveal differential cooling across the Yingxiu-Beichuan and Erwangmiao faults during the Miocene. Forward modelling of apatite fission track data from the northern Longmen Mountains Thrust-Nappe Belt suggests relatively slow regional cooling through the Mesozoic and early Tertiary, followed by accelerated cooling during the Miocene, beginning at ca. 20 Ma, to present day. Regional cooling is attributed to erosion during exhumation of the evolving Longmen Mountains Thrust-Nappe Belt (LMTNB) following the Indosinian Orogeny. Differential cooling across the Wenchuan-Maouwen Shear Zone and the Yingxiu-Beichuan and Erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults. Thermochronological data from the Longmen Mountains Thrust-Nappe Belt reveal a greater amount of differential exhumation across thrust faults from north to south. This observation is in accord with the prevalence of Proterozoic and Sinian basement in the hanging walls of thrust faults in the central and southern Longmen Mountains. The two most recent phases of reactivation occurred following the initial collision of India with Eurasia, suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the LMTNB.

Tectonic evolution of the Lachlan Orogen, southeast Australia: historical review, data synthesis and modern perspectives

The Lachlan Orogen,like many other orogenic belts,has undergone paradigm shifts from geosynclinal to plate-tectonic theory of evolution over the past 40 years. Initial plate-tectonic interpretations were based on lithologic associations and recognition of key plate-tectonic elements such as andesites and palaeo-subduction complexes. Understanding and knowledge of modern plate settings led to the application of actualistic models and the development of palaeogeographical reconstructions, commonly using a non-palinspastic base. Igneous petrology and geochemistry led to characterisation of granite types into ‘I’ and ‘S’, the delineation of granite basement terranes, and to non-mobilistic tectonic scenarios involving plumes as a heat source to drive crustal melting and lithospheric deformation. More recently, measurements of isotopic tracers (Nd, Sr, Pb) and U–Pb SHRIMP age determinations on inherited zircons from granitoids and detrital zircons from sedimentary successions led to the development of multiple component mixing models to explain granite geochemistry. These have focused tectonic arguments for magma genesis again more on plate interactions. The recognition of fault zones in the turbidites, their polydeformed character and their thin-skinned nature, as well as belts of distinct tectonic vergence has led to a major reassessment of tectonic development. Other geochemical studies on Cambrian metavolcanic belts showed that the basement was partly backarc basin- and forearc basin-type oceanic crust. The application of 40Ar–39Ar geochronology and thermochronology on slates,schist and granitoids has better constrained the timing of deformation and plutonism,and illite crystallinity and bo mica spacing studies on slates have better defined the background metamorphic conditions in the low-grade parts. The Lachlan deformation pattern involves three thrust systems that constitute the western Lachlan Orogen, central Lachlan Orogen and eastern Lachlan Orogen. The faults in the western Lachlan Orogen show a generalised east-younging (450–395 Ma), which probably relates to imbrication and rock uplift of the sediment wedge, because detailed analyses show that the décollement system is as old in the east as it is in the west. Overall, deformation in the eastern Lachlan Orogen is younger (400–380 Ma), apart from the Narooma Accretionary Complex (ca 445 Ma). Preservation of extensional basins and evidence for basin inversion are largely restricted to the central and eastern parts of the Lachlan Orogen. The presence of dismembered ophiolite slivers along some major fault zones, as well as the recognition of relict blueschist metamorphism and serpentinite-matrix mélanges requires an oceanic setting involving oceanic underthrusting (subduction) for the western Lachlan Orogen and central Lachlan Orogen for parts of their history. Inhibited by deep weathering and a general lack of exposure, the recent application of geophysical techniques including gravity, aeromagnetic imaging and deep crustal seismic reflection profiling has led to greater recognition of structural elements through the subcrop, a better delineation of their lateral continuity, and a better understanding of the crustal-scale architecture of the orogen. The Lachlan Orogen clearly represents a class of orogen, distinct from the Alps, Canadian Rockies and Appalachians, and is an excellent example of a Palaeozoic accretionary orogen.

Chronology of deformation within the turbidite‐dominated, Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana

Ar-Ar data from fabric-forming white mica in slates, syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism, deformation, and exhumation in the Lachlan orogen, Australia. These data also help define the tectonic evolution of the Tasmanides during Paleozoic time. The Lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to Gondwana during the closing of a small marginal ocean or back arc basin. This tectonic setting was similar to the present western and southwestern Pacific region. Accretion of the Lachlan orogen to Gondwana occurred by closing of the basin system by subduction-accretion processes and some translation. The process is typified in the western Lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement/melange zone and is recorded by Ar-Ar mica ages ranging from ∼455 Ma in the western part to ∼390 Ma in the eastern part. In the central Lachlan orogen, deformation migrated southwestward from ∼440–430 to 405 Ma away from the high-temperature Wagga-Omeo metamorphic complex, where deformation/metamorphism occurred between >440 and 400 Ma. In the north, ∼400 Ma mica ages record deformation and inversion of structures in the Cobar basin. In the eastern Lachlan orogen, Ar-Ar mica dates range from 450 to 340 Ma. Ages of 455–445 Ma are yielded by the Narooma accretionary complex, 405–390 Ma ages are found along the major thrust faults bounding high-grade metamorphic complexes, and 360–340 Ma cooling ages are found in the inverted extensional basins (e.g., Hill End) and related structural zones. The Ar-Ar results also document periods of reactivation on early-formed structures during later deformation elsewhere in the orogen.

Refrigeration of the western Cordilleran lithosphere during Laramide shallow-angle subduction

The Laramide orogeny has generally been attributed to a shift from normal-angle to shallow-angle subduction beneath the western margin of the North American plate. In addition to important mechanical effects, this shift may have had important thermal effects on the lithosphere in the western Cordillera. Before the Laramide, geothermal gradients in the western Cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge. With the shift to shallow subduction, the wedge was probably expelled and replaced by a cold subducting slab that extended just below the Cordilleran lithosphere. This would shift the western Cordillera into a cold, forearc-like thermal setting dominated by the refrigeration effects of the subducting slab. Thermochronologic data from the Sierra Nevada, Great Basin, Mojave Desert, western Arizona, and perhaps the Colorado Plateau record latest Cretaceous-early Tertiary cooling that may be evidence of such refrigeration. If regional refrigeration occurred, it would have several important implications: (1) metamorphism in the western Cordillera would have waned with the start of the Laramide orogeny; (2) the crust in the western Cordillera would have strengthened and become much more resistant to deformation (e.g., gravitational collapse and extension of thickened crust in the Sevier hinterland would have been impeded); and (3) latest Cretaceous-early Tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng.

Chronology and tectonic framework of turbidite-hosted gold deposits in the Western Lachlan Fold Belt, Victoria: – results

Abstract 40 Ar – 39 Ar data constrain the history of metamorphism, deformation and mineralization in the western subprovince of the Lachlan Fold Belt. Rocks in the Mount Stavely Volcanic Complex and the Bushy Creek Pluton give 40 Ar – 39 Ar ages of about 500 Ma for hornblende and biotite. The dates from the Mount Stavely Volcanic Complex limit the age of mineralization in the metavolcanic rocks of the Stavely Belt. Whereas, the date for the Bushy Creek pluton extends the area of Cambrian–Ordovician plutons, of the Delamerian Orogeny, to east of the Grampians Mountains and helps define the surface expression of the western border of the Paleozoic Lachlan Fold Belt. 40 Ar – 39 Ar dating of metamorphic mica growth (phengite) in cleaved slates and phyllites within the Landsborough Fault, Avoca Fault zone, Heathcote Fault zone and Mount Wellington Fault zone gives ages of 453±2, 440±2, 426±4 and 410–390 Ma, respectively. These data indicate a progression of deformation from west to east starting in Late Ordovician–Early Silurian time in the Stawell and Bendigo-Ballarat structural zones. The major phase of deformation in the western subprovince of the Lachlan Fold Belt is therefore Silurian and not Devonian. Sericites from major gold deposits in the Stawell zone and Bendigo-Ballarat zone give 40 Ar – 39 Ar dates of about 440 Ma, that are coincident with folding and thrusting. A second episode of mineralization and fault reactivation at about 420 to 410 Ma is found in some deposits in these zones, based on data from a previous study. In the Melbourne zone (including the Woods Point Dike Swarm) and locally in the Bendigo-Ballarat zone, some gold deposits give sericite dates of about 380–360 Ma. These ages are consistent with the close association of many Melbourne zone gold deposits with Devonian dikes and plutons. The results indicate that gold mineralization in the Stawell zone, Bendigo-Ballarat zone and Melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism. Initial stages of mineralization in the province, comprising many of the large deposits in western and central Victoria (e.g. Bendigo, Ballarat, Stawell, Wattle Gully, Tarnagulla), accompanied metamorphism, folding and thrusting. Subsequent mineralization is widespread but produced mainly smaller deposits (

A Damara orogen perspective on the assembly of southwestern Gondwana

Abstract The Pan-African Damara orogenic system records Gondwana amalgamation involving serial suturing of the Congo–São Francisco and Río de la Plata cratons (North Gondwana) from 580 to 550 Ma, before amalgamation with the Kalahari–Antarctic cratons (South Gondwana) as part of the 530 Ma Kuunga–Damara orogeny. Closure of the Adamastor Ocean was diachronous from the Araçuaí Belt southwards, with peak sinistral transpressional deformation followed by craton overthrusting and foreland basin development at 580–550 Ma in the Kaoko Belt and at 545–530 Ma in the Gariep Belt. Peak deformation/metamorphism in the Damara Belt was at 530–500 Ma, with thrusting onto the Kalahari Craton from 495 Ma through to 480 Ma. Coupling of the Congo and Río de la Plata cratons occurred before final closure of the Mozambique and Khomas (Damara Belt) oceans with the consequence that the Kuunga suture extends into Africa as the Damara Belt, and the Lufilian Arc and Zambezi Belt of Zambia. Palaeomagnetic data indicate that the Gondwana cratonic components were in close proximity by c. 550 Ma, so the last stages of the Damara–Kuunga orogeny were intracratonic, and led to eventual out-stepping of deformation/metamorphism to the Ross–Delamerian orogen (c. 520–500 Ma) along the leading edge of the Gondwana supercontinental margin.

Structural and thermal constraints on the initiation angle of detachment faulting in the southern Basin and Range: The Chemehuevi Mountains case study

The Cenozoic normal fault system exposed in the Chemehuevi Mountains of the southern Cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching. There, three stacked, brittle, low-angle normal faults that formed at depths as great as 10-12 km cut gently down section northeastward through deformed Proterozoic and Mesozoic crystalline basement. Hanging-wall blocks are displaced relatively northeastward. The upper crust above the Chemehuevi detachment fault was pulled apart along high-angle normal faults that rotated to more gentle dips through time. In contrast, rocks of originally mid-crustal affinity in the footwall were only gently rotated and accommodated minor extension ( Application of 40 Ar/ 39 Ar and fission-track thermochronology to rocks in the footwall of the Chemehuevi detachment fault system provides further constraints on the timing and initiation angle of regional detachment faulting. At the onset of extension between 22 and 24 Ma, granitic rocks exposed in the southwestern and northeastern portions of the footwall were at ∼200 °C and 350-400 °C, respectively, separated by a distance of some 23 km down the known slip direction. This gradual increase in temperature with original depth is attributed to the gentle southwest tilting of broadly planar pre-extension isothermal surfaces and constrains the exposed part of the Chemehuevi detachment fault to have had a regional dip initially of about 15° to 30°. The fault system apparently cut gently down through the upper crust, to a minimum depth of ∼10-12 km, the deepest exposed parts of the system today, and was domed from midcrustal depths and locally denuded during continued slip. Together the structural and thermochronologic data confirm the suggestion that faults accommodating large-magnitude slip can be initiated and move within the seismogenic regime at moderate to low angles (that is, ≤30°).

Shaping the Australian crust over the last 300 million years: insights from fission track thermotectonic imaging and denudation studies of key terranes

Apatite fission track thermochronology is a well‐established tool for reconstructing the low‐temperature thermal and tectonic evolution of continental crust. The variation of fission track ages and distribution of fission track lengths are primarily controlled by cooling, which may be initiated by earth movements and consequent denudation at the Earths surface and/or by changes in the thermal regime. Using numerical forward‐modelling procedures these parameters can be matched with time‐temperature paths that enable thermal and tectonic processes to be mapped out in considerable detail. This study describes extensive Australian regional fission track datasets that have been modelled sequentially and inverted into time‐temperature solutions for visualisation as a series of time‐slice images depicting the cooling history of present‐day surface rocks during their passage through the upper crust. The data have also been combined with other datasets, including digital elevation and heat flow, to image the denudation history and the evolution of palaeotopography. These images provide an important new perspective on crustal processes and landscape evolution and show how important tectonic and denudation events over the last 300 million years can be visualised in time and space. The application of spatially integrated denudation‐rate chronology is also demonstrated for some key Australian terranes including the Lachlan and southern New England Orogens of southeastern Australia, Tasmania, the Gawler Craton, the Mt Isa Inlier, southwestern Australian crystalline terranes (including the Yilgarn Craton) and the Kimberley Block. This approach provides a readily accessible framework for quantifying the otherwise undetectable, timing and magnitude of long‐term crustal denudation in these terranes, for a part of the geological record previously largely unconstrained. Discrete episodes of enhanced denudation occurred principally in response to changes in drainage, base‐level changes and/or uplift/denudation related to far‐field effects resulting from intraplate stress or tectonism at plate margins. The tectonism was mainly associated with the history of continental breakup of the Gondwana Supercontinent from Late Palaeozoic time, although effects related to compression are also recorded in eastern Australia. The results also suggest that the magnitude of denudation of cratonic blocks has been significantly underestimated in previous studies, and that burial and exhumation are significant factors in the preservation of apparent ‘ancient’ features in the Australian landscape.