Hugh D. Sinclair

Tectonostratigraphic model for underfilled peripheral foreland basins: An Alpine perspective

Advances in the development of quantitative models of foreland basin stratigraphy have outpaced the observational data used to constrain the input parameters in such models. Underfilled peripheral foreland basins comprise a broad threefold subdivision of depositional realms that translates into three stratigraphic units which are commonly superimposed during basin migration; these units are here termed the “underfilled trinity.” The three units of the trinity reflect (1) carbonate deposition on the cratonic margin of the basin (the lower unit), (2) hemipelagic mud sedimentation offshore from the cratonic margin of the basin (the middle unit), and (3) deep water turbiditic siliciclastic sedimentation toward the orogenic margin of the basin (the upper unit). Theoretical predictions of how such a complex basin fill initiates and evolves through time are not currently available; hence this study reviews the stratigraphy of underfilled peripheral foreland basins and provides a unique data set comprising rates of thrust advance and basin fill migration for the Tertiary foreland basin of the European Alps. The Paleocene to Oligocene Alpine foreland basin of France and Switzerland comprises a well-developed underfilled trinity that is preserved within the outer deformed margins of the Alpine orogen. Structural restorations of the basin indicate a decrease in the amount of basin shortening from eastern Switzerland (68%) to eastern France (48%), to southeastern France (35%). Structurally restored chronostratigraphic diagrams allow rates of basin migration to be calculated from around the Alpine arc. Paleogeographic restorations of the Nummulitic Limestone (lower unit) illustrate a radial pattern of coastal onlap on to the European craton. Time-averaged rates for northwestward coastal onlap of the underfilled Alpine basin across Switzerland were between 8.5 and 12.9 mm/yr. Time-equivalent westward to southwestward coastal onlap rates in France were between 4.9 and 8.0 mm/yr. The direction of migration of the cratonic coastline of the basin was parallel to the time-equivalent thrust motions, and oblique to the Africa-Europe plate motion vector. By comparing rates of thrust propagation into the orogenic margin of the basin to rates of coastal onlap of the cratonic margin of the basin, it is possible to suggest that the Alpine foreland basin of central Switzerland migrated with an approximately steady state geometry for at least 210 km northwestward over the European craton. The westward and southward decrease in the basin migration rate around the Alpine arc was associated with an increase in the degree of syndepositional normal faulting on the European plate; this is thought to relate to the opening of the Rhine-BresseRhone graben system.

SIMULATION OF FORELAND BASIN STRATIGRAPHY USING A DIFFUSION-MODEL OF MOUNTAIN BELT UPLIFT AND EROSION - AN EXAMPLE FROM THE CENTRAL ALPS, SWITZERLAND

Foreland basin stratigraphy can be considered as the result of three interacting processes: thrust deformation, which builds the tectonic load, sedimentary and erosional processes which redistribute that load, and the flexural response of the lithosphere. The resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities. To understand the development of these unconformities, we present a simple model of these three processes, coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography. Our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere. The model describes the thrust wedge-foreland basin system in terms of four parameters: (1) the effective elastic thickness of the foreland plate (Te), (2) the sediment transport coefficient (K), (3) the thrust wedge advance rate, (4) the surface slope of the thrust wedge. The model is applied to the Oligocene-Miocene North Alpine Foreland Basin (NAFB) of eastern Switzerland. The stratigraphy of the NAFB can be simplified into two large-scale shallowing upward cycles separated by an unconformity at the base of the Burdigalian (22 Ma). Geological information is taken from the NAFB to estimate suitable values for the parameters listed above. Assuming a linear elastic lithospheric rheology, the Te value is estimated at 10±5 km from decompacted sediment columns. Data to constrain the sequential development of the thrust wedge come from structural geology. The early stages (40–24 Ma) of compression involved a relatively low-angle thrust wedge with an advance rate of approximately 2–4 mm/yr. At about 24 Ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate. The value for the transport coefficient has been estimated from previous studies. Prior to attempting to simulate the broad-scale geometry of the NAFB the role of each parameter was assessed individually. The values for Te and K are held constant throughout the simulation of the NAFB at 7.5 km and 500 m²/yr, respectively. The geometry of the base Burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge. The surface slope angle of the wedge is increased from 2.5° to 4° over 0.2 m.y., and the rate of thrust advance is decreased from 2 mm/yr to 0.2 mm/yr, during the thickening event between 24 and 23.8 Ma. The rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and, after a time lag, erosion of the distal stratigraphy over the forebulge, so simulating the base Burdigalian unconformity.

Climate-induced rebound and exhumation of the European Alps

Foreland basins record the regional isostatic compensation of mountain belts; during periods of crustal thickening, they subside, and when erosion unloads the mass of the mountains, the basins rebound and are eroded. In order to evaluate this mechanism for rebound, it is critical that the timing and magnitude of erosion are documented. We present data estimating the timing and magnitude of late orogenic or postorogenic erosion in the North Alpine Foreland Basin of Switzerland. Mineral cooling ages demonstrate that the basin underwent 1-3 km of erosion soon after 5 Ma. This erosion coincided with a decline in structural deformation in the Swiss Alps, and a doubling of sediment accu- mulation rates in surrounding depocenters. We propose that accelerated erosional un- roofing of the Swiss Alps triggered isostatic rebound and erosion of the foreland basin after 5 Ma. A projection of the isostatic rebound of the basin into the mountains suggests that at least 6.5 km of erosion should have occurred in the high topography of the Aar Massif. Accelerated erosion in the Swiss Alps at that time is explained by an increase in atmospheric moisture driven by an intensification of the Atlantic Gulf Stream at 4.6 Ma. Consequently, we propose that the changing erosional capacity of the climate triggered late orogenic to postorogenic mass reduction and isostatic rebound of the Swiss Alps and their neighboring foreland basin.

Depositional Evolution of Confined Turbidite Basins

ABSTRACT Confined turbidite basins are a common feature of many structurally complex continental slopes, but their depositional history has never been characterized using outcrop data. A synthesis of outcrop data from Tertiary Alpine basins with subsurface data from the Gulf of Mexico indicates that the progressive infill of confined turbidite basins can be characterized by four phases: (1) Flow ponding, where incoming flows are totally trapped, depositing thick, sheet-like sand-mud couplets. (2) Flow stripping, where the finer, more dilute portion of the flow is able to escape over the confining topography to be deposited elsewhere, causing increased sand/mud ratio within the basin. (3) Flow bypass, either by flows traversing over the filled basin or by switching of feeder channels away from the basin; the former resulting in incision, the latter in abandonment. (4) Blanketing, of the basin and surrounding topography due to base-level rise; this usually takes the form of meandering channel-levee complexes with low sand/mud ratios. Confined basin sequences may be stacked during the episodic growth of the confining topography to a basin, and may appear similar to sea-level-induced depositional sequences.

FLYSCH TO MOLASSE TRANSITION IN PERIPHERAL FORELAND BASINS : THE ROLE OF THE PASSIVE MARGIN VERSUS SLAB BREAKOFF

The initiation of continental collision and the inception of peripheral foreland basins occur by the deformation and flexure, respectively, of the inherited passive margin of the foreland plate. During progressive plate convergence, peripheral foreland basins develop from an underfilled flysch stage to a filled or overfilled molasse stage. Classically, this flysch to molasse transition is interpreted as recording the migration of the thrust wedge and foreland basin over the hinge line of the inherited passive margin. It is demonstrated that during the development of the North Alpine foreland basin neither inherited paleobathymetry nor changing lithospheric strength of the underthrust European passive margin played a significant role in the flysch to molasse transition. Sediment supply from the Alps increased at least 30% from the time of flysch to molasse deposition. At about the same time as the flysch to molasse transition (mid-Oligocene), the inner parts of the mountain belt experienced accelerated exhumation, uplift of high-pressure metamorphic rocks, lower lithospheric melting, and the onset of major backthrusting, all of which have been linked via a model of slab breakoff. A further consequence of the model is isostatic surface uplift and erosion. It is proposed that slab breakoff may have been responsible for the increased sediment supply that resulted in the flysch to molasse transition in the North Alpine foreland basin, and that this provides an alternative to the passive margin model.

Sedimentology of the Indus Group, Ladakh, northern India: implications for the timing of initiation of the palaeo-Indus River

The upper reaches of the Indus River form a longitudinal network that drains a large portion of the western Himalaya. It plays an important role as a sediment routing system, feeding the Indus Delta and submarine fan, and has played a controlling role in the denudational history of the western Himalaya. Given the rivers long-term significance, its timing of initiation remains poorly constrained. The facies, palaeocurrents and provenance of the post-early Eocene Indus Group preserved along the upper reaches of the modern Indus reveal a history of fluvial/deltaic and deep-water sedimentation in an intermontane basin dominated by internal drainage. Hence, the Indus Group is not considered to represent the deposits of a palaeo-Indus River as previously thought. Illite crystallinity values and apatite fission track dating of the Indus Group suggest that the succession was buried to temperatures of 155–280°C, and that unroofing started in early Miocene times and proceeded at 0.1–0.4 mm a−1 to the present. The youngest sedimentary rocks preserved along the Indus Basin are early Miocene in age from the tectonostratigraphically equivalent deposits around Kargil to the west of the study area. The transition from sediment accumulation to erosional unroofing in early Miocene times coincides with accelerated regional unroofing of the High Himalayas to the south, and the initiation of the Indus Delta/submarine fan to the west. Differential uplift between the northward thrusting of the Zanskar and Indus sedimentary succession against the undeformed Ladakh Batholith provides a mechanism for post-early Miocene initiation of a major longitudinal river. Hence, early Miocene times is believed to represent the earliest possible age of initiation of the palaeo-Indus river following this course.

Tectonic forcing of longitudinal valleys in the Himalaya: morphological analysis of the Ladakh Batholith, North India

Abstract Longitudinal valleys form first order topographic features in many mountain belts. They are commonly located along faults that separate tectonic zones with varying uplift histories. The Indus Valley of Ladakh, northern India, runs northwestwards following the boundary between the relatively undeformed Ladakh Batholith to the north–east and the folded and thrusted Zanskar mountains to the south–west. In this region the Shyok Valley, on the northern side of the batholith, approximately parallels the course of the Indus. This study investigates geomorphic variations in transverse catchments that drain the Ladakh Batholith, into the Indus and Shyok rivers. The batholith has been divided into three zones based on varying structural characteristics of its northeastern and southwestern boundaries. Morphometric analysis of 62 catchments that drain into the Indus and Shyok valleys was carried out using three digital datasets, and supported by field observations. Morphometric asymmetry is evident in the central zone where the Shyok valley is considered tectonically inactive, but the Indus Valley is bound by the northeastwardly thrusting Indus Molasse and the batholith. In this zone the catchments that drain into the Indus Valley are more numerous, shorter, thinner and have lower hypsometric integrals than those that drain into the Shyok. By linking these observations with the regional geology and thermochronological data it is proposed that high sediment discharge from the deformed Indus Molasse Indus Valley has progressively raised base levels in the Indus Valley and resulted in sediment blanketing of the opposing tectonically quiescent catchments that drain southwestwards off the batholith. The Indus Molasse thrust front has propagated at least 36 km towards the Ladakh Batholith over the last 20 Ma. Hence it is proposed that this long term asymmetric structural deformation and exhumation has forced the Indus longitudinal valley laterally into the Ladakh Batholith resulting in the morphometric asymmetry of its transverse catchments.

The Influence of Lateral Basinal Slopes on Turbidite Sedimentation in the Annot Sandstones of SE France

ABSTRACT The deep-water deposits of the lower Oligocene Annot Sandstones of SE France were deposited in topographically dissected basins. Axially derived turbidites show marked lateral onlap onto the underlying marls. The original lateral slopes were at least 250 m high, and up to 12° prior to compaction-induced steepening; they commonly show a stepped or undulatory profile. Lateral tracing of individual beds away from their position of onlap termination down the paleoslope reveals a characteristic succession of microfacies: thin-bedded, nongraded fine sandstones with upper-flow-regime planar lamination pass downslope into medium-bedded, partially graded medium sandstones with ripple and planar lamination. At the base of the depositional slope, the turbidites become thick to very thick-bed ed, fully graded (commonly stepwise) coarse sandstones to mudstones. The most likely interpretation of the lateral transition from lower-flow-regime to upper-flow-regime conditions from the axial trough to the lateral margin of the basin involves pulsatory flow conditions. Episodes of increased flow thickness and velocity deposited upper-flow-regime bed forms on the higher parts of lateral ramps, while periods of waning current deposited lower-flow-regime bed forms in the axial trough. During accumulation of the sands, the underlying marls were differentially compacted, with maximum compaction under the axis of the basin, decreasing to zero compaction at the edges of the basin. This resulted in secondary slopes on the upper surface of the sandstones overlying the marl slopes, which may have been important in controlling the accumulation of successive sand packages. Successive onlap and overstep of sand layers onto the marls resulted in a vertical profile that shows the same succession of facies as traced downslope in individual beds. The resulting vertical sections coarsen and thicken upwards over 5-10 m. The characteristics of the thin-bedded fine sandstones at the base of cycles may be used to distinguish between prograding and laterally onlapping turbidite packages. Sandstone packages in the basin axis are thick (30-60 m), sharp based, and sandstone rich. The laterally equivalent part of a package overlying a confining slope is thinner, asymmetric with a coarsening upward signature, and more mudstone rich.

Punctuated thrust deformation in the context of doubly vergent thrust wedges: Implications for the localization of uplift and exhumation

Understanding uplift and exhumation in orogenesis requires integrating the effects of both climatic and tectonic forcing. Punctuated thrust activity is widely documented from field studies in a range of mountain belts. The potential for climatic forcing of thrust activity has been a subject of recent debate. Here we use a modeling approach to analyze the behavior of individual thrust units in the context of asymmetric doubly vergent thrust wedges. The model predicts that rates of surface uplift, frontal accretion, and exhumation should be punctuated on a time scale linked to thrust sheet geometry and convergence rates. This time scale ranges from 0.1 to 5 m.y. for various settings, and should be calculated before external forcings such as climate are invoked.

Rapid early Miocene exhumation of the Ladakh batholith, western Himalaya

Zircon, apatite (U-Th)/He, and apatite fission-track age data record a rapid cooling event in the Ladakh batholith of northwest India ca. 22 Ma. A combination of inverse and forward modeling of the data confirms this qualitative interpretation. Combining the thermochronometric data with structural evidence, we propose that exhumation was due to south-directed thrusting of the batholith along a north-dipping structure, coupled with erosion to bring the rocks to the surface. The rapid exhumation recorded in Ladakh is contemporaneous with exhumation of the High Himalaya. This focused surface denudation and structural shortening north of the Indus suture zone in early Miocene time implies that the actively deforming and eroding Himalayan thrust wedge extended farther north than channel flow models currently predict.