Joachim Déramond

Quaternary transfer faulting in the Taiwan Foothills: evidence from a multisource approach

Abstract The major structures of the Western Foothills of Taiwan mainly consist of NNE-SSW-trending folds and imbricated west-vergent thrust systems. The additional occurrence of N140°E-striking oblique structures was revealed through a multisource approach involving a Digital Elevation Model (DEM), a study of drainage network anomalies, aerial photographs, Side-Looking Airborne Radar (SLAR) images and SPOT-P and Landsat images. These structures are described from north to south based on new field analyses (including stratigraphy and tectonics studies). They are also compared to seismic data and geodetic reconstruction, in order to evaluate their present-day activity. These N140°E major morphostructures are interpreted as left-lateral transfer fault zones, either inherited from the Eurasian passive margin and/or newly formed in the cover in response to the presence of basement highs within the foreland basin (Peikang and Kuanyin highs). The Sanyi and the Chishan transfer fault zones display a high seismic activity; the distribution of earthquakes and the related focal mechanisms confirm the left-lateral movement along N140°E directions. The Chiayi, Chishan, and Fengshan fault zones act presently as transfer fault zones, as indicated by GPS data. The associated N70°E- to N100°E-trending faults result from the reactivation of normal faults of the Eurasian passive margin as right-lateral strike-slip faults in the Foothills during the Plio-Quaternary collision in Taiwan. We conclude that multisource and multiscale geomorphic studies combined with tectonic analysis in the field yield a significant contribution to the understanding of the structural and kinematic development of the Western Foothills at the front of the Taiwan collision belt.

Relationships between thrust tectonics and sequence stratigraphy surfaces in foredeeps: model and examples from the Pyrenees (Cretaceous-Eocene, France, Spain)

Abstract A new method of chronological and geometrical analysis is used for understanding the sedimentary and tectonic evolution of perisutural basins (foredeep and foreland basins). It may be applied to synorogenic basins whose infilling was controlled by sea-level fluctuations and where, as a result, the tectonic structures are well preserved. This method is based on the mapping of depositional sequence boundaries and maximum flooding surfaces defined by sequence stratigraphy. These planar markers are considered as synchronous and their numerical ages are determined by classical chronostratigraphic methods, by dating and by comparison with the eustatic global cycle chart. The evolution of the studied perisutural basins is controlled by the propagation in space and time of a syn-depositional thrust-and-fold system including a fault-propagation anticline and two synclines (a foreland and a hinterland syncline) over and in front of a blind basal sole thrust. The propagation of this thrust-and-fold system toward the foreland is connected to unconformable depositional sequence sets, which indicate for each system a time span for thrusting of several eustatic cycles. The evolution of a single thrust-and-fold system corresponds to a shortening accommodated by upward imbrication of second-order thrusts inside the anticline. Each of these thrusts is related, at least, to one depositional sequence lying unconformably on the external side of the fold and deformed by the successive development of the branch lines toward the hinterland. This tectono-sedimentary correlation indicates a time span sometimes as short as one eustatic cycle for each second-order thrust. Thrusting and sea-level changes are not exactly synchronous, as the depositional sequence sets (which are tectonic in origin) do not systematically correlate with eustatic supercycles. The apparent time correlation between the two groups of independent phenomena is an artefact of the method which calibrates the tectonic evolution by comparison with eustatic fluctuations. Applied to the South Central Pyrenees, the method has specified the structure of the Late Cretaceous foredeep basin and has revealed the successive stages of forward thrust-and-fold system migration (4 to 5 Ma for each one from 89 Ma) and of hindward second-order thrust migration (1 to 4 Ma for each one). In the North Central Pyrenees as in the South Central Pyrenees, Late Cretaceous foredeeps are characterized by syntectonic turbidite deposits (from 89 Ma onward) and by a significant shortening due to a symmetrical and synchronous structural evolution. The Eocene foreland is a foreland syncline developed in the footwall of the North Pyrenean Thrust. This syncline is due to the occurrence of blind thrusts and it is infilled by fluvial deposits.

Morphotectonic evidence from lateral propagation of an active frontal fold; Pakuashan anticline, foothills of Taiwan

Abstract The Pakuashan anticline is uniquely suited for study of the forward and lateral growth of fault-related folds. The Pakuashan ridge development arises from the late Quaternary uplift of the most external thrust zone of the western foothills of Taiwan. From Kaoshiung to Taichung, recent and active westward thrusting occurs at the front of the foothills. The Pakuashan anticline, trending N 150°E in the northern part to N 000° in the southern part, has been active throughout the Quaternary period. This activity is marked by geological structures, tectonic geomorphology and seismicity. A multisource and multiscale approach to study of the continental collision setting has been undertaken to combine tectonics, sedimentology and geomorphology. Studies of fracture patterns allow identification of two main features of stress orientations: a WNW/ESE compression direction, and E–W and N–S extension directions. Quantitative geomorphic parameters have been used to define the morphotectonic evolution and to infer tectonic style along the mountain front. Geomorphic evidence provides significant information on the processes that govern lateral propagation of an active anticline. Quaternary terraces are uplifted, tilted and folded over the Pakuashan ridge. Drainage systems in areas of active compression give information on the thrust zone structures and their development. Steep drainage and high local relief indicate that the Pakuashan anticline forms a well-defined zone of high uplift, especially in the southern part. The two main controls on drainage in that area are rock strength in the hanging wall and propagation of the deformation towards the south.

A dynamic model of the structural evolution of the Hercynian Pyrenees

Abstract The structural history of the Hercynian Pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations, the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes, the migmatization and the intrusions of plutonic and basic and ultrabasic rocks. The metamorphism can be shown to have progressed upwards with time, attaining the presently exposed levels just before, during and a short time after the major penetrative regional deformation. The intrusion of major plutonic, anatectic and gneissic massifs, which has determined the metamorphic domes in country rocks, was roughly synchronous with the progression of the metamorphism, but seems to have continued later in the uppermost levels. The basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation, thus showing that their intrusion predated this deformation in the lowermost levels. These results, together with the presently available sedimentological data, lead us to propose a dynamic model of the tectonic evolution of the Hercynian Pyrenees. The first step was a crustal extension and thinning, attaining its maximum during Frasnian time, marked in the upper levels by a “ horst and graben” controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low-pressure metamorphism and anatexis characteristic of the Hercynian Pyrenees. The second step was a crustal shortening marked by large-scale crustal thrusting followed by widespread regional penetrative deformation. The flysch troughs formed in front of the progressing and emerging thrusts. In lower levels, the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels.

Thrust-related, diapiric, and extensional doming in a frontal orogenic wedge: example of the Montagne Noire, Southern French Hercynian Belt

Abstract The Montagne Noire, which is situated at the toe of the orogenic wedge of the French Massif Central South European Variscides, appears to be a well-suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins. The data reported in the present paper show that the Montagne Noire dome is a particular type of basement-involved frontal culmination in an orogenic wedge and foreland basin system. This frontal culmination is characterized by a syn-contractional HT decompression recorded by clockwise PTt paths and widespread strata overturning in thrust and fold structures, which controlled the sedimentation in the adjacent foreland basin. These unusual characteristics are interpreted to be a result of the succession of thrusting, diapirism and extensional collapse. Antiformal stacking of syn-metamorphic thrust sheets controlled the first stages of the foreland basin development. Diapirism was essentially responsible for the HT decompression and widespread strata overturning. Extensional doming was a result of late- to post-metamorphic collapse acting on the pre-existing high-amplitude dome. Diapirism and associated isothermal decompression metamorphism, which constitute the essential difference between the Montagne Noire and ‘ordinary’ frontal ridges in orogenic wedges, were probably enhanced by a local partial melting of the upper to middle crust. It is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over-thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline.

Paleodeformations d'un massif orthogneissique: Massif des Cammazes, Montagne Noire occidentale, France

Abstract The Cammazes orthogneissic massif in the western Montagne Noire (France) has been affected by numerous tectonic phases during the Hercynian orogeny. The principal deformation, synchronous with a mesozonal metamorphism, is represented by different structural features such as foliation, ‘augen’ structures and deformation of megacrystals. These structural features are essentially varied with the concentration of feldspar megacrystals in the gneisses. The structural studies of these gneisses have allowed the recognition of the nature and the orientation of the principal deformation. To determine the intensity of this deformation, large undeformed feldspar crystals have been used as markers. These crystals have been assimilated to rigid ellipsoidal elements disposed in a random fashion in the initial stage of the mesostase which is considered as a viscous Newtonian fluid. The statistical studies of the orientation of these crystals with reference to the plane of flattening (foliation) has caused the appearance of an anisotropy in the final deformed state. This anisotropy, when compared to that obtained theoretically from a population of rigid elements included in a Newtonian fluid flowing at a slow speed, could evaluate the rate of the finite strain.

Time, place and mode of propagation of foreland basin systems as recorded by the sedimentary fill: examples of the Late Cretaceous and Eocene retro-foreland basins of the north-eastern Pyrenees

Abstract The relationship between tectonics and sedimentary fill has been studied in two syncontractional basins of the western Corbières (eastern North Pyrenean retro-foreland basin). The Late Cretaceous basin formed during c. 10–12 Ma as a result of left-lateral transpressional deformation, and is composed of forward-younging sub-basins characterized by reworking of the forelimbs of growing fold-propagation folds. Thrust-wedge advance and cratonward migration of the platform are recorded by a deepening-upward stacking pattern indicating increased regional subsidence with a limited contribution of the submarine orogen. Tectonic quiescence and erosional unloading lasting 29–30 Ma are recorded by a shallowing-upward stacking pattern, and fluvial sedimentation issued from widespread sources in the emerging inner orogen. The Early to Middle Eocene basin formed as a result of pure shortening normal to the range. The marine Early Eocene basin developed during c.2 Ma by widening of a single basin provoked by the two-step propagation of a basement duplex. This is recorded by growth-stratal patterns and coarsening-upward depositional sequences indicating the increasing contribution of the emerged orogen. The Middle Eocene continental deposits infilled two sub-basins working synchronously and were transported by alluvial fans with a provenance in the inner orogen, during decreasing thrust-wedge advance and increasing erosional unloading.

Partition between collision and subduction accretionary prisms along an inherited transcurrent fault zone: New insights on the Taiwan fold and thrust belt

A new geotectonic framework of the Taiwan orogen is presented in accordance with the hypothesis of an oblique arc- arc collision. The colliding Luzon arc is physically connected to the eastern Coastal Range in which a subduction complex remnant is preserved and backthrust with intra-arc sediments in a small retroforeland basin. A southern and extinct extension of the Ryukyu arc is characterized in western Taiwan. It displays a duplex structure (Tananao and Backbone horses and Lishan triangle zone) between a buried floor thrust located in the arc crust and a roof thrust developed in the arc cover (Hsfiehshan Range and South Backbone Range). Westward the basal thrust climbs in the sedimentary series of the western proforeland (Foothills and Hengchun Peninsula) and dies out in a buried tip line. The northern part of the orogen, including all the Tananao arc core, is shown as an intra-oceanic-continental arc-arc collision belt characterized by an unroofed duplex culmination above a leading floor thrust and both proforeland and retroforeland basins. The southern part, which displays a roof thrust sequence above a buried duplex, is shown as an accretionary prism built in a transition zone between continent and oceanic subduction (transition from the Asian continental crust, including the former Ryukyu arc, to the oceanic Old Philippine Sea crust). The partition is believed to be induced by a deep intracontinental transcurrent fault zone able to influence the difference in shortening, duplex pattern, and leading thrust depth. The evolution was controlled by the Ryukyu subduction (backarc extension, arc magmatism extinction, and cooling and intra-arc collapse) until the early middle Miocene (around 15 Ma) and then it was controlled by the Luzon arc progression (continental subduction, collision, indentation, and hinterland uplift and frontal thrust propagation).

Constant thickness overthrust on a visco-plastic sole

Abstract The overthrust model developed by Hubbert and Rubey, and Hsu is based on the concept of a rigid-plastic block, lightened by a fluid overpressure whereas the model developed by Kehle consists of a number of viscous layers with high viscosity contrasts. The mechanical features of these two different models are summarized and a third model is proposed. This model consists of a thick rigid-plastic block sliding on a thin visco-plastic layer. This model seems more realistic to the geometric problems posed by the Gavarnie overthrust in the central Pyrenees and we have obtained relationships between geometrical and mechanical parameters of this overthrust mass.