J.-F. Stephan

New results from physical modelling of arc–continent collision in Taiwan: evolutionary model

Abstract Both the analysis of the geological data and the results from 2D and 3D experimental modelling of arc–continent collision in Taiwan reveal the following major stages in this process: (1) failure and thickening of the subducting crust of the Chinese continental margin under the frontal part (accretionary prism) of the Philippine Sea Plate (PSP) since ca. 6–7xa0Ma; (2) uplift (squeezing) and exhumation of the subducted crustal slice of the Chinese margin and initiation of the Central Range about 5xa0Ma ago; (3) flexural buckling (subsidence) of the Luzon arc/fore-arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming Central Range; (4) failure of the PSP along the west-vergent Longitudinal Valley Fault dipping beneath the arc and closure of the fore-arc; (5) subduction reversal and stoppage of continental subduction which has already occurred in northern Taiwan and propagates to the south. The southward propagation of the collision is not stationary. Currently occurring initial collision in southern Taiwan and offshore to the south does not follow the same (described above) evolution as in central and northern Taiwan. The evolution of the collision in time and space should be considered in the framework of a 3D model where the PSP rupture is preparing in the central and northern Taiwan during a long time, passing through a number of evolutionary stages. Once it has occurred, the rupture (Longitudinal Valley Fault) rapidly propagates to the south into the domain, which did not undergo the pre-rupture evolution. The rupture front is currently located near 21°30′N.

The denudation history of the Argentera Alpine External Crystalline Massif (Western Alps, France-Italy): an overview from the analysis of fission tracks in apatites and zircons

Apatite/zircon fission track (FT) records of the Argentera external crystalline massif (Western Alps) show three tectonic pulses, respectively at 22 Ma (zircons), 6 and 3.5 Ma (apatites). The first pulse is consistent with the basement exhumation and initiation of the major deformation recorded in the foreland of the belt from Middle to early Upper Miocene. The two others might be respectively local expressions of the syncollisional extension mainly controlled by a westward sedimentary cover detachment and a Plio-Quaternary uplift acceleration. Zircon ages of 50-80 Ma in a limited NW area and evidence of an uplift elsewhere show that in a large fraction of the massif, temperatures in post-Variscan times never reached 320°C. Finally, FT data show that the Argentera massif did not behave as a single block during its denudation. First, in the NW of the massif, a small fault-limited block was already separated since the Cretaceous and later on recorded the 6 Ma denudation event, the 22 Ma pulse being recorded only in the remaining part of the massif. Second, less than 3.5 Ma ago, the northeastern part of the massif overthrust the southwestern block along the Bersézio-Veillos fault zone.

Impact of arc-continent collision on the conditions of burial and exhumation of UHP/LT rocks: experimental and numerical modelling

Abstract A 2-D physical and finite-element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere. The experimental technique allowed us to model the whole subduction/collision process from oceanic subduction to deep subduction of the continental crust. With the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process. The results obtained by both techniques are coherent and mutually complementary. They show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction. The conditions for such a failure (the weakening of this plate) are prepared during oceanic subduction. The weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates, and due to back-arc spreading. In oceanic subduction zones with a compressional regime (no back-arc opening, thick and strong back-arc lithosphere), the weakest zone is volcanic arc area. When weakening becomes sufficient during subduction, the lithosphere fails in this area. The failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore-arc. Almost half of the presently active subduction zones are characterised by a tensional subduction regime with back-arc spreading. In such subduction zones, the weakest place is not the volcanic arc but the back-arc spreading centre. When a subduction regime changes from tensional to compressional, failure occurs in the vicinity of the extinct spreading centre. This process can occur during oceanic subduction again along either a trench-vergent or trenchward-dipping fault, but the formation of a trench-verging fault is most likely. In this latter case, which is a principal subject of our study, the failure is followed by partial subduction of the arc plate. Complete subduction occurs during arc-continent collision (subduction of the continental margin) when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle. The arc itself can be subducted completely or be partially or completely scraped-off and accreted. A deeply subducted material (including continental margin) is preserved at relatively low temperatures between the lithospheric mantle and the “cold” subducted arc plate to about 150-km depth. Subduction of the arc plate is a major phenomenon, which affects all processes associated with continental subduction from deep burial and HP/LT metamorphism to exhumation of subducted material. Does this process occur in nature? Future investigations will allow us to answer this question. In this paper, we analyse the conditions of emplacement of a very young oceanic lithosphere (Samail ophiolite) on the continental crust in Oman in the late Cretaceous and argue that this lithosphere formed in a back-arc basin. It reached and overthrust the Arabian continent after complete subduction of the arc plate.

Cretaceous black shales of Venezuelan Andes: preliminary results on stratigraphy and paleoenvironmental interpretations

Abstract This paper deals mainly with the Late Cretaceous La Luna Formation on the Venezuelan Andean Platform. The studied area (the so-called Transversale de Barquisimeto) is located at the contact zone between the Andes of Merida and allochthonous slabs thrusted from the Caribbean Domain and is crosscut by a major active transform fault (the Bocono Fault), which is part of the contact zone between the Caribbean and South-American plates. The La Luna Formation is made of dark, organic-rich, laminated limestones and shales and, though always appearing with black shale facies, is different in the areas located in the north or in the south of the Bocono Fault. We provide here some new information about the biostratigraphic frame of the La Luna Formation and of the overlying Colon Formation. In the studied area, the depositional environment of the La Luna Formation was certainly rather shallow (near storm wave base) and stratified. During deposition of La Luna Formation, dysaerobic to anoxic conditions prevailed, which may be linked with a regional anoxic event, perhaps related to seasonal upwelling along the northern coast of South America during Late Cretaceous time.

The magnetic fabric of weakly deformed Late Jurassic shales from the southern subalpines chains (French Alps): evidence for SW-directed tectonic transport direction

Abstract Extensive magnetic fabric investigations have been conducted on the Late Jurassic sedimentary black shales (37 sites, 362 samples), the so-called Terres Noires Formation, from the Southern Subalpine Chains (western Alps). This study completes a previous study done in the Northern Subalpine Chains which demonstrated that the Terres Noires Formation preserves a subtle magnetic lineation related to tectonic transport direction. In the Southern Subalpine Chains (Diois, Devoluy, Embrunais, Digne nappe), similar magnetic fabrics have been found. The shape of the magnetic fabric is oblate and locally mirrors the gradient of deformation in the Embrunais footwall. The magnetic foliation is parallel to the bedding or to the slaty cleavage. About 50% of the magnetic lineations have down-dip magnetic directions while the other magnetic lineations are sub-horizontal or parallel to the intersection bedding/cleavage. The tectonic magnetic fabrics recorded in the Embrunais confirm the ability of the Terres Noires to record tectonic transport direction. The magnetic fabric geometry suggests SW-directed tectonic transport in the Southern Subalpine Chains. The Diois appears as a tip zone where transport directions are abruptly clockwise rotated from NW-directed in the Northern Subsalpine Chains to SW-directed. The magnetic fabric from the Digne nappe exhibits a fan pattern of transport directions in close agreement with the arcuate shape of the thrust.

The middle america trench as an example of a subduction zone

Abstract Short Sea-Beam survey cruises were conducted by the R.V. “Jean Charcot” in march 1980 along the Middle America Trench between Panama and Acapulco, with particular emphasis on the IPOD Legs 66 and 67 areas. These detailed mappings have revealed that the Cocos horst- and graben pattern, related to the bending of the oceanic plate, strikes generally 20°–30° oblique to the trench direction. This en-echelon pattern, closely parallel to the magnetic anomalies, is here proposed as being due to the reactivation of inherited fractures generated at the East Pacific Rise. Sea-Beam data, together with UTMSI multichannel seismic reflection profiles and IPOD drilling results, clearly show that at least two distinct geodynamic processes are presently working along the same trench: accretion and no accretion. They also show a relative structural homogeneity of the oceanic side of the trench in contrast to the dual aspect of the continental side.

Advection of fluids at the front of the Sicilian Neogene subduction complex

Abstract The Sicilian accretionary prism shows a large, southward imbricated system of thrust sheets. The frontal part of the belt has been tectonically emplaced in Plio-Pleistocene times. Major decollement surfaces and sole thrusts at the base of and inside the prism consist of melanges with a “blocks in matrix” fabric. In the frontal part, two superposed decollement levels are recognized, the upper one located at the base of the Paleogene-Neogene sedimentary series, and the basal one at the base of duplexes (Triassic-Miocene sedimentary series). Syntectonic dewatering of the sedimentary sections occurred along decollements and thrust faults, involving hydrofracture and mineralized vein development. Inside the deep sole thrusts (rooted in the basal decollement); hydrofractured blocks show different generations of syntectonic quartz and calcite extensional veins, while in the upper decollement only calcite veins have been observed. Both quartz and calcite veins show aqueous primary fluid inclusions of low salinity, with low trapping temperature for fluids in the upper decollement ( If we assume that, at the time of deformation, the main heat source was the regional conductive geothermal gradient, the trapping temperatures of the fluids inside veins of the deep sole thrusts are higher than those predicted by the Pilo-Pleistocene gradient for this subduction zone and correspond to a thermal anomaly. We suggest that the frontal Sicilian prism supported localized transient fluid flow of deep freshwater sourced from two different origins: a shallow one for fluids now trapped in the upper decollement, and a deep internal origin (6–10 km minimum depth) for the hot fluids channelized along the basal decollement and now trapped inside the sole thrust of the Mt. Scalpello duplex area.

Northern and Southern Caribbean Festoons (Panama, Colombia-Venezuela and Hispa-Niola - Puerto Rico), Interpreted as Pseudosubductions Induced by the East-West Shortening of the Pericaribbean Continental Frame

Abstract The Caribbean plate is moving eastward, relatively to and between the two Americas, at about 2 cm/year. This motion is expressed : - laterally by large strike-slip movements, sinistral on the North side, dextral on the right one; - at the leading edge, to the East, by the Lesser Antilles subduction zone and the Barbados accretionary prism. The Caribbean plate is made of a central domain with oceanic crust, more or less thickened, surrounded by a continental frame. This frame is deformed and draws arched structures (“festoons”) with a Caribbean convexity : the Panama and the Colombia - Venezuela festoons to the South; the Hispaniola-Puerto-Rico (= Los Muertos) festoon, to the North. These structures orientated at 90° to the Lesser Antilles Arc, geometrically mime subduction zones, with their frontal accretionary prism similar to the Barbados or Nankai one. Nevertheless, they do not have the seismicity nor the volcanism tied to subductions : we propose to call them “pseudosubductions”. These arched structures are classically interpreted by the authors as the result of the light convergence (

Geology and tectonic history of Southeastern Luzon, Philippines

Abstract Southeastern Luzon belongs to the eastern Philippine Mobile Belt. Geological and structural studies revealed that it could be divided into three major structural units limited by two subparallel NW-SE trending left-lateral strike-slip faults: the Hilawan Fault in the NE and the Minas Fault in the SW. The North-central Catanduanes Structural Unit (NCSU) is characterized by a Middle to early Late Cretaceous volcanic arc sequence unconformably overlain by a Middle to Late Eocene volcanic arc sequence followed by Early Oligocene intrusives. The Median Structural Unit (MSU), limited from the NCSU by the Hilawan Fault, is underlain by a Late Cretaceous volcanic arc sequence followed by two distinct chaotic sequences from the end of Cretaceous-Paleocene and latest Middle Eocene-earliest Late Eocene. It is limited to the southwest by the Minas Fault. The Western Caramoan Structural Unit (WCSU), pre-Late Cretaceous ophiolitic suite unconformably overlain by Late Cretaceous volcanic arc sequence and Middle Eocene limestones are exposed. These structural units are overlain by carbonate and detrital sequence from the Late Oligocene to Pliocene. Constrained by angular unconformities and deposition of olistostromes, polyphase left-lateral strike-slip faulting was recorded since the end of Cretaceous up to the limit of Early-Late Oligocene. These faults probably represent the traces of a Proto-Philippine Fault System in Southeastern Luzon.

An unknown active fault revealed by microseismicity in the south‐east of France

[1]xa0A seismic swarm occurred in the south-east of France in December 2000, about 15 km north of the densely populated cities of the French-Riviera. More than 300 events were located by a dense temporary seismic network that had been installed in the epicentral region one month before. We evidence an alignment of the seismicity that we interpret as an unknown active fault. On the basis of regional tectonics, we propose that this fault results from basement faulting cutting through the detached overlying sedimentary cover.