Pierre Souquet

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.

Basement blocks and tecto-sedimentary evolution in the Pyrenees during Mesozoic times

The Alpine tectonics in the Pyrenean Range is controlled by motions of basement blocks (High Primary Range, North Pyrenean blocks). These blocks belong to different tectonic units; they are amygdaloid, wedge-shaped, squeezed at their roots, upthrust and displaced with their cover (detached or not) in a convergent shear zone between the European and Iberian plates. These basement blocks are distributed within isopic (isofacial) units with diachronous terrigenous prisms. The stratigraphical analysis of their Mesozoīc cover reveals their relative position during the Mid-Cretaceous continental break-up which induced the separation of the two previously joined plates.

Facies Sequences in Large-Volume Debris- and Turbidity-Flow Deposits From the Pyrenees (Cretaceous; France, Spain)

Facies types and depositional models for thick gravity-flow deposits are inferred from Cretaceous Pyrenean examples occurring in different sedimentary settings: basin-plain during relative low sea level stand; slope-apron or deep valleys during relative sea level rise; and slope during relative sea level rise. The depositional units are interpreted as debrites, debrite-turbidite couplets, and megaturbidites. They are a function of transport distance and hydraulic jump by liquefaction and phase separation in large subaqueous flows.

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).

Le gisement vraconnien (albien superieur) de la selva de bonansa (pyrenees espagnoles): interet stratigraphique, paleoecologique et paleobiogeographique

Abstract The Vraconian deposit (Upper Albian Stoliczkaia dispar zone) of La Selva de Bonansa in the South Pyrenean Zone (Spain), has been revised by a group of paleontologists. More than fifty species have been found: ammonites, lamellibranchs, gastropods, brachiopods, echinoids and foraminifers. Some of these organisms belong to the North Temperature Realm and others to the Tethyan Realm. The deposit is situated on a hard-ground which was the external limit of the calcareous continental shelf, i.e. the equivalent upper circalittoral bathymetric zone and the external part of the infralittoral zone. This is shown by the associations of invertebrates. The fauna identified at Bonansa also shows the stratigraphical value of some species which lived only during the Late Vraconian, or in the Early Cenomanian, or crossing the Albian-Cenomanian boundary.

Sequence Stratigraphy and Grade-Dating in the Senonian Series from the South-Pyrenees (Spain), the Sedimentary Record of Eustacy and Tectonics

A new stratigraphic procedure that combines sequence stratigraphy and grade dating based on chronometric calibration of Rosita fornicata lineage evolution is applies to the analysis of the South Pyrenean Senonian Basin. These methods give eustatic and biometric curves both constructed according to Haq’s chronology and they permit time-correlations without variations in accuracy. In this way, two kinds of stratigraphic units are recognized and numerically dated from the Senonian-Early Danien in the South-Pyrenean foredeep Basin. These units are: eustatically-controlled depositional sequences; and tectonically-controlled depositional sequence sets.