Frédéric Mouthereau

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.

Structural, geodetic and seismological evidence for tectonic escape in SW Taiwan

Abstract Recent structural, geodetic and seismological data in SW Taiwan are analysed and discussed in terms of present-day tectonic escape occurring in response to the active N100° collisional shortening. The escaping area corresponds to the onland extension of the Manila accretionary wedge; this region comprises a rheologically weak, thick muddy cover which is decoupled from the underlying basement by a decollement and which deforms mainly by aseismic creep. It is separated from the northern actual collisional area by a major WNW- to NW-trending structural and kinematic transition zone oblique to the structural grain of the belt, the Chishan Transfer Fault Zone. Geodetic data are further used to define several poorly deforming blocks undergoing nearly uniform displacement velocities and bounded by kinematic discontinuities that fit the major faults, and to determine the present-day across-strike and along-strike motions on these major faults. Although direct onland structural evidence of tectonic escape is poor, reconstruction of Quaternary paleostress patterns demonstrate that this escape probably began during the late Pleistocene, later than in northeastern Taiwan as a result of the southward migration of the collision through time. Offshore structural data help to constrain the geometry and the southern extension of the escaping blocks. Finally, a tentative model of lateral extrusion in SW Taiwan is proposed.

Placing limits to shortening evolution in the Pyrenees: Role of margin architecture and implications for the Iberia/Europe convergence

Estimating shortening in collision belts is critical to reconstruct past plate motions. Balanced cross-section techniques are efficient in external domains but lack resolution in the hinterland. The role and the original extent of the continental margins during the earliest stages of continental convergence are debated. Here we combine existing and new sequentially restored cross sections in the central Pyrenees, with Iberia/Europe (IB/EU) plate kinematic reconstructions and new apatite fission track, zircon (U-Th)/He, and U/Pb ages to discuss higher and lower bounds of crustal shortening and determine the amount of distal margin sutured during collision. We show that after extension in the Albian (~110 Ma), a 50 km wide extremely thinned crustal domain underwent subduction at 83 Ma. Low-temperature data and thermal modeling show that synorogenic cooling started at 75–70 Ma. This date marks the transition from suturing of the highly extended margin to collision of the more proximal margin and orogenic growth. We infer a relatively low crustal shortening of 90 km (30%) that reflects the dominant thick-skinned tectonic style of shortening in the Pyrenees, as expected for young (Mesozoic) and weak lithospheres. Our proposed reconstruction agrees with IB/EU kinematic models that consider initially rapid convergence of Iberia, reducing from circa 70 Ma onward. This study suggests that plate reconstructions are consistent with balanced cross sections if shortening predicted by age-dependent properties of the continental lithosphere is taken into account.

Timing of uplift in the Zagros belt/Iranian plateau and accommodation of late Cenozoic Arabia–Eurasia convergence

The motion of Arabia was stable with respect to Eurasia over the past 22 Ma. Deformation and exhumation in the Zagros is seen to initiate at the same time as argued by new detrital thermochronologic constraints and increasing accumulation rates in synorogenic sediments. A recent magnetostratigraphic dating of the Bakhtyari conglomerates in the northern Fars region of the Zagros further suggests that shortening and uplift in the Zagros Folded Belt accelerated after 12.4 Ma. Available temporal constraints from surrounding collision belts indicate that shortening and uplift focused in regions bordering the Iranian plateau to the south between 15 and 5 Ma. As boundary velocity was kept constant this requires concomitant decreasing strain rates in the Iranian plateau. Slab detachment has been proposed to explain the observed changes as well as mantle delamination, but the insignificant change in the Arabian slab motion and lack of unambiguous constraints make both hypotheses difficult to account for. It is proposed based on a review of shortening estimates provided throughout the Arabia–Eurasia collision that the total 440 km of convergence predicted by geodesy and plate reconstruction over the past 22 Ma can be accounted for by distributed shortening. I suggest that the topography and expansion of the Iranian plateau over Late Miocene–Pliocene time can be reproduced by the progressive thickening of the originally thin Iranian continental lithosphere presumably thermally weakened during the Eocene extensional and magmatic event.

Variations along the strike of the Taiwan thrust belt: Basement control on structural style, wedge geometry, and kinematics

A model of imbricate thrust wedges based on the conceptual model of the critically tapering wedge is discussed and applied to the case of the Taiwan thrust belt. This model takes into account (1) the occurrence of compressional features located far from the foreland of the orogen and (2) the occurrence of deep-crustal decoupling that allows both the regional stress field to be transmitted in the foreland and the basement to be involved in the orogenic wedge. Accordingly, three different belt fronts are considered, a mountain front, a reactivation front, and a deformation front, based on topographic, kinematic, and mechanical criteria, respectively. The reactivation front located at the outermost reactivated extensional structure displays large curvatures in areas where structural inversion occurs. The mountain front is usually distinct from the reactivation front and localizes the emergence of a shallow décollement. Serial geologic sections of the thrust belt provide strong arguments in favor of the superimposition of deepand shallow-décollement tectonics at the thrust-belt front in agreement with the model proposed. The along-strike structural changes are usually accompanied by changes in tectonic regimes due to local effects such as frontal contraction and lateral movement in response to indentation by the basement highs. The record of orogenic stresses in the Taiwan Strait allows us to define and locate a deformation front west of the Penghu Islands. Our results suggest that single-minded models based solely on the principles of either thin-skinned or thick-skinned tectonics may be unrealistic in the case of the Taiwan thrust belt. Mouthereau, F., Deffontaines, B., Lacombe, O., and Angelier, J., 2002, Variations along the strike of the Taiwan thrust belt: Basement control on structural style, wedge geometry, and kinematics, in Byrne, T.B., and Liu, C.-S., eds., Geology and Geophysics of an Arc-Continent collision, Taiwan, Republic of China: Boulder, Colorado, Geological Society of America Special Paper 358, p. 35–58. INTRODUCTION The Taiwan thrust belt has been taken as a key example for what is usually called thin-skinned tectonics (Suppe, 1976; Namson, 1981). The structure of the Western Foothills units of the Taiwan thrust belt has been investigated and described in terms of balanced cross sections based on the geometric principles of fault-related folds (Suppe and Namson, 1979). Analyses of recent structural data, however, argued in favor of basement-involved tectonics for the foreland fold-and-thrust belt (Lee et al., 1993), and new geophysical works defended the thick-skinned model for the whole thrust belt (Ellwood et al., 1996; Wu et al., 1997). Understanding the structure of the Taiwan thrust belt and the kinematic processes prevailing at depth is still an objective to be met. The first goal of this paper is to decipher the possible inF. Mouthereau et al. 36 volvement of the basement in the tectonics of the western frontal units. Second, we aim at relocating the thrust-belt front and determining the type of related structures. In addition, we demonstrate that along-strike variations occur in the wedge geometry. To this purpose, we have first investigated the overall structural framework of the foreland basement and established a new basement map of the western foreland. The accurate location of the thrust-belt front was first approached by a consideration of the nature and the geometry of the thrust wedge that is based on morphostructural and basement-topography analyses. These investigations provided further information on the geometry of structures and their relationship with basementinvolved tectonics. On the basis of these considerations, serial geologic cross sections of the frontal thrust units have been constructed. Finally, we have carried out a kinematic analysis of thrust emplacement by means of synthetic paleostress reconstruction. FORELAND THRUST-BELT FRONT: NATURE AND SIGNIFICANCE Previous work on thrust-belt fronts The study of thrust belts has been greatly spurred by the development of thin-skinned tectonics theory, especially the fundamental work of Bally et al. (1966) and Dahlstrom (1969), conducted in the Rocky Mountains of North America. These pioneering works largely contributed to the understanding of the structural framework of thrust systems as a whole (Boyer and Elliott, 1982). Furthermore, because of the increase in petroleum investigation, many structural geologists are focused on the frontal zones of thrust belts such as the one in Taiwan (Suppe and Namson, 1979). Consequently, thrust-belt fronts (or “mountain fronts”) have been described by various terminology. Basically, the terms refer to the topographic boundary of the regional foreland-dipping monocline, elevated above its initial structural level, i.e., the foreland of the thrust belt (Vann et al., 1986). This obvious morphologic frontier was regarded as the result of the emplacement of the outermost thrust. A first attempt at classifying the fronts of thrust belts, in terms of thin-skinned tectonics and based on the belts’ geometries, has led geologists to distinguish two main types of fronts: (1) buried thrust fronts and (2) emergent thrust fronts. The occurrence of each of these types depends on geometric and mechanical factors, such as the lateral termination of strata suitable for hosting a décollement and/or the presence of a broad area of weakly strained rocks (Morley, 1986). Significance of front in the critically tapering wedge model Improvements in the understanding of thrust-belt mechanics have enabled the fold-and-thrust belt to be modeled. According to the thin-skinned tectonics theory, Chapple (1978) put emphasis on the following key assumption: most fold-andthrust belts exhibit a basal décollement that gently dips toward the interior of the thrust belt. This basal décollement is usually sited at a relatively weak level, for example, within salt or shale units in the sedimentary cover. Above the basal décollement, compressional deformation occurs whereas in the rigid basement below, the deformation remains limited. These hypotheses led Davis et al. (1983) to describe the mechanical behavior of fold-and-thrust belts and accretionary wedges in terms of critically tapering wedges of Coulomb material (Fig. 1A). According to this model, as the critical taper is achieved, the wedge deforms internally by thrust-sheet imbrication. In order to apply this model to mountain building, Davis et al. (1983) implicitly considered that the toe of the thrust wedge corresponds to the thrust-belt front defined by structural geologists, i.e., the topographic front. However, the thrust-belt front thus determined in the critical-taper theory (Fig. 1A) is in fact an assemblage of three distinct and fundamental boundaries. First, there is a topographic boundary, because one of the two parameters characterizing the critical-taper angle (c) is the surface-slope angle (Fig. 1A). Second, a kinematic boundary is easily defined because as the critical taper is attained, sliding occurs along the basal décollement, resulting in the development of a new frontal thrust. Beyond this limit, i.e., at the toe, the propagation of the wedge ceases. Consequently, the front in the simple model of Davis et al. (1983) could also be viewed as a pin line (the nail in Fig. 1A) that marks the position of no movement. Third, a mechanical boundary can be defined between the fold-and-thrust belt hinterland (which deforms internally by faulting and folding) and the undeformed foreland domains (where low stress magnitudes prevail). Therefore, according to the critical-taper theory, defining a front in a foldand-thrust belt must refer to these three different considerations, and three types of fronts are thus possible: a mountain front, a reactivation front, and a deformation front, based on topographic, kinematic, and mechanical criteria, respectively. Hereafter, we propose an alternative model, based on the critically tapering wedge theory, in which the different fronts are distinguished and the involvement of the basement is considered (Fig. 1B). This model also takes into account the presence of inversion tectonics—usually documented in the forelands of orogens—as well as imbricate thrust wedges (Fig. 1B). The upper thrust wedge is restricted to the cover (“thinskinned” tectonics) and corresponds to the classical steady-state thrust wedge (Fig. 1A) considered by Davis et al. (1983). Its equilibrium depends on a critical-taper angle (c1 in Fig. 1B), which is controlled by the dip of the shallow décollement in the cover and the topographic slope (see Fig. 2). This thrust wedge develops by propagation of newly formed frontal thrust sheets toward the foreland; its front, the thrust-wedge front (1) in Figure 1B, corresponds to the outermost limit of the allochthonous units and is equivalent to the topographic front ( the mountain front). However, because the deformation is limited to the upper levels, the thin-skinned tectonics cannot acFigure 1. Determination of the different front types of foreland thrust belts on the basis of the décollement tectonics model. (A) Nature of a front in classical critically tapering wedge model after Chapple (1978) and Davis et al. (1983). (B) Alternative to criticaltaper model. This alternative considers imbricate thrust wedges bounded by either a shallow décollement (thin-skinned tectonics) or a deep décollement (thick-skinned tectonics) and defines different types of structural fronts: a mountain front (the front of the critically tapering wedge), a reactivation front (the outermost reactivated preexisting extensional feature), and a deformation front. Differentiation of the front types is on the basis of topographic, kinematic, and mechanical criteria. A—shallow thrust wedge, B—inner domain of thrust wedge, C—outer domain of thrust wedge, c1—the critical-taper angle controlled by the dip of the shallow décollement, and

Geometry and Quaternary kinematics of fold‐and‐thrust units of southwestern Taiwan

Structural and paleostress analyses provide new insights into the Quaternary kinematics of the outermost fold-and-thrust units of southwestern Taiwan Foothills. The frontal folds are interpreted as fault-related folds, and their tectonic evolution through space and time is tightly constrained. Fold development is correlated with reef building on top of the anticlines. Moreover, we provide field evidence that NW–SE fault zones oblique to the structural grain of the belt probably acted as transfer fault zones during the the Quaternary fold-thrust emplacement. Two successive Quaternary stress regimes are evidenced in southwestern Taiwan: A NW–SE compression, followed by a recent nearly E–W compression. The latter shows an along-strike change from pure E–W contraction to the north to perpendicular N–S extension in the south. This southward decrease in N–S confinement probably represents the on-land signature of the incipient Quaternary tectonic escape predicted by analogue and numerical modelling and evidenced at present-day by Global Positioning System data.

Fracture patterns in the Zagros Simply Folded Belt (Fars, Iran): constraints on early collisional tectonic history and role of basement faults

Pre-/early folding fracture patterns were recognized in several anticlines from three structural domains in the Simply Folded Belt of the Fars arc. These fracture sets were characterized in terms of opening mode and relative chronology and used to reconstruct the main compressional trends related to the early Zagros collisional history. The palaeostress reconstructions based on these fracture sets were further refined by combination with newly collected or already available fault-slip and calcite twin data. As an alternative to previous models of rigid block rotations or regional stress rotation, we propose that the complex pattern of pre-folding fractures and the contrasting palaeostress orientations through time in the different domains investigated are related to the presence of basement faults with N–S and WNW trends, above which basement and cover were variably coupled during stress build-up and early deformation of the Arabian margin. Beyond regional implications, this study draws attention to the need to carefully consider pre-existing fractures, possibly unrelated to folding, to build more realistic conceptual fold–fracture models.

Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt

Mechanically induced calcite twins in veins and host rocks of Late Cretaceous to Miocene age in Iran have been used to determine regional Arabia-Eurasia collisional stresses. A late folding stress regime with a compression oriented 025° (±15°) has been identified across the Zagros belt and the southern Iranian Plateau. This late Neogene stress pattern agrees with the current stress field determined from the focal mechanisms of basement earthquakes and suggests that the Hormuz salt decollement poorly decouples the basement and cover stress fields. Our data show that the collisional state of stress has been relatively constant since ca. 5 Ma. The magnitudes of the stresses obtained from the twinning analysis are unexpectedly low, and, to a first approximation, they are constant across the Zagros simply folded belt. This result supports an overall mechanism of buckling of the detached Zagros cover. Internal viscous-plastic processes help to relieve stress within this cover, thus lowering its seismogenic potential. Beyond these regional implications, this study underlines the potential of paleostress analyses in constraining both the tectonics and the mechanics of ancient and active foreland fold belts.

Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran

Thin-skinned fold-and-thrust belts are generally considered as the result of contractional deformation of a sedimentary succession over a weak decollement layer. The resulting surface expression frequently consists of anticlines and synclines spaced in a fairly regular manner. It is thus tempting to use this spacing along with other geological constraints to obtain insights into the dynamics and rheology of the crust on geological time scales. Here we use the Zagros Mountains of Iran as a case study, as it is one of the most spectacular, well-studied thin-skinned fold-and- thrust belts in the world. Both analytical and numerical models are employed to study what con- trols fold spacing and under what conditions folding dominates over thrusting. The models show that if only a single basal decollement layer is present underneath a brittle sedimentary cover, deformation is dominated by thrusting, which is inconsistent with the data of the Zagros fold belt. If we instead take into account additional decollement layers that have been documented in the fi eld, a switch in deformation mode occurs and crustal-scale folding is obtained with the correct spacing and time scales. We show that fold spacing can be used to constrain the friction angle of the crust, which is ~5° the Zagros fold belt. This implies that on geological time scales, the upper crust is signifi cantly weaker than previously thought, possibly due to the effect of fl

Quaternary transfer faulting and belt front deformation at Pakuashan (western Taiwan)

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along-strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along-strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left-lateral tear fault or transfer fault that offsets two distinct frontal thrust-and-fold sheets.