Olivier Lacombe

Characterization of stress perturbations near major fault zones: insights from 2-D distinct-element numerical modelling and field studies (Jura mountains)

Abstract We consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations. Based on 2-D distinct-element modelling, we reconstruct the local stress field around a vertical discontinuity in various geological contexts. The resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 °. We establish simple relationships controlling stress changes around a pre-existing fault zone as a function of (1) the remote differential stress magnitude, (σ1 − σ3), (2) the friction coefficient on the discontinuity, and (3) the strike of the discontinuity relative to the far-field stress. As a geological example, we present the Morez Fault Zone in the internal Jura. Paleostress reconstruction in forty-two sites indicates that the trends of the Mio-Pliocene compression are N110 ° on average near the fault, whereas they are N130 ° in the surrounding areas. A comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 ° counterclockwise deviation is directly related to the reactivation of this large weak zone. We thus evaluate the role of mechanical decoupling along pre-existing zones of weakness, especially with consideration to the accommodation of the Alpine deformation in the Jura belt.

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.

Determination of deviatoric stress tensors based on inversion of calcite twin data from experimentally deformed monophase samples: preliminary results

Abstract Following previous successful (paleo)stress determinations from numerically generated aggregates and naturally deformed samples, preliminary results about stress reconstructions based on inversion of calcite twin data from experimentally deformed monophase samples are reported. This new study carried out on samples of Carrara marble and crinoidic limestone from eastern France provides a test of the validity of stress reconstructions using inverse methods, which had not been properly calibrated before. The percentages of correlated twins are discussed for each stress tesnor, and the range of uncertainties on the determination of stress orientations and differential stress magnitudes is evaluated.

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.

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

Evidence for pre‐folding vein development in the Oligo‐Miocene Asmari Formation in the Central Zagros Fold Belt, Iran

In order to understand the interplay between vein development and folding in the carbonates of the Oligo-Miocene Asmari Formation (one of the main hydrocarbon reservoir rocks) in Iran, several anticlines have been investigated in the central part of the Zagros folded belt. Combining observations of relative chronology between veins based on calcite-filling phases and crosscutting/abutting relationships, as well as aerial/satellite image interpretation on several anticlines allowed proposing a tectonic model highlighting the widespread development of veins and other extensional micro/meso-structures in the Central Zagros folded belt. Our data suggest that most of the veins affecting the Asmari formation predated the main Miocene-Pliocene folding episode. An early regional vein set striking N50° marked the onset of collisional stress build-up in the region. Then, N150° and N20° trending vein sets were initiated in response to local extension caused by large-scale flexure/drape folds above N-S and N140° basement faults reactivated under the regional NE compression. At the onset and during Miocene-Pliocene folding of the sedimentary cover, the early formed veins were reactivated (reopened and/or sheared) while duplexes, low angle reverse faults and thrusts formed. Beyond regional implications, this study puts emphasis on the need of carefully considering regional/local vein development predating folding as well as influence of underlying basement faults in models of folded-fractured reservoirs in fold-thrust belts.

Early Reactivation of Basement Faults in Central Zagros (SW Iran): Evidence from Pre-folding Fracture Populations in Asmari Formation and Lower Tertiary Paleogeography

Early reactivation of basement faults and related development of flexures/forced-folds in the Central Zagros are discussed based on fracture populations observed in outcrops and aerial photographs/satellite images and paleogeographic maps. The presence of pre-folding joint sets slightly oblique to anticline axes and observed even within synclines or the occurrence of N-S (and E-W) trending fracture sets near N-S trending basement faults and strongly oblique to cover folds are not compatible with simple fold-related fracture models in this region. These early fractures are proposed to have formed within the cover above deep-seated basement faults in response to the formation of flexures/forced folds whose geometries and orientations may be different from the present-day folds in the Central Zagros. This early stage of intraplate reactivation of the NW-SE and N-S trending basement faults likely marks the onset of collisional deformation and stress build-up in the Zagros basin. This reactivation led to facies variations and development of different sub-basins in the Central Zagros during the sedimentation of the Oligocene- Miocene Asmari Formation. The evaporitic series of the Kalhur Member within the Asmari Formation resulted from the development during Aquitanian times of a long and narrow restricted lagoon environment, between two main basement faults (i.e., DEF and MFF), and provide one of the main key constraints on the beginning of deformation in the region. Finally, based on observed fracture populations and proposed geodynamic evolution in the Central Zagros basin, it is suggested that partitioning of N-S Arabia-Eurasia convergence into a belt-perpendicular NE-SW shortening and a beltparallel right-lateral strike-slip motion (as currently along the Main Recent Fault) in the Central Zagros may have started as early as Oligocene (?)-Lower Miocene times.

Paleostress magnitudes associated with development of mountain belts: Insights from tectonic analyses of calcite twins in the Taiwan Foothills

Determinations of stress magnitudes based on inversion of calcite twin data from fault-related folds in the Taiwan Foothills show a decrease of differential stresses during synfolding erosion and exhumation, suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks. Regional frictional conditions linked to shallow and/or deep decollement tectonics probably also influenced stress magnitudes. Combination of the twin inversion technique with fracture analysis and rock mechanics data provides first-order estimates of principal stress values related to the major Pleistocene shortening event in Taiwan. These stress estimates are compared to previous stress estimates in fold-thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens. At the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland. This decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation. Differential stresses estimated from natural deformation consequently provide an independent support to the depth-dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes.

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.