Benoit Deffontaines

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.

The Mindanao collision zone: a soft collision event within a continuous Neogene strike-slip setting

Abstract Two volcanic belts are presently juxtaposed on Mindanao Island in the southern Philippines. Southward, the collision is still active in the Molucca Sea which is commonly regarded as a region of doubly verging subduction, plunging eastward below the Halmahera arc and westward below the Sangihe arc. In the Molluca Sea, tectonic features related to the incipient collision appear only in the very thick sediments of the basin, and the morphology of the parallel Halmahera, Talaud and Sangihe ridges is closely controlled by recent N-S strike-slip faults. Among these faults, the Philippine Fault is a neotectonic feature crosscutting the Agusan-Davao Basin which seals tectonic events not younger than Eocene. In addition, the Central Cordillera shows strong similarities with the Pacific Cordillera for both stratigraphy and tectonic evolution, and several indications favour a Eurasian margin affinity for the Daguma Range (Southern and Eastern Kudarat Plateau that may be part of the Sangihe arc, as inferred for the Zamboanga Peninsula and the Northern Arm of Sulawesi. Thus the island of Mindanao can be divided into two composite terranes, the western one (northward extension of the Sangihe arc) being restricted to the Kudarat Plateau and the Zamboanga Peninsula. The apparent continuation of the Sangihe arc into the Central Cordillera of Mindanao is thus the result of post collision tectonics. The portion of the suture where the collision is completed curves westward north of the southern peninsula and extends beneath the sediments of the Cotabato Basin or the volcanic plateaus of the Lanao-Misamis-Bukidnon Highlands. In the northern part, the contact is linear and suggests, together with the absence of compressional deformation, a docking of the eastern oceanic terrane (Philippine Mobile Belt-Halmahera arc) against the western continental terrane (Zamboanga-Daguma) in a strike-slip environment. Prior to Early Pliocene, the eastern and the western terranes were subject to different tectonic regimes with direction of extension perpendicular to the present one. From Late Pliocene to present, both terranes are affected by NNE and E-W compression.

Principles of drainage basin analysis from multisource data: Application to the structural analysis of the Zaire Basin

Abstract Classifications which attempt to combine descriptive and genetic aspects of types of drainage basins must take into account the application of new techniques (satellite imagery, high-precision topographic maps and digital terrain models). In addition, the use of new concepts such as neotectonics and drainage basin history will lead to a reassessment of classification schemes. After a review of current classifications, we propose a genetic scheme that is based on the development of drainage networks and which includes integration of local and regional-scale perturbations. State-of-the-art methods of drainage basin analysis are applied to the Zaire Basin to provide new information on deep structures. The influence of basin and block tectonics is revealed in the context of major Late Proterozoic shear faults which have been rejuvenated up to the present day. This is demonstrated from an analysis of the drainage network in the light of geological and geophysical observations.

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

New geomorphic data on the active Taiwan orogen: A multisource approach

A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital elevation model (DEM), side-looking airborne radar (SLAR), and satellite (SPOT) imagery, aerial photographs, and control from independent field data. This analysis enables us not only to present an integrated geomorphic description of the Taiwan orogen but also to highlight some new geodynamic aspects. Well-known, major geological structures such as the Longitudinal Valley, Lishan, Pingtung, and the Foothills fault zones are of course clearly recognized, but numerous, previously unrecognized structures appear distributed within different regions of Taiwan. For instance, transfer fault zones within the Western Foothills and the Central Range are identified based on analyses of lineaments and general morphology. In many cases, the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently. In turn, the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic features in other areas. Examples are the conjugate pattern of strike-slip faults within the Central Range and the oblique fold-and-thrust pattern of the Coastal Range. Furthermore, neotectonic and morphologic analyses (drainage and erosional surfaces) have been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in Taiwan, such as for the Longitudinal Valley Fault. Next, at a more general scale, numerical processing of digital elevation models, resulting in average topography, summit level or base level maps, allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance. Finally, a preliminary morphotectonic sketch map of Taiwan, combining information from all the sources listed above, is presented.

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.

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.

Genetic relations between the central and southern Philippine Trench and the Sangihe Trench

We surveyed the junction between the central and southern Philippine Trench and the Sangihe Trench near 6oN using swath bathymetry, gravity, and magnetics. These data, along with seismicity, allow us to discuss the genetic relations between these trenches and the forces acting on converging plates. Our final model favors the northern extension of the Halmahera Arc up to 8oN, with three segments offset left-laterally along NW-SE transform faults. Accretion of the northern segment to Mindanao Island 4 to 5 m.y. ago resulted in the failure within the Philippine Sea Plate east of the arc. Initiation of the Philippine Trench between 7oN and 10oN agrees with the maximum recorded depth of the Philippine Trench floor (10,000 m below sea-level) and Philippine Sea slab (200 km). South of 6oN (trench junction), another segment of the arc is being subducted beneath the Sangihe margin, while south of 3oN, the southern segment of the Halmahera Arc is still active. The rapid southward shallowing of the trench floor along the southern Philippine Trench, the type of faulting affecting both sides of the trench, the lack of significant interplate seismicity, and the concentration of the seismicity beneath the Miangas- Talaud Ridge are interpreted as a slowing down of the subduction along this branch of the Philippine Trench compared with the rest of the subduction zone. The Sangihe deformation front has been recognized up to 7oN but seems active only south of 6oN.

Upper plate deformation induced by subduction of a volcanic arc: the Snellius Plateau (Molucca Sea, Indonesia and Mindanao, Philippines)

Abstract The northern Molucca Sea shows the incipient subduction of a composite oceanic and arc volcanic block, the Snellius–Halmahera block (SHB). Multi-beam, reflectivity, seismic and gravity data obtained during the MODEC marine survey showed that the SHB disappears beneath the accretionary wedge and the outer ridge of the Sangihe Arc. To the north in Mindanao Island, ongoing convergence generated shortening of the forearc basin and the backthrusting of the SHB; meanwhile a classical system of paired subduction (Philippine Trench) and strike-slip fault (Philippine Fault) was installed. The transition from lithospheric subduction to crustal overthrusting is located where the Philippine Trench sensu stricto begins, and also coincides with the off-shore extension of the Philippine Fault. We observe a reversal of the thrusts from an eastward vergence in the Molucca Sea to a westward vergence in Mindanao Island. This reversal takes place at the latitude where the forearc area emerges by uplift, and the downgoing crust (SHB) deepens, resulting in a strong gravity low centered above the accretionary wedge. The Philippine Fault initiated in a place where the crust was sliced off by a transfer zone which marked the northern termination of the Molucca Sea, and drags northward a sliver of the previously accreted SHB. The northward drifting of this sliver created an extension, which, however, cannot account for the gravity low. We propose that the shortening and the uplift of the upper plate were induced by the buoyancy of the subducted unit (SHB), and triggered the thrust reversal.