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Decoupling along plate boundaries: Key variable controlling the mode of deformation and the geometry of collisional mountain belts Ernst Willingshofer, Dimitrios Sokoutis Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands gence rates (Faccenda et al., 2008). In addition, the increasing resistance of buoyant continental material to subduction may strengthen the stress coupling between the foreland and the orogenic wedge (Ziegler et al., 2002). In this study we discuss the orogenic wedge–foreland plate interaction as a consequence of increasing mechanical coupling between the orogen and the foreland. Our findings have implications for collisional mountain belts in general, and their relevance is discussed for the case of the European Alps in particular. cm MODELING SETUP The experimental continental lithosphere consists of three layers representing the brittle crust, ductile crust, and ductile mantle. (See Fig. 1 for the geometrical setup, the analogue materials used, and the length and time scales.) Material properties and scaling parameters were described in Willingshofer et al. (2005). The 2.5 13 nt In de ic ge n ro O 10 cm Yield strength (Pa) 15 cm Brittle crust Brittle crust Visc. crust Dry quartz sand Viscous crust & mantle Silicone mix I Strong viscous mix I Silicone mix II upper mantle 30o Low-viscosity lower mantle lithosphere + Asthenosphere (liquid) Zone of decoupling 0 Depth (cm) 1.0 1.5 15 cm Indenter Orogenic wedge er dg e pl la re Fo 0 we at e 18 Lateral confinement nd cm 22 Depth (cm) INTRODUCTION Decoupling within plates or at plate boundaries has implications for transmission of stresses within the lithosphere and the resulting strain, which is a reflection of the different deformation processes at work. Vertical decoupling within the lithosphere is mainly a function of its composition and thermal state. Investigations of the consequences of decoupling at or close to the Moho (e.g., Ellis, 1996; Royden, 1996) have emphasized that crust-mantle decoupling affects the shape, height, and length scales of collisional mountain belts. Horizontal decoupling, however, relates to strain accommodated along weak plate contacts, such as subduction channels (e.g., Stöckhert, 2002) or weak intraplate faults (Handy et al., 2005). The weakness at plate interfaces seems to be critical for the exhumation of high- and ultrahigh-pressure rocks (e.g., Gerya et al., 2002), the development and stability of one-sided terrestrial subduction (Faccenda et al., 2008; Gerya et al., 2008), or the amount of supported upper-plate topography (Lamb, 2006). The results of deep seismic profiling, imaging the crustal-scale geometry of mountain belts like the Himalayas (Zhao et al., 1993), also demands strong decoupling to enable underthrusting of the Indian continent. The strength of the interface between plates or between plates and orogenic wedges as considered in this study can change through time because lubrication between the plates may become inefficient as the incoming plate carries less sediment and the availability of water decreases, or as shear heating and hence thermally driven softening of the plate interface decrease as a function of slowing of conver- strength of the model lithosphere varies laterally such that a wedge-shaped weak zone is bordered on both sides by strong lithospheres, referred to as foreland plate and indenter, respectively. In analogy to natural orogenic systems, the weak zone represents imbricated sediments and upper crustal slices, which have been scraped off the incoming plate. Lateral changes of lithospheric strength were incorporated in the model by varying the thickness of the brittle and ductile crustal layers and the rheology of the ductile upper mantle. This study discusses cases where the weak wedge is strongly coupled to the adjacent plates (experiment A) and where the same weak wedge is decoupled from the strong plate along its inclined boundary (experiment B). It is argued that strong decoupling along the wedge–foreland plate interface is conditioned by shear heating (e.g., Faccenda et al., 2008) or by fluid activity in response to the dehydration of cover sediments of the incoming plate, similar to what is known from subduction systems (e.g., Peacock, 1990). Strong decoupling is achieved by lubricating the inclined boundary with a weak 2-mm-thick silicone layer, differing from previously published lithosphere-scale analogue models where crust-mantle coupling has been implemented by varying the shortening velocity (e.g., Davy and Cobbold, 1991). Solid walls confined three sides of the model while the fourth, a layer of silicone putty labeled “lateral confinement” (Fig. 1), allowed limited amounts of lateral escape. During modeling, the surfaces of all experiments were scanned with a threedimensional video laser every 30 min, and the data were converted to digital elevation models (DEMs). All experiments were performed under normal gravity conditions. 300 600 900 1.0 1.5 2.5 Yield strength (Pa) 0 Depth (cm) ABSTRACT The consequences of decoupling between weak orogenic wedges and strong adjacent foreland plates are investigated by means of lithospheric-scale analogue modeling. Decoupling is implemented in the three-layer models by lubrication of the inclined boundary between a strong foreland and a weak orogenic wedge. Plate boundaries are orthogonal to the convergence direction. Experimental results show that strong decoupling between the foreland and the orogenic wedge leads to underthrusting of the former underneath the orogenic wedge and deformation of the orogenic wedge by folding, shearing, and minor backthrusting. Shortening is mainly taken up along the main overthrust, the decoupled boundary, and within the orogenic wedge, leaving the indenter devoid of deformation. In contrast, strong coupling between the foreland and the orogenic wedge favors buckling, involving both the weak zone and the strong plates. The results of these end-member models have implications for collision zones, for example, the Eastern Alps in Europe, such that the switch from localized deformation within the orogenic wedge during the Oligocene–middle Miocene to orogen-scale uplift and deformation during the late Miocene–Pliocene involving the foreland and indenter plates, respectively, is interpreted as reflecting a change from a decoupled to a coupled system. 300 600 900 1.0 1.5 2.5 Length scale: 1cmEXP ~ 20 kmNATURE Shortening velocity: 0.7 cm/hr Figure 1. Modeling setup and initial strength variation among modeling domains as shown by strength envelopes. © 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY, January 2009 Geology, January 2009; v. 37; no. 1; p. 39–42; doi: 10.1130/G25321A.1; 2 figures; Data Repository item 2009010. 39 MODELING RESULTS Experiment A—Strong Coupling Deformation commenced at the vertical boundary between the weak wedge and the indenter as backthrusting (thrust 1 in Fig. 2A). Subsequently, deformation stepped into the weak wedge, leading to the activation of forethrust 2, which together with backthrust 1 defines a pop-up structure. At ~10% bulk shortening, deformation within the brittle crust jumped to the other side of the orogenic wedge, where backthrusts and forethrusts (4, 5, and 6) developed above the initially inclined boundary (Fig. 2B). From 12% bulk shortening onward most of the shortening within the brittle crust was taken up by forethrust 1′, which coincides at the surface with former backthrust 1 (cf. Figs. 2B and 2C). Thrusting thereafter remained confined to near the transition zones between the weak and adjacent strong plates, leading to narrowing of the zone between thrusts 1 and 3 (Fig. 2D). The cross section reveals the geometry of a folded lithosphere (Fig. 2E), showing that the process of folding controls the locus of thrust formation (at the inflection points of folds, as also shown by Davy and Cobbold, 1991; e.g., thrusts 1′ and 8), leading to the rotation of thrusts into a steep dip (thrusts 1–3) and the development of pop-down–type structures similar to those described by Sokoutis et al. (2005). Although the rheological contrast between the weak zone and the adjacent plates is on the order of one magnitude, coupling between the plates is strong, as documented by the lack of shear across the inclined and vertical boundaries and the fact that deformation affected all parts of the model throughout the model run (Fig. 2F). The latter can be deduced from the DEMs in Figures 2A–2D and the topographic profiles (Fig. 2F), which show that the weak zone developed from a broad uplifted region with thrust-controlled relief (topographic profile after 5% bulk shortening) to form a single antiform. Broad antiform highs are separated by narrow longitudinal basins and the asymmetry in the model is conditioned by the advancing wall. Experiment B—Strong Decoupling In experiment B, deformation started at the decoupled boundary (thrust 1 in Fig. 2G). Thereafter, shortening was accommodated within the weak zone by folding and by forethrusting and backthrusting (thrust 2 in Fig. 2G and thrusts 6 and 10 in Figs. 2H–2J). Close to the transition to the lateral confinement, shortening of the weak zone was also accommodated by minor strike-slip faulting. In contrast to experiment A, deformation remained localized on thrust 1 throughout the simulation, and contraction of the weak wedge caused overturning of the fold in the hanging wall to thrust 1. 40 Sections reveal an asymmetric lithospheric structure, which is dominated by underthrusting of the foreland plate (Fig. 2K). As shortening is mainly taken up by motion along thrust 1, and to a lesser extent within the brittle layer of the weak wedge, the indenter remained nearly undeformed. The foreland-facing side of the weak zone forms a recumbent fold cored by the ductile crust. The foreland and indenter plates are separated by a broad ductile shear zone. The Moho topography of the foreland plate documents bending, whereas that of the indenter remained flat. At the end of the experiment (24% bulk shortening) a mature foreland-type basin had developed on the underthrusting plate (DEMs in Figs. 2H–2J, 2L). The evolving topography is asymmetric, with steeper slopes facing the foreland plate where migration of thrust 1 and the progressive deepening of the basin can be deduced from the topographic profiles. INTERPRETATION OF MODELING RESULTS Our results show that decoupling between orogenic wedges and adjacent strong plates is a variable that can steer the structural and topographic evolution of collision zones as it controls the mechanisms by which the crust and lithosphere deform (see GSA Data Repository Fig. DR11). Our end-member models indicate that strong mechanical coupling between plates and orogenic wedges favors deformation of the lithosphere by folding, involving the strong indenter and foreland plates as well as the weak orogenic wedge, differing in the relative amount of vertical motion as a function of plate and wedge strengths and the distance to the stress source (moving wall). Consequently, stress transfer from the indenter to the foreland plate is hampered by the intervening weak wedge, leading to lower amplitude folding of the latter with respect to the indenter. Hence, lateral strength variations within the continental lithosphere do not necessarily lead to decoupling between plates or parts of plates, but merely influence the wavelength and amplitude of deformation. In contrast, strong decoupling between the orogenic wedge and the foreland plate leads to crustal- and lithospheric-scale thrusting. As most of the shortening is taken up by shear along the decoupled boundary, minor backthrusting along the indenter–orogenic wedge boundary does not lead to the development of a mature retroforeland basin. Similarly, internal deformation of the indenter and foreland plate is negligible, 1 GSA Data Repository item 2009010, Figure DR1 (cartoon illustrating the consequences of decoupling and coupling between the foreland and an orogenic wedge), is available online at www.geosociety. org/pubs/ft2009.htm, or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. except for bending of the latter due to loading by the evolving orogen. Our results are consistent with numerical modeling studies of Faccenda et al. (2008) and Burov and Yamato (2008), who emphasized that decoupling between colliding plates produces one-sided mountain belts (our experiment B), whereas strong coupling favors the formation of two-sided orogens, with largescale folding being the dominant deformation mechanism (our experiment A). IMPLICATIONS FOR THE EVOLUTION OF COLLISIONAL MOUNTAIN BELTS AND THEIR FORELANDS The models discussed in this study represent end-member scenarios with constant mechanical boundary conditions throughout the simulations. In nature mechanical coupling between orogenic wedges and adjacent plates is likely to be subject to changes through time as the amount of water lubricating the contact zone decreases, the buoyancy of the incoming continental lithosphere, resisting subduction, increases, or the effect of shear heating at plate contacts diminishes as shortening velocities decrease (Faccenda et al., 2008). In such cases, an increase of coupling among the orogen and the foreland plates is expected to occur, leading to the change of the dominant deformation mechanism through time. Folding of an orogenic wedge with a complex rheological stratification is difficult to reconcile in natural examples on the basis of structural observations because folding is typically disharmonic and is accompanied by thrusting in the brittle layers of the lithosphere (Davy and Cobbold, 1991). The pattern of vertical motions in the mountain belt and the surrounding plates, however, may help to constrain increasing coupling between the wedge and the foreland plates (Ziegler et al., 2002). Taking the Eastern Alps (German, Austrian, Italian) as an example, we argue that the following events in the evolution of the Alps reflect a change from a decoupled to a coupled system. Following consumption of a small ocean (Alpine Tethys), late Eocene underthrusting of the distal European continental margin, now exposed in the core of the mountain belt, the Tauern Window (Frisch et al., 1998; Lammerer and Weger, 1998), occurred under weakly coupled conditions with calcareous clays and silts from the ocean and the sedimentary cover as decoupling material. Loading of the European plate is reflected by strong subsidence in the foreland basin during the late Eocene–early Oligocene (Genser et al., 2007) as the response to the northward-propagating mountain belt. Experiment B depicts such a situation where the foreland plate is thrust under the orogenic wedge, leading to crustal thickening and uplift of the internal part of the orogen and subsidence of the foreland plate. GEOLOGY, January 2009 Experiment B - strong decoupling Experiment A - strong coupling 1 1.6% bulk shortening (bs) ~ 14 km Digital elevation model Indenter Weak wedge Foreland plate 2 1.6% bs ~ 14 km Lateral confinement 1 2 2 1 2 1 22 A 4 21 5 3 4 10% bs ~ 84 km 10% bs ~ 84 km 6 1 21 4 G 8 3 7 5 8 2 5 2 6 1 62 8 3 7 5 3 5 8 2 4 6 5 6 1 2 H B 6 4 421' 5 15% bs ~ 126 km 83 7 8 8 20% bs ~ 174 km 3 64 17 42 62 8 1' 5 1 3 6 8 7 8 5 8 1 5 3 I 6 4 31' 56 2 22% bs ~ 182 km 17 64 2 3 1' 5 837 8 10 9 1 5 6 24% bs ~ 210 km 6 3 8 A A 6 4 Brittle crust Strong viscous upper man 4 cm tle 3 2 1' 2 1 D 8 6 1 8 10 5 9 5 6 J 6 10 1 1 A Initially inclined boundary Viscous crust and upper mantle Topographic evolution 7 9 8 8 17 6 C Model Moho 4 cm K E Relief along cross section A after: 0%, 5%, 10%, 15%, 20%, 22% bulk shortening Vertical exaggeration: 2x Topographic evolution Relief along cross section A after: 0%, 5%, 10%, 15%, 20%, 24% bulk shortening Vertical exaggeration: 2x Steep slopes facing the foreland Gentle slopes facing the indentor 4 cm 4 cm Uplift F Subsidence Thrust fault Strike-slip fault Normal fault Migration of thrust front (1) Anticline/syncline Outline of weak wedge L Black colors = non-active Figure 2. A–F: Structural and topographic evolution of experiment A. G–L: Structural and topographic evolution of experiment B. Original top-view photographs, digital elevation models (DEMs), structural interpretation and cross sections along line A, and snapshots of evolving topography along same section are displayed. Numbers indicate sequence of deformation. Grid spacing is 4 cm. Green rectangles in A and G delineate areas covered by DEMs. GEOLOGY, January 2009 41 The coupling between the orogen and the foreland plate thereafter increased gradually, as documented by the following. 1. Oligocene–Miocene strong internal deformation of the wedge led to folding and stacking of tectonic units, now exposed along the central axes of the Eastern Alps (Lammerer and Weger, 1998). During this period of time the strongest vertical motions occurred in the core of the orogen, as documented by pressure-temperaturetime paths from the Tauern Window (e.g., Cliff et al., 1985; Fügenschuh et al., 1997). Convergence was partitioned into a component of north-south shortening and concomitant eastwest extension (Ratschbacher et al., 1991). Eastward material transfer was conditioned by the Periadratic fault system, leading to crustalscale decoupling between the orogen proper and the indenter (Handy et al., 2005). 2. The switch from dominant north-directed thrusting of the orogenic wedge onto the European foreland to south-directed thrusting and internal deformation of the indenter occurred during the late Miocene (ca. 12–10 Ma). Such behavior is consistent with coupled examples as shown in experiment A and the experiments by Sokoutis et al. (2005). 3. Isotope data and, in particular, apatite fission track data from the Eastern Alps compiled by Luth and Willingshofer (2008) document a switch from localized uplift and cooling in the core zone of the orogenic wedge in pre–middle Miocene time to regional, orogen-scale uplift from the late Miocene onward. This important change is also reflected in the increase of sediment discharge from the Alps (Kuhlemann et al., 2001). 4. Middle to late Miocene cooling probably led to strengthening of the orogenic wedge, which facilitated the stress transmission from the indenter to the northern foreland, triggering its uplift, which started ~6 m.y. ago as documented by subsidence analyses from the foreland basin (Genser et al., 2007). Folding of the foreland plate (experiment A) in cases of mechanical coupling between the foreland and the orogenic wedge is a potential mechanism to explain the late-stage uplift of the foreland basin without the need to call upon weakly constrained processes like delamination of the lithospheric root (Genser et al., 2007). Based on our results and in accord with the regional studies of Ziegler et al. 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