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8 Foreland Basin Evolution and the Growth of an Orogenic Wedge

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8 Foreland Basin Evolution and the Growth of an Orogenic Wedge A a) 8 Foreland basin Foldandthrustbelt Marginal(rem nant) ocean basin evolution and the Forelandbasin nant) ocean basin growth of an orogenic Marginal(rem Forebulge wedge Craton B b) Foldand thrustbelt 8.1 Introduction Forelandbasin Forebulge A B Peripheral foreland basin systems (DeCelles and Giles, 1996) result from the flexural down- bending of continental lithosphere in response Forelandbasinsystem to tectonic and topographic loading during Orogenicwedge continent-continent collision (Price, 1973). The TF DF Wedge-top Foredeep Forebulge Backbulge spatial evolution of the associated depozones, i. e., Foldandthrustbelt wedge-top, foredeep, forebulge and backbulge D (Fig. 8.1) and their respective sedimentary infill, Passivemargindeposits c) is strongly dependent on (i) the effective elastic Figure 8.1: (a) Schematic map view of a foreland basin, thickness (Te) of the involved lithospheres; (ii) the bounded longitudinally by a pair of marginal ocean basins. The scale is not specified, but would be of the order of 102 level of horizontal stresses; (iii) the magnitude to 103 km. Vertical line at right indicates the orientation of of the loads imposed on the foreland by the oro- a cross-section that would resemble what is shown in (b). genic wedge and the subducted lithospheric slab; (b) The generally accepted notion of foreland-basin geome- (iv) the dip-angle of the latter; (v) the rate and try in transverse cross-section. Note the unrealistic geometry direction of convergence; (vi) the amount of ero- of the boundary between the basin and the thrust belt. Ver- tical exaggeration is of the order of 10 times. (c) Schematic sion of the orogenic wedge and the dispersal sys- cross-section depicting a revised concept of a foreland basin tem within the foreland and (vii) eustasy (Beau- system, with the wedge-top, foredeep, forebulge and back- mont, 1981; Turcotte and Schubert, 2002; Allen bulge depozones shown at approximately true scale. Topo- et al., 1991; Sinclair, 1997b; Ziegler et al., 2002). graphic front of the thrust belt is labeled TF. The foreland basin system is shown in dark grey; area in light grey indi- Numerous field studies have demonstrated that al- cates passive margin deposits, which are incorporated into most all foredeeps evolve from an underfilled to a (but not shown within) the fold-thrust belt toward the left of filled or overfilled depositional state (Covey, 1986; diagram. A schematic duplex (D) is depicted in the hinterland Sinclair, 1997a). The underfilled state is charac- part of the orogenic wedge. Note the substantial overlap be- tween the front of the orogenic wedge and the foreland basin terised by deep-marine (Flysch type) sediments, system. Modified after DeCelles and Giles (1996). high thrust advance rates, and low exhumation rates. In contrast, the overfilled state shows shal- low marine to continental (Molasse type) deposits line of the inherited passive margin of the under- and a dominance of exhumation versus frontal ad- thrusted plate (Dewey, 1982). Numerical simula- vance of the orogen (Sinclair and Allen, 1992). tions in conjunction with field studies in the Swiss Classically, the Flysch to Molasse transition is in- Alps however, suggest that the rate of frontal ad- terpreted as recording the migration of the thrust vance of the orogenic wedge and the sediment wedge and the associated foredeep over the hinge- transport coefficient (K) are the main control on 105 106 8. Foreland basin evolution and the growth of an orogenic wedge the state of the foredeep infill, whereas an increase 8.2 Method of the flexural rigidity or the surface slope of the orogenic wedge is only of minor importance (Sin- Time series of the horizontal distance between clair et al., 1991; Sinclair, 1997b). the deformation front of the pro-wedge and the singularity [L f p=H0] as well as the height of the Additionally, most forward modelling studies, axial-zone above the singularity [H=H0] provide which are aimed at unravelling the influence of an approximation of the triangular shape of the the above parameters on the stratigraphic archi- pro-wedge throughout its evolution. Although tecture of foreland basins (Flemings and Jor- these data represent dimensionless lengths, they dan, 1990; Jordan and Flemings, 1991; Sinclair are scaled by 105, a factor commonly used in sand- et al., 1991; Crampton and Allen, 1995; Galewsky, box experiments (e. g., Malavieille, 1984; Storti 1998; Allen et al., 2001; Clevis et al., 2004), as- et al., 2000). Both converted time series (L and sume that the respective orogenic load results from H) are used to calculate the flexure of a hypothetic either a lithospheric scaled fault-bend fold (Flem- foreland lithosphere in response to orogenic load- ings and Jordan, 1990) or from a pro-wedge sensu ing for each time (convergence) step according to Willett et al. (1993). However, sandbox simula- Turcotte and Schubert (2002): tions of bivergent orogens (like this study) have demonstrated that strain is partitioned between the V0 = WrorogengLH (8.1) pro- and the retro-wedge. It follows that processes acting either upon or within the retro-wedge con- trol the load distribution within the pro-wedge as µ ¶ 4D 1=4 well, which in turn influences the geometry of a = (8.2) the pro-foredeep. Consequently, cause and effect (rm ¡ rs)g would be considerably offset in space. ³ ´3=4 4D Thus, the purpose of this study, which is based (r ¡r )g V0 w = m s (8.3) on scaled-sandbox simulations as well as ana- 0 4D lytical considerations, is twofold. First, we ex- plore how the lateral growth of an orogenic wedge ¡x=a controls the spatio-temporal evolution of the pro- wx = w0e cosx=a (8.4) foredeep and thus the Flysch to Molasse transi- tion. Second, we focus on the influence of the whereby V0 is the vertical load in N, W is coupling between the pro- and the retro-wedge on the width of the hypothetical orogen = 1m, L is the evolution of the pro-foredeep. In order to ad- the converted length of the pro-wedge in m, H dress both issues we consider a reference experi- is the converted height of the axial-zone above ment (9.05) and one experiment with pro- and an- the singularity in m, a is the flexural parame- other with retro-wedge erosion (9.09, 9.06). Fi- ter, D is the flexural rigidity of the plate, chosen nally, sedimentary basins and thus foreland basins to be 1022 Nm, g is the acceleration due to grav- 2 provide the most significant sources of energy- ity (9:81m=s ), rorogen is the density of a hypo- 3 related commodities, such as hydrocarbons, coal, thetical orogen (2600kg=m ), rm is the density of 3 uranium and many metals (Kyser and Hiatt, 2003). the mantle (3300kg=m ), rs is the density of sedi- 3 Consequently, the formulation of conceptual mod- ments, filling the foredeep (2300kg=m ), w0 is the els and the detection of far-field relations may help deflection of the plate at x = 0 and w(x) is the de- to constrain future exploration strategies. flection of the plate at point x. Density values are 8.3. Results and discussion 107 Foredeep Forebulge Backbulge ton and Allen, 1995). In a sediment-filled basin (x) the forebulge will be less high, because the flex- Distancenormaltoconvergencex ural response to the sediment load interferes with the one from the orogenic load, resulting in a less Flexuralprofile well developed forebulge. Since we are interested Deflectionofplatew in the Flysch to Molasse transition, we assume that the foredeep is completely filled with sediments. atx=0;V0 ,w (0) Figure 8.2: Theoretical deflection of an elastic broken plate under a line load applied at its end. Modified from Allen and 8.3 Results and discussion Allen (2005). Flexural profiles derived from the above calcula- mean values and have been taken from Turcotte tions image three out of the four depozones asso- and Schubert (2002). A schematic flexural profile ciated with foreland basin systems, i. e., the fore- is shown in figure (8.2). As in previous modelling deep, the forebulge and the backbulge. Although, studies (Karner and Watts, 1983; Sinclair et al., the shape of the flexural profile depends on a mul- 1991; Crampton and Allen, 1995; Roddaz et al., titude of factors as outlined above, the correspond- 2005) orogenic overthrusting of the foreland plate ing magnitudes of either downbending or uplift is simulated by two-dimensional a priori loading agree with field observations. According to De- of a broken elastic plate. Flexural response to Celles and Giles (1996) foredeep depozones are loading is treated as instantaneous, since the re- commonly 2 to 8km thick, forebulges are » 10m sponse time for isostatic adjustment is on the order to several 100m high and backbulges are gen- of 103 to 105 years (Crampton and Allen, 1995). erally not deeper than 200m. Maximum calcu- Lateral migration of the pro-wedge gravity- lated values are 2km, 150m and 20m, respectively center was not taken into account. This would (Fig. 8.3). The temporal evolution of the flexural have resulted in slight changes of the foredeep profiles further indicates that: geometry only. Effects of horizontal stresses on the foreland basin system are neglected, since they i. During early stages of orogenic evolution, in- would only slightly magnify the effects of flex- cremental deepening of the foredeep as well ure and would not change the overall geometries. as incremental uplift of the forebulge is high, Sea level variations would have a similar negligi- but decreases with further convergence. This ble effect on plate flexure (Crampton and Allen, is to be expected, since both the width and the 1995).
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