8 Foreland basinevolution and thegrowth of an orogenicwedge

8.1 Introduction

Peripheral foreland basin systems (DeCelles andGiles, 1996) result from the flexural down-bending of continental lithosphere in responseto tectonic and topographic loading duringcontinent-continent collision (Price, 1973). Thespatial evolution of the associated depozones, i. e.,wedge-top, foredeep, forebulge and backbulge(Fig. 8.1) and their respective sedimentary infill,is strongly dependent on (i) the effective elasticthickness (Te) of the involved lithospheres; (ii) thelevel of horizontal stresses; (iii) the magnitudeof the loads imposed on the foreland by the oro-genic wedge and the subducted lithospheric slab;(iv) the dip-angle of the latter; (v) the rate anddirection of convergence; (vi) the amount of ero-sion of the orogenic wedge and the dispersal sys-tem within the foreland and (vii) eustasy (Beau-mont, 1981; Turcotte and Schubert, 2002; Allenet al., 1991; Sinclair, 1997b; Ziegler et al., 2002).Numerous field studies have demonstrated that al-most all foredeeps evolve from an underfilled to afilled or overfilled depositional state (Covey, 1986;Sinclair, 1997a). The underfilled state is charac-terised by deep-marine (Flysch type) sediments,high thrust advance rates, and low exhumationrates. In contrast, the overfilled state shows shal-low marine to continental (Molasse type) depositsand a dominance of exhumation versus frontal ad-vance of the orogen (Sinclair and Allen, 1992).Classically, the Flysch to Molasse transition is in-terpreted as recording the migration of the thrustwedge and the associated foredeep over the hinge-

Fold and thrust belt

Foreland basin

Forebulge

Craton

Marg

inal (

rem

nant)

oce

an

basi

n

Marg

inal (re

mnant)

oce

an

basin

A

B

a)

Foreland basin

Forebulge

Fold andthrust belt

A B

b)

Wedge-top Foredeep Forebulge Backbulge

Orogenic wedge

Foreland basin system

TF

D

Fold and thrust belt

DF

Passive margin deposits c)

Figure 8.1: (a) Schematic map view of a foreland basin,bounded longitudinally by a pair of marginal ocean basins.The scale is not specified, but would be of the order of 102

to 103 km. Vertical line at right indicates the orientation ofa cross-section that would resemble what is shown in (b).(b) The generally accepted notion of foreland-basin geome-try in transverse cross-section. Note the unrealistic geometryof the boundary between the basin and the thrust belt. Ver-tical exaggeration is of the order of 10 times. (c) Schematiccross-section depicting a revised concept of a foreland basinsystem, with the wedge-top, foredeep, forebulge and back-bulge depozones shown at approximately true scale. Topo-graphic front of the thrust belt is labeled TF. The forelandbasin system is shown in dark grey; area in light grey indi-cates passive margin deposits, which are incorporated into(but not shown within) the fold-thrust belt toward the left ofdiagram. A schematic duplex (D) is depicted in the hinterlandpart of the orogenic wedge. Note the substantial overlap be-tween the front of the orogenic wedge and the foreland basinsystem. Modified after DeCelles and Giles (1996).

line of the inherited passive margin of the under-thrusted plate (Dewey, 1982). Numerical simula-tions in conjunction with field studies in the SwissAlps however, suggest that the rate of frontal ad-vance of the orogenic wedge and the sedimenttransport 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 increaseof the flexural rigidity or the surface slope of theorogenic wedge is only of minor importance (Sin-clair et al., 1991; Sinclair, 1997b).

Additionally, most forward modelling studies,which are aimed at unravelling the influence ofthe above parameters on the stratigraphic archi-tecture of foreland basins (Flemings and Jor-dan, 1990; Jordan and Flemings, 1991; Sinclairet al., 1991; Crampton and Allen, 1995; Galewsky,1998; Allen et al., 2001; Clevis et al., 2004), as-sume that the respective orogenic load results fromeither a lithospheric scaled fault-bend fold (Flem-ings and Jordan, 1990) or from a pro-wedge sensuWillett et al. (1993). However, sandbox simula-tions of bivergent orogens (like this study) havedemonstrated that strain is partitioned between thepro- and the retro-wedge. It follows that processesacting either upon or within the retro-wedge con-trol the load distribution within the pro-wedge aswell, which in turn influences the geometry ofthe pro-foredeep. Consequently, cause and effectwould be considerably offset in space.

Thus, the purpose of this study, which is basedon scaled-sandbox simulations as well as ana-lytical considerations, is twofold. First, we ex-plore how the lateral growth of an orogenic wedgecontrols the spatio-temporal evolution of the pro-foredeep and thus the Flysch to Molasse transi-tion. Second, we focus on the influence of thecoupling between the pro- and the retro-wedge onthe evolution of the pro-foredeep. In order to ad-dress both issues we consider a reference experi-ment (9.05) and one experiment with pro- and an-other with retro-wedge erosion (9.09, 9.06). Fi-nally, sedimentary basins and thus foreland basinsprovide the most significant sources of energy-related commodities, such as hydrocarbons, coal,uranium and many metals (Kyser and Hiatt, 2003).Consequently, the formulation of conceptual mod-els and the detection of far-field relations may helpto constrain future exploration strategies.

8.2 Method

Time series of the horizontal distance betweenthe deformation front of the pro-wedge and thesingularity [L f p/H0] as well as the height of theaxial-zone above the singularity [H/H0] providean approximation of the triangular shape of thepro-wedge throughout its evolution. Althoughthese data represent dimensionless lengths, theyare scaled by 105, a factor commonly used in sand-box experiments (e. g., Malavieille, 1984; Stortiet al., 2000). Both converted time series (L andH) are used to calculate the flexure of a hypotheticforeland lithosphere in response to orogenic load-ing for each time (convergence) step according toTurcotte and Schubert (2002):

V0 = WρorogengLH (8.1)

α =(

4D(ρm−ρs)g

)1/4

(8.2)

w0 =

(4D

(ρm−ρs)g

)3/4V0

4D(8.3)

wx = w0e−x/αcosx/α (8.4)

whereby V0 is the vertical load in N, W isthe width of the hypothetical orogen = 1m, L isthe converted length of the pro-wedge in m, His the converted height of the axial-zone abovethe singularity in m, α is the flexural parame-ter, D is the flexural rigidity of the plate, chosento be 1022 Nm, g is the acceleration due to grav-ity (9.81m/s2), ρorogen is the density of a hypo-thetical orogen (2600kg/m3), ρm is the density ofthe mantle (3300kg/m3), ρs is the density of sedi-ments, filling the foredeep (2300kg/m3), w0 is thedeflection of the plate at x = 0 and w(x) is the de-flection of the plate at point x. Density values are

8.3. Results and discussion 107

Distance normal to convergence x

De

fle

ctio

n o

f p

late

w(x

)

at x = 0; V , w0 (0)

Flexural profile

Foredeep Forebulge Backbulge

Figure 8.2: Theoretical deflection of an elastic broken plateunder a line load applied at its end. Modified from Allen andAllen (2005).

mean values and have been taken from Turcotteand Schubert (2002). A schematic flexural profileis shown in figure (8.2). As in previous modellingstudies (Karner and Watts, 1983; Sinclair et al.,1991; Crampton and Allen, 1995; Roddaz et al.,2005) orogenic overthrusting of the foreland plateis simulated by two-dimensional a priori loadingof a broken elastic plate. Flexural response toloading is treated as instantaneous, since the re-sponse time for isostatic adjustment is on the orderof 103 to 105 years (Crampton and Allen, 1995).

Lateral migration of the pro-wedge gravity-center was not taken into account. This wouldhave resulted in slight changes of the foredeepgeometry only. Effects of horizontal stresses onthe foreland basin system are neglected, since theywould only slightly magnify the effects of flex-ure and would not change the overall geometries.Sea level variations would have a similar negligi-ble effect on plate flexure (Crampton and Allen,1995). The influence of different flexural rigiditieson the foreland basin system is well known (e. g.,Flemings and Jordan, 1989; Sinclair et al., 1991).High flexural rigidities lead to the formation of abroad, shallow depression with a low, wide fore-bulge, whereas low flexural rigidities give rise toa deep, narrow peripheral trough and a relativelyhigh forbulge. This means that, as the load ad-vances across the plate, the onset of subsidence ata given point on the profile will be later, and therate of subsidence higher for a low compared to ahigh flexural rigidity (Sinclair et al., 1991; Cramp-

ton and Allen, 1995). In a sediment-filled basinthe forebulge will be less high, because the flex-ural response to the sediment load interferes withthe one from the orogenic load, resulting in a lesswell developed forebulge. Since we are interestedin the Flysch to Molasse transition, we assume thatthe foredeep is completely filled with sediments.

8.3 Results and discussion

Flexural profiles derived from the above calcula-tions image three out of the four depozones asso-ciated with foreland basin systems, i. e., the fore-deep, the forebulge and the backbulge. Although,the shape of the flexural profile depends on a mul-titude of factors as outlined above, the correspond-ing magnitudes of either downbending or upliftagree with field observations. According to De-Celles and Giles (1996) foredeep depozones arecommonly 2 to 8km thick, forebulges are ∼ 10mto several 100m high and backbulges are gen-erally not deeper than 200m. Maximum calcu-lated values are 2km, 150m and 20m, respectively(Fig. 8.3). The temporal evolution of the flexuralprofiles further indicates that:

i. During early stages of orogenic evolution, in-cremental deepening of the foredeep as wellas incremental uplift of the forebulge is high,but decreases with further convergence. Thisis to be expected, since both the width and theheight of the pro-wedge, are best describedby a power law (section 5.3.7).

ii. Each thrust initiation phase is followed bya deepening of the foredeep and uplift ofthe forebulge, an observation, which hasbeen documented by Jordan and Flemings(1991) as well. Thrust episodes lead to depthchanges of ∼ 1m of the backbulge region.

iii. For a given lateral position, thrust inducedchange of the depth of the foredeep decreaseswith increasing proximity to the orogen. In

108 8. Foreland basin evolution and the growth of an orogenic wedge

contrast, each thrust event has a profound ef-fect on the height of the forebulge, whichis consistent with previous analytical studies(e. g., Jordan and Flemings (1991)).

Although associated with lower magnitudes,these three phenomena can be as well observedin both the pro- and the retro-wedge erosion case(Fig. 8.3b, c). Furthermore, we found that alsoretro-wedge erosion has a significant influence onthe spatio-temporal evolution of the pro-foredeep.The above results confirm thus, the expected andwell established link between the evolution of oro-genic belts and their foreland basin system (De-Celles and Giles, 1996). There are however, someimplications, which deserve further discussion.

The CCW concept predicts and sandbox sim-ulations confirmed that the lateral and verticalgrowth of an orogenic wedge follows a power law(Dahlen, 1990; Mulugeta and Koyi, 1992; Hothet al., 2006). The disagreement between theoreti-cally predicted and experimentally derived powerlaw coefficients is not considered here, but wasdiscussed in section (5.3.7). If the width and theheight of an orogenic belt grow proportional tothe convergence (t) by t0.5, then the respective in-cremental change, which is described by the firstderivation (−0.5t0.5), decreases with time. It fol-lows that the associated increase of the load ofthe orogenic wedge onto its foreland and thusthe resulting increase of the deflection decreasesthrough time as well. At the same time how-ever, the surface of the wedge, prone to be eroded,grows proportional to the convergence (t) by 2t0.5

and is thus twice as fast as the lateral and verti-cal growth. If one further assumes a constant ero-sion rate of the wedge and a constant sedimenta-tion rate within the foredeep, a change from un-derfilled (fast addition of new accommodation), tooverfilled (slow addition of new accommodation)would result. This transition would be thus anemergent consequence of the imposed kinematicboundary conditions. We therefore propose a two-staged evolutionary model. During early stages of

convergence the rate of orogenic growth/advanceis high and so is the rate of flexure induced sub-sidence within the foredeep. Debris derived fromthe orogen is deposited in a deep and probably un-derfilled foredeep (Flysch-type). At a later stageof convergence the rate of flexure induced subsi-dence within the foredeep decreases. The latteris successively filled and may reach a point in itsevolution where all sediments are bypassed (over-filled or Molasse-type). There is thus no need toinvoke a halt of convergence or a slab breakoff toexplain the Flysch to Molasse transition. A slow-down of thrust front advance associated with theFlysch to Molasse transition has been documentedfor the Swiss Alps and the Longmen Shan ThrustBelt (Sinclair, 1997b; Yong et al., 2003; Kempfand Pfiffner, 2004) and has been postulated for thePyrenees as well (Labaume et al., 1985).

The influence of erosion on the orogen-foredeepsystem would be twofold. First, erosion controlsthe geometry and the propagation of an orogenicwedge and thus determines the incremental addi-tion of load responsible for the flexure. Second,erosion provides the debris with which the fore-deep is filled. Consequently, intense erosion of thepro-wedge promotes the Flysch to Molasse transi-tion within the pro-foredeep and would thus leadto a short-lived underfilled foreland basin system.This might explain the scarcity of early Flysch de-posits in the foredeep of the Himalayas, Taiwanand the Pyrenees, which were subject to intenseerosion on their respective pro-wedges (Covey,1986; Fitzgerald et al., 1999; Najman et al., 2004).

We further highlight the far-field connectionbetween retro-wedge erosion and the spatio-temporal evolution of the pro-foredeep. In a pre-vious section (6.2) we demonstrated that retro-wedge erosion influences thrust activity withinthe pro-wedge and thus seismicity. Consequently,retro-wedge erosion does also change the load dis-tribution of the pro-wedge and thus the flexureof the pro-lithosphere. Given that the magnitudeof strain transfer between the pro- and the retro-

8.3. Results and discussion 109

10

0

-10

0

-50

0

-10

00

-15

00

Episodes of thrusting

Tim

e

a)

50

100

Tim

e

Foreland basin Forebulge Backbulge

b)

50

100

-2500 -2000 -1500 -1000 -500 0

Incremental flexure [m]

Distance along strike of foreland [km]

50

100

050 100 150 200 250 300

Tim

e

c)

Figure 8.3: Spatio-temporal evolution of hypothetic foreland basins. See text for derivation. (a) Reference experiment; Dis-tributed erosion of: (b) Retro-wedge, (c) Pro-wedge.

wedge decreases while the former grows laterally,the above far-field influence might be only de-tectable during early stages of convergence.

Episodes of thrust activity result intransgression-regression cycles, where trans-gressions on the distal side correlate withregressions on the proximal or forebulge side(Fig. 8.3, Flemings and Jordan (1990)). Sucha scenario is supported by observations fromthe Karoo Basin, which formed in response tothe advance of the Cape Fold Belt (Catuneanuand Elango, 2001). The Late Permian to EarlyTriassic Balfour Formation, which was depositedduring the overfilled phase of the Karoo Basin,consists of six third-order depositional sequencesseparated by prominent subaerial unconformities.

In the absence of any evidence for a climatic oreustatic forcing, these six sequences are thoughtto result from thrust episodes within the CapeFold Belt (Catuneanu and Elango, 2001). Theaverage duration of each cycle was calculated tobe 0.66Ma.

Similarly, thrusting within the Swiss Alps mightexplain the stepwise nature of transgressionswithin the North Alpine Foreland basin between50Ma and 37Ma (Kempf and Pfiffner, 2004).This observation has been previously attributedto crustal-scaled inhomogeneities, which tend tofocus plate bending (Waschbusch and Royden,1992). Thus, further analysis of the kinematic evo-lution of the advancing Alpine orogen is requiredto address this issue.

110 8. Foreland basin evolution and the growth of an orogenic wedge

Migration of an orogen towards its forelandis associated with a coevally migrating flexuralwave. In submarine foreland basins forebulgesare preferred sites for carbonate platforms to de-velop (Crampton and Allen, 1995; DeCelles andGiles, 1996). Consequently, sites of active car-bonate deposition may first experience uplift anderosion, associated with karstification and are fi-nally drowned due to the submergence beneath thephotic zone (Galewsky, 1998). Carbonate plat-form drowning is controlled by the maximum up-ward growth potential of the platform-building or-ganisms and the rate of sea level rise relative to theplatform. The latter depends on the tectonicallyinduced subsidence and on eustatic sea-level vari-ations. It follows that the interplay between defor-mation and surface processes within the advanc-ing orogen controls the duration of forebulge up-lift and thus the degree of karstification (Cramptonand Allen, 1995) and finally the rate of drowning.In the North Alpine Foreland basin, karstificationbelow the drowning unconformity surface reachesup to 100m down into the limestones, resultingin an extensive interconnected system of macro-pores, which may finally form hydrocarbon reser-voirs (Crampton and Allen, 1995). Thus, the un-derstanding of the eroding orogen may finally helpto predict forbulge plays. Furthermore, forebulgeunconformities are preferred sites of MississippiValley Type deposits (Leach et al., 2001; Bradleyand Leach, 2003). Their formation might be in-duced by an eroding orogen, hundreds of kilome-ters away.