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Growth of the South Pyrenean Orogenic Wedge 241 TECTONICS, VOL. 16, NO. 2, PAGES 239-258, APRIL 1997 Growthof the South Pyrenean orogenicwedge AndrewJ. Meigs DivisionofGeological and Planetary Sciences, Calif•a Instituteof Technology,Pasadena DouglasW. Burbank DepartmentofEarth Sciences, University.of Southern California, Los Angeles Abstract. A six-stepreconstruction of the South Pyrenean DeCelles et al., 1993; DeCettes, 1994; DeCe!!es and Mitra, forelandfold-and-thrust belt in Spaindelineates the topographic 1995]. Othershave argued, in contrast,that critical-taper models slope,basal dtcollement angie, internal deformation, and thrust- have limited applicabilityto fold-and-thrustbelts becauseof front advance from the Early Eocene until the end of mechanicaland deformationalincompatibilities between the contractionaldeformation in the Late Oligocene. Style of thrust- geologicrecord and the model [Woodward, 1987; Price, 1988; frontadvance, dip of the basaldtcollement, slope of the upper Meigsand Burbank, 1993; Bombolakis, !994]. surface,and internal deformationare decoupledand not simply This paper is a retreat to geologic data from the South related. Internal deformation increased, decreased, and Pyreneanforeland fold-and-thrust belt in Spain. A uniquelyhigh- maintainedsurface slope angle at differentstages. From the onset resolutionrecord of deformation and topography,contained to the cessationof deformation, the basal dtcollement angle within sedimentsdeposited during thrusting, enablesdetailed decreasedoverall suggestingtranslation of the thrust belt onto reconstructionof the structuraland topographicdevelopment of strongercrust with time. Taper angle of the Pyreneanthrust thisorogen. An increasinglypopular approach is to interpretthe wedgewas fundamentally controlled by the flexuralrigidity of deformationaland depositional histories of thrustbelts in termsof thelower plate, the relative rate of creationof structuralrelief in critical-tapermaintenance [DeCelles and Mitra, 1995]. Our therear versus the front of the wedge,the extentof depositionof approach,in contrast,is to determine(1) how taper angle varies erodedmaterial within the deformingwedge, and the taperof the through time and (2) how that variation relates to spatial and pretectonicstratigraphic wedge. temporalpatterns of deformation,erosion, and deposition. We present a six-part sequential restoration that recovers the predeformationalgeometry of the thrust belt from its present, Introduction postdeformationalgeometry. Each sequentialrestoration is a time slice constrainedby abundant surface and subsurface One of the most significant, and controversial,theories for geologicdata and absoluteage data from syntectonicsediments. f01d-and-thrustbelt developmentduring the past 20 yearsis the The surface slope and basal d6collementvaried independently critical-taperwedge model [Chappie, 1978; Davis etal., 1983; andmused continuous taper-angle variation with time. Stochnal,1983; Davis and Engelder, 1985; Platt, 1986;Dahlen andSuppe, 1988; Jaumd and LilIie, 1988;Dahlen, !990; CoIletta GeologicalPredictions of the Critical-Taper eta!., 199!; Davis and Lillie, 1994]. Mechanicalproperties of Wedge Model rockswithin the thrustwedge, spatial and temporalpatterns of deformation,and topographic evolution are coupledaccording to In orderthat evolutionof the SpanishPyrenees be compared themodel. Thrust-belt growth revolves around maintenance of a with the critical-taperwedge model, explicit definition of the criticaltaper angle between the basaldtcollement and the surface model and of the geometric-kinematicpredictive framework it slope. Feedbackbetween internal deformation, which acts to provides is required. Six variables define the boundary buildtaper, and frontal accretion, tectonic thinning, and erosion, conditionsof the model. Two definethe taperangle itself: (1) the w!•ichact to decreasetaper, dictates the kinematichistory of a dip of basal dtcollement and (2) the surface slope; three criticallytapered fold-and-thrust belt. Only rarelyis datafrom determine the critical-taper value for a specific belt: (!) the thegeologic record of sufficientresolution to permitsimultaneous strengthof material within the wedge, (2) the strengthof the calibrationof material properties, deformational patterns, patterns material within which the basaldtcollement is localized, and (3) ofsyntectonic erosion and deposition, and topography [DeCelles the pore fluid pressmethroughout the wedge;the final variable, andMitra, 1995]. As a consequence,the modelis generally the backstop,provides the pushon and geometryof therear of the thoughtto be applicablebased on qualitativecompatibility wedge[Chapple, 1978; Davis etal., !983; Stockreal,1983; Dav/s betweenspatial and temporalpatterns of thrustingin some and Engelder, 1985; Platt, 1986; Byrne and Hibbard, 1987; 0rogensand the distributionof deformationpredicted by critical- Mulugetaand Koyi, 1987;Datden and Suppe,1988; Jaumd and tapermodels [Boyer and Geiser,!987; Geiserand Boyer,1987; LiIlie, 1988;Mulugeta, 1988; Dahlen, 1990;Colletta etal., 1991; Morley,1988; Lucas, 1989; Boyer, 1992; Burbank eta/., 1992b; Byrne eta/., 1993; Davis and Lillie, 1994]. If thrust-belt developmentfollows critical-taper wedge theory,a critical angle Copyright1997 by the American Geophysical Union. betweenthe topographicslope and the basaldtcollement must be maintained[Davis etal., 1983; Stockmai• 1983; Davis and Papernumber 96TC03641. Engelder,1985; Platt, 1986;Dahlen and Suppe, 1988; Jaumd and 0278-7407/97/96TC-03641$12.00 Litlie, 1988; Dahlen, 1990; Davis and Lillie, 1994]. Patternsof 239 240 MEIGS AND BURBANK: GROWTH OF THE SOUTH PYRENEAN OROGENICWEDGE A. Stables•ng rrater•l to be no/nrernal (2) •ccret•to toe ..... •. -.. •?.'-.':.•-?...-........ -....... ,?• (3) i•tema•restoreuer•r•t/on taper to ...... .?-•-,.,.-,..'.,...,. -..-.. ... - . ...... •... Figure 1. Schematicdiagram contrasting modes of thrust-frontadvance. (a) A stable-slidingadvance (position 2) from an initial position(position 1) occursconcurrently with aggradationof materialin frontof advancingwedge (open dot pattern). In the absenceof erosion,accretion, or extension,the wedge maintains its taper. Co)Thrust-front advance by accretion(position 3) occursafter aggradationof materialin front (opendot pattern)and on top (vertical line pattern)of wedge(,position 2). Additionof materialto frontof wedgedecreases taper angle. internal deformation are modulated by changesin the surface to thecritical angle. Alternatively, if thethrust front advances by slope and in material propertieswithin and at the base of the stablesliding between t 1 and t2 (Figure 2b), the wedgeremains at wedge. Stresstransmission to and accretionof material at the toe critical taper, internal deformation ceases,and the thrust front occursonly when the wedgeis critically tapered. advancesgradually toward the foreland. Although theseend- Subcritical,critical, and supercriticalare the possiblewedge- members imply a simple relationship between internal taper states given by the model [Davis et al., 1983]. A deformation and thrust-front advance, they do not accountfor subcriticallytapered wedge has a taper angle below that required processessuch as erosion,isostatic depression from loading,or for slip transmissionon the basewhereas a supercriticallytapered tectonicthinning of the hinterlandthat alsoimpact taper with time wedge'staper angle is greaterthan the critical value [Davis et al., [Davis eta/., 1983; Platt, 1986; Woodward, 1987; DeCelles and 1983; Woodward,1987; Dahlen, 1990;Dahlen and Suppe,1988; Mitra, 1995]. DeCellesand Mitra, 1995]. Once a wedge has attainedcritical A particularset of geologicobservations are required,based on taper, two contrasting modes of thrust-front advance, that is, this framework,to comparea foreland fold-and-thrustbelt with repositioningof the deformation front toward the foreland, are thecritical-taper wedge model. Explicitly,the necessary geologic implied by themodel [Davis et al., 1983] (Figure 1). Translation data that must be determinedsimultaneously throughout an of a critically tapered wedge along its base without additionor orogenyare (1) the dip of basald6collement; (2) the dip of the removal of material to either its toe or baserequires no internal topographicsurface; (3) the spatial and temporaldistribution of deformation because the taper angle is maintained (herein deformation;(4) the contrastingstrength of materialswithin and referred to as stable sliding; Figure la). Alternatively, a at the baseof the wedge (includingdown dip changesin the critically tapered wedge whose thrust-front advances by the strengthof thebasal d6collemen0; and (5) thepore fluid pressure. accretion of material to its toe must experience concurrent Whereas the first three variables can be constrained with internal deformationin order to maintain the critical taper angle confidenc6in thePyrenees and the comparative strengths of rocks (hereinreferred to as accretion,Figure lb). withinthe wedge and at its basecan be qualitativelyassessed, the If periods of thrust-front advance by accretion can be pore fluid conditionscan not be estimated. Below we document differentiatedfrom periodsof stablesliding, a contemporaneous the changes(or lack thereof)in thesevariables between six well- and appropriate internal deformational responseis expected constrainedtemporal and geometricalreconstructions of the within the wedge. During the courseof a hypotheticalorogeny, SpanishPyrenean foreland fold-and-thrust belt. the couplingbetween thrust front advance,internal deformation,
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