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752A.B. Weil et al. / Journal of Structural Geology 22 (2000) 735±756each group, which was then rotated about a verticalaxis until the small-circle contained the reference Bdirection. The Euler pole was then used to correct themagnetizations back to their reference orientation.After correcting all of the Proza in-situ bedding totheir pre-F 3 con®guration, a linear north±south anticlinewas restored (compare Fig. 12a and d).5. DiscussionThe magnetization history of the CAA as describedabove allows discrimination between regional fold generationsand determination of the timing and regionalkinematics of the main phases of Variscan deformationthat a€ected our study area in northern Spain (Fig. 15).5.1. Temporal and spatial constraintsUtilizing local and regional fold tests within structuraldomains, age determinations can be made forfolding events by comparing directions of characteristicmagnetizations to the late Paleozoic magnetic directionsfor the stable interiors of Iberia and Europe(Van der Voo, 1993). The ®rst generation of Variscandeformation, F 1 , is bracketed by the acquisition of theC magnetization that is Westphalian in age and coincideswith a change in sedimentation from shallowmarine to clastic (molasse-type) deposits associatedwith initial orogenic uplift. The second generation, F 2 ,is bracketed by the age of the C magnetization and theage of the B magnetization, and represents an unconformityand hiatus at the Westphalian±Stephanianboundary (Fig. 15). The associated clastic deposits ofStephanian age form wedges that are predominatelymade up of carbonate conglomerates, coal measures,and thin red-bed layers that unconformably overlieolder stratigraphic units (Julivert, 1971; PeÂ rez-EstauÂ net al., 1990; Martinez-Garcia, 1991). The deformationof the shallow marine and clastic units constrains therelative ages of the ®rst movements of di€erent thrustsheets, with a lower Westphalian age for the westernmostthrusts and an early Stephanian age for the easternmostthrusts (PeÂ rez-EstauÂ n et al., 1988). The F 3deformation phase is separated from earlier deformationby the acquisition of the B magnetization, andresulted in a radial fold set and tightening of the arcduring Sakmarian to Kungurian (Lower Permian)times. The F 3 phase ended before the Late Permian, asindicated by Permo-Triassic cover rocks that show nomajor rotation since their deposition (PareÂ s et al.,1996). Thus, the B and C magnetization componentsfound in the hinge zone of the CAA are constrainedby local and regional fold-tests as post-F 2 folding andpost-F 1 folding, respectively. This di€ers from the interpretationof PareÂ s et al. (1994), Stewart (1995), andVan der Voo et al. (1997), who did not distinguishbetween F 1 and F 2 folding and combined them intoone generation, and interpreted the acquisition of theC magnetization as synfolding during this single foldingevent.The geographical distribution of magnetization componentsbetween structural domains suggests regionaltectonic control on remagnetization. The B magnetizationis present throughout the hinge area in both theouter Somiedo-Correcilla thrust unit and inner LaSobia thrust unit, except for the La Queta domain,whereas the C magnetization seems to be restricted tothe outer Somiedo-Correcilla thrust unit only. Thispattern, to a ®rst approximation, correlates with thepaleotemperature maps of Raven and van der Pluijm(1986) and Bastida et al. (1999) based on the conodontalteration index, suggesting that orogenic ¯uids mayhave been responsible for the CAA's variable remagnetizationhistory. Because there is no signi®cant internalstrain observed in Devonian limestones of the CAA,rock±¯uid interaction during orogenesis was also proposedas the remagnetizing mechanism by Van derVoo et al. (1997). It has been well documented thatPaleozoic limestones throughout the Variscan andAlleghanian orogenic belts and foreland basins haveexperienced widespread remagnetizations (e.g. McCabeand Elmore, 1989; Thominski et al., 1993; Molina-Garza and Zijderveld, 1996).5.2. Nature of deformation phases in the CAAPrevious studies in the CAA documented an earlydeformation event that generated both thrusts andlongitudinal folds, followed by a second event thatFig. 15. The Variscan deformation history experienced by the CAAas represented by the three folding phases (F 1 , F 2 and F 3 ), sedimentationrecord, and the two recorded ancient remagnetizations (B andC components). Age in millions of years is plotted on the horizontalaxes for the late Carboniferous and Permian. Deformation phasesare bracketed by the age of magnetization and sedimentation as statedin text. Sedimentation record is bracketed by unconformities (zigzaglines) and hiatuses (lighter gray). The times of acquisition of thetwo magnetizations found in the CAA (darker gray blocks) areknown by the comparison of observed mean inclinations with thosepublished for stable Europe.
A.B. Weil et al. / Journal of Structural Geology 22 (2000) 735±756 753tightened the region into the arcuate structure weobserve today (e.g. Julivert and Marcos, 1973; Julivertand Arboleya, 1984, 1986). PeÂ rez-EstauÂ n et al. (1988)argued that the earlier propagation of thrusts andfolds was from west to east, away from the hinterland,as typically observed in fold-and-thrust belts. However,they suggested that the ®nal distribution ofthrusting had the spatial properties of a photographiciris, each thrust stacked (and carried inward) in an arcuatearrangement atop the next. Similarly, Julivert andArboleya (1984) argued for a mechanism in which theindividual thrusts had positioned themselves, by a rotationalmotion, towards the center of the arc duringearly deformation (F 1 and F 2 in this paper). Importantly,both of these models involve considerable curvatureof the CAA prior to F 3 folding. Others haveargued that the CAA originated as a linear belt, thatlater experienced substantial counterclockwise rotationof a single limb during late folding (Ries and Shackleton,1976; Ries et al., 1980; Bonhommet et al., 1981).Our work based on paleomagnetism shows that F 3deformation resulted in arc tightening, refolding, reactivationof major north±south-trending thrusts, andfolding of syntectonic Stephanian sediments. Moreover,the need for additional vertical-axis rotation tocorrect for the C magnetization relative to the B magnetizationin those structures carrying both magnetizations,mandates a distinction within the previouslyde®ned early deformation phase into two fold generations:F 1 and F 2 . This sequence for the initial deformationphase has not been documented in previousstudies. In our model F 3 remains geometrically a`radial' fold set (Julivert and Marcos, 1973), but it ischaracterized by variably plunging fold-axes, from(sub)horizontal to (sub)vertical, that are imposed on apre-existing (F 1 and F 2 ) structural fabric. Thus, F 1 andF 2 structures control the orientation of subsequent F 3fold axes, which restricts F 3 axes to distinct orientationswithin a given structural domain. Speci®cally,F 3 fold axes will vary in orientation based on their locationin early folds (F 1 and F 2 ), but collectively de®nean axial surface that re¯ects the regional shorteningdirection. This contrasts with Stewart (1995), who postulatedthat the kinematics of F 3 folding was only controlledby the westward-dipping sedimentary layerswithin the initial thrust stacks and ignored the largescalefolding superimposed on these thrust sheets.We conclude that the present-day curved geometryof the CAA's hinge zone is established by the interferenceof original north±south-trending structures,Fig. 16. (a) Schematic structure map of the four structural domains studied in their present-day con®guration, including Van der Voo et al.'s(1997) Lagos del Valle Syncline. Highlighted are the refolded F 1 and F 2 axial traces (heavy lines) and F 3 fold axes (dashed lines) calculated inthis study. Notice that the F 3 axes are near orthogonal in all cases. (b) Stereonet projection of correction fold axes, as described in text and aslisted in Table 2. Black diamonds represent those axes that correct for both F 2 and F 3 deformation, and closed circles represent those axes thatcorrect for only F 3 deformation. (c) Schematic map of pre-F 3 CAA con®guration based on bedding orientation calculations from this study. (d)Stereonet projection of pre-F 3 fold axes restored after correction for F 3 deformation. Notice the shallow north±south trend of axes and resultantsteep axial surface.
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