740A.B. Weil et al. / Journal <strong>of</strong> Structural Geology 22 (2000) 735±756<strong>an</strong>d thickening (Bonhommet et al., 1981). <strong>The</strong> schematicgeologic map in Fig. 4 shows the eastern limb <strong>of</strong>the Proza <strong>an</strong>ticline truncated <strong>by</strong> the western limb in responseto <strong>an</strong> eastward propagating thrust. <strong>The</strong> <strong>an</strong>ticlineitself has a small interlimb <strong>an</strong>gle <strong>of</strong> 30±458, <strong>an</strong>d ahinge line that is oblique to the direction <strong>of</strong> thrustmovement (Julivert <strong>an</strong>d Arboleya, 1984). Julivert <strong>an</strong>dArboleya (1984) proposed that the Proza Anticline isthe result <strong>of</strong> thrust emplacement <strong>by</strong> rotational motioncentered at the northern fault terminus. Such a kinematicscenario would require that the thrust sheet beprogressively curved as it was emplaced, rather th<strong>an</strong>moving as a rigid plate. Thus, according to this model,the tightening <strong>of</strong> the structure <strong>an</strong>d its curvature wouldbe contempor<strong>an</strong>eous with thrust emplacement <strong>an</strong>d notthe result <strong>of</strong> multiple de<strong>formation</strong> events. This viewc<strong>an</strong> be tested with detailed paleomagnetic data.3. Sampling <strong>an</strong>d experimental methodsApproximately 500 cores were collected in the summers<strong>of</strong> 1996 <strong>an</strong>d 1997 at 22 sites in the Portilla Formation<strong>an</strong>d 38 sites in the S<strong>an</strong>ta Lucia Formation. Sitelocations were distributed throughout all three structuraldomains <strong>an</strong>d severe outcrop weathering <strong>an</strong>dhighly faulted areas were avoided where possible.Oriented samples were collected in the ®eld using aportable drill <strong>an</strong>d a magnetic compass. Bedding directions<strong>an</strong>d shear-sense indicators were measured in the®eld using a magnetic compass.On average, from seven to ten st<strong>an</strong>dard 2.54-cm-diameterpaleomagnetic specimens from each site wereprogressively demagnetized in <strong>an</strong> Analytical ServiceCo. (ASC) thermal demagnetizer <strong>an</strong>d measured in athree-axes cryogenic 2G magnetometer in the ®eld-freeroom at the University <strong>of</strong> Michig<strong>an</strong>'s paleomagneticlaboratory. Heating was carried out in 508C incrementsto a maximum temperature <strong>of</strong> 5508C, withreduced increments at temperatures above 3508C.Alternating Field (AF) demagnetization was performedwith a Sapphire Instruments SI-4 AF demagnetizer;however, the AF treatment was unable to isolate theprincipal components <strong>of</strong> the rem<strong>an</strong>ence properly. Inthe thermally treated samples, rem<strong>an</strong>ence directionswere calculated from Principal Component Analysis(Kirschvink, 1980) <strong>of</strong> linear demagnetization vectorspicked from paleomagnetic orthogonal projection plots(Zijderveld, 1967). In those cases in which a site hadmore th<strong>an</strong> two magnetization components, the Ho€m<strong>an</strong><strong>an</strong>d Day (1978) great-circle method was used toisolate the rem<strong>an</strong>ence directions.Site me<strong>an</strong>s were calculated <strong>by</strong> averaging the sampleset directions, using the method <strong>of</strong> Fisher (1953). Inclination-only(Pare s et al., 1994) <strong>an</strong>d local fold tests(McElhinny, 1964) were used to characterize the age <strong>of</strong>the magnetization components with respect to foldingin the individual domains. However, because <strong>of</strong> thelocal declination scatter caused <strong>by</strong> (sub)vertical axis rotations,the inclination-only test was predomin<strong>an</strong>tlyused. With this test it is possible to account for thestatistically signi®c<strong>an</strong>t clustering <strong>of</strong> inclinations afterpartial or full tectonic correction, independent <strong>of</strong> thedeclination scatter produced <strong>by</strong> vertical axis rotation(McFadden <strong>an</strong>d Reid, 1982).4. Results<strong>The</strong> S<strong>an</strong>ta Lucia <strong>an</strong>d Portilla limestones have similardemagnetization characteristics <strong>an</strong>d the two <strong>formation</strong>swill be treated together in the following discussion.<strong>The</strong> results from demagnetization for all three structuraldomains are presented in Table 1. Two magnetizationsare distinguished, in addition to the presentday®eld overprint: <strong>an</strong> early Permi<strong>an</strong> (B) component,<strong>an</strong>d a Late Carboniferous (C) component. This ageassignment is the same as that used <strong>by</strong> V<strong>an</strong> der Voo etal. (1997). <strong>The</strong> present-day ®eld A magnetization,which is northerly <strong>an</strong>d down, has a low unblockingtemperature <strong>an</strong>d is removed <strong>by</strong> 2508C. This is followed<strong>by</strong> the intermediate B magnetization, which has ame<strong>an</strong> inclination <strong>of</strong> +1.5823.18 (as calculated <strong>by</strong> V<strong>an</strong>der Voo et al., 1997), <strong>an</strong>d <strong>an</strong> unblocking temperatureup to 4808C. <strong>The</strong> C magnetization has a me<strong>an</strong> inclination<strong>of</strong> +5.0826.68 (as calculated <strong>by</strong> V<strong>an</strong> der Vooet al., 1997), <strong>an</strong>d unblocks at temperatures up to 508C.4.1. Tectonic correctionsDue to the complexity <strong>of</strong> superimposed-folding inthe area, a method was devised to determine the optimaltectonic correction for individual site me<strong>an</strong> directions.Each sampling site was evaluated in the context<strong>of</strong> the local structures to determine the best possiblecorrection to undo post-magnetization tilts <strong>an</strong>d rotations.Within individual structural domains, de<strong>formation</strong>axes were determined <strong>by</strong> calculating the best-®trotation axes to cluster in-situ magnetic vectors. Thisis similar to the approach used <strong>by</strong> Setiabudidaya et al.(1994) <strong>an</strong>d Stewart (1995) to reconstruct complex foldgeometries in comparable structures. <strong>The</strong> observed girdledistributions represent the di€erential rotation <strong>of</strong>F 3 fold limbs in the case <strong>of</strong> the B component, <strong>an</strong>d therotations produced <strong>by</strong> F 2 <strong>an</strong>d F 3 de<strong>formation</strong> for theC component subsequent to magnetization acquisition.By de®nition, the distributions <strong>of</strong> in-situ magnetic vectorsproduced <strong>by</strong> folding will track small-circles thatcontain the original reference magnetic direction (i.e.the me<strong>an</strong> B or C magnetic direction acquired prior t<strong>of</strong>olding). Coincidentally, in most structures studied itwas found that the reference direction was positioned
A.B. Weil et al. / Journal <strong>of</strong> Structural Geology 22 (2000) 735±756 741Table 1Paleomagnetic <strong>an</strong>d structural site in<strong>formation</strong> <strong>an</strong>d statistical parametersSite No. Cores Tectonic Corrections In Situ Site Me<strong>an</strong>Strike Inv. Dip Dip Dir. Dip Dec(Low-Temp)Inc(Low-Temp)alpha 95 K Dec(High-Temp)Inc(High-Temp)alpha 95KLa Queta SynclinePL26 8 318 48 48 142 11 7 108PL27 a 8 170 99 80 81 134 4 6 110PL28 8 145 235 84 123 2 4 284PL29 8 97 187 52 102 43 3 938PL30 a 8 170 95 80 85 111 27 5 170PL31 a 8 137 92 47 88 88 17 10 44PL32 8 300 30 58 140 5 3 437PL33 8 30 120 52 73 53 4 326PL34 8 335 65 55 88 59 5 177PL35 8 275 5 35 141 20 6 187PL36 8 310 40 40 134 1 6 75PL37 8 110 200 38 110 38 6 48PL38 8 191 281 78 143 8 9 48La Vega de los Viejos SynclineSL56 b 6 (108 198 58) (88 22) 9 60 (46 3) 9 61SL57 b 6 (80 170 85) (44 34) 16 19SL58 d 7 112 202 62 125 14 11 27 77 20 9 67SL59 b 7 (166 256 60) (66 42) 10 38SL60 ad 7 (243 135 153 45) (321 52) 14 15 (357 67) 8 66SL61 5 135 225 68 80 41 5 117SL62 6 143 53 90 103 34 3 487SL64 d 8 114 204 50 110 14 9 42 93 13 7 66SL65 8 120 210 84 122 13 9 37SL66 8 164 254 85 126 15 4 167SL67 6 344 74 80 143 2 6 66SL68 6 352 82 80 117 32 13 54SL69 d 9 355 85 77 125 11 6 76 97 20 6 71SL70 6 355 85 70 119 4 5 273SL71 8 354 84 74 140 5 12 21PL49 7 238 328 69 117 47 5 125PL50 5 120 210 51 82 32 10 64PL51 d 8 145 235 78 132 24 9 32 93 27 6 86PL52 ad 8 146 94 56 86 145 39 14 20 114 42 10 33PL53 6 105 195 64 106 6 13 34PL54 a 8 152 99 62 81 129 34 6 70PL55 d 6 125 215 60 120 2 3 626 102 4 8 80PL56 d 5 114 204 41 112 10 5 154 104 14 5 130PL57 8 21 111 55 108 18 10 40Proza AnticlineSL38 8 250 340 54 190 40 6 114SL39 a 8 142 105 52 75 123 16 4 281SL40 c 9 20 110 74 202 12 11SL41w 4 200 290 70 147 3 11 35SL41e 5 360 90 76 129 6 11 29SL42 b 8 358 88 80 (194 41) 6 128 (180 63) 8 48SL43 8 45 135 78 215 24 7 73SL44 9 155 245 39 136 12 5 182SL45 7 238 328 45 175 12 7 71SL46 8 148 238 47 125 15 8 53SL47 8 205 295 47 174 37 8 70SL48 8 216 306 67 168 11 3 704SL72 c 9 218 308 63 186 22 8SL73 6 214 304 62 187 5 8 80SL74 6 221 311 48 177 8 4 246SL75 ab 7 353 102 263 78 (193 27) 6 75SL76 8 45 135 63 225 59 5 294(continued on next page)
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