Analysis of Variscan dynamics; early bending of theCantabria^Asturias Arc, northern Spain
J.M. Kollmeier, B.A. van der Pluijm, R. Van der Voo *Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA
Received 28 January 2000; accepted 6 June 2000
Abstract
Calcite twinning analysis in the Cantabria^Asturias Arc (CAA) of northern Spain provides a basis for evaluatingconditions of Variscan stress and constrains the arc's structural evolution. Twinning typically occurs during earliestlayer-parallel shortening, offering the ability to define early conditions of regional stress. Results from the Somiedo^Correcilla region are of two kinds: early maximum compressive stress oriented layer-parallel and at high angles tobedding strike (D1c1) and later twin producing compression oriented sub-parallel to strike (D2c1). When all D1compressions are rotated into a uniform east^west reference orientation, a quite linear, north^south trending fold^thrust belt results showing a slight deflection of the southern zone to the south^southeast. North^south-directed D2c1
compression was recorded prior to bending of the belt. Calcite twinning data elucidate earliest structural conditions thatcould not be obtained by other means, whereas the kinematics of arc tightening during D2 is constrained bypaleomagnetism. A large and perhaps protracted D2c1 is suggested by our results, as manifested by approximately 50%arc tightening prior to acquisition of paleomagnetic remagnetizations throughout the CAA. Early east^westcompression (D1c1) likely resulted from the Ebro^Aquitaine massif docking to Laurussia whereas the north-directedcollision of Africa (D2c1) produced clockwise bending in the northern zone, radial folding in the hinge, and rotation ofthrusts in the southern zone. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: Hercynian orogeny; calcite; twinning; oroclines; stress
1. Introduction
The highly curved Cantabria^Asturias Arc(CAA) of northern Spain forms the core of thegreater Ibero^Armorican Arc as it traces westernVariscan zonations through France (Brittany)into northern Spain (Fig. 1). The Somiedo struc-
tural unit, also known as the fold nappe belt [1],marks the ¢rst occurrence of unmetamorphosedPaleozoic sedimentary units east of the basementcored Narcea Antiform [2]. Deformation contin-ued eastward (toward the foreland) producing theSobia unit, the Central Coal Basin (C.C.B.) [3],Ponga Unit [4], and the more easterly Picos deEuropa (P.D.E.) and Pisuerga^Carrion province(P.C.P). These individual structural units vary incharacter commensurate with variations in lithol-ogy and changing deformation conditions. Basedon deformation style and structural orientation,
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 0 3 - X
* Corresponding author. Tel. : +1-734-764-8322;Fax: +1-734-763-4690; E-mail: [email protected]
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we have divided the sampled Somiedo into threezones: a northern zone (NZ), hinge zone (HZ),and southern zone (SZ).
The kinematics of arc formation, particularlyarcuate foreland fold^thrust belts, has gained con-siderable interest since the advent of the platetectonics [5^11]. Unraveling arc development re-quires contributions from as many analytical ap-proaches as possible. However, despite extensivework in the CAA (structural, stratigraphic, pale-omagnetic, seismic, and paleotemperature analy-ses), critical questions persist concerning the ear-liest conditions of foreland deformation, whichcorrespond on a larger scale to the overall char-acter and coeval tectonism in the convergencezone between Laurussia and Gondwana, whichproduced the Ibero^Armorican Arc.
Tectonostratigraphic evidence indicates that in-itial deformation was foreland propagating, be-ginning with the rear-most Somiedo structuralunit in Westphalian B times and a¡ecting theeastern-most structural units in the upper Stepha-nian ([12], and references therein). Spatially, awest to east geometric gradation can be seen
with arcuate structural trends, averaging to anorth^south trend, that gives way to ultimatelyeast^west trending structures in the P.D.E. unit.This change in arc character has been interpretedwith two end-member models: (1) an originallylinear fold^thrust belt that was later bent intoits present geometry (e.g. [13]), and (2) a fold^thrust belt that evolved into today's structureunder a continuously changing stress ¢eld (e.g.[12]). Other interpretations have suggested an ini-tial arc that is partially curved and later tightened[14^16] based on unfolding the radial fold set (re-sponsible for an aerial reduction of 35^40%), dis-placement directions, and agreement with earlier,somewhat erroneous paleomagnetic conclusions.Paleomagnetic studies have generally concludedthat (at least) half of the present curvature is sec-ondary [17^21], rendering this an `orocline' as de-¢ned by Carey [22]. Pares et al. [20] reevaluatedpublished Paleozoic paleomagnetic data by apply-ing inclination-only tilt tests and concluded thatall remagnetizations are syn-tilting to post-tiltingin age, thereby changing the earlier paleomagneticconclusions to some degree. Results from Devon-
Fig. 1. Structural sketch of the Cantabrian Arc showing the traces of principal thrusts and the traces of axial fold planes that re-sulted from longitudinal F1 (closed) and radial F2 (open) fold sets. The Somiedo structural unit has been divided into threezones (NZ, HZ, and SZ), based on variation in structural style and orientation.
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ian limestones speci¢cally, reveal that magnetiza-tions were recorded during and after early tiltingand were a¡ected by late vertical-axis rotations[20,23]. Thus the more recent paleomagnetic ¢nd-ings indicate that the horseshoe-shaped CAA re-sulted from late tightening of a partially curved(V50%) fold^thrust belt, leaving the amount ofstructural rotations prior to these remagnetiza-tions unknown.
Calcite twinning analysis can o¡er an e¡ectivemethod for quantifying rotations in curved oro-genic belts (e.g. [7,8]) complementary to paleo-magnetic analysis. Previous ¢eld applications ofcalcite twinning analyses in fold^thrust belts in-clude the Alps [7,8,24,25], the Idaho^Wyomingsalient [26] and the Appalachians [27^30]. Exper-imentally deformed limestones show that individ-ual twins record the orientation of the local stateof stress as well as the orientation and magnitudeof the induced intragranular strain. It is consid-ered that stress recorded over the whole aggregateof a sample is representative of the regional stressstate. Calcite twinning requires a low critical re-solved shear stress (V10 MPa) [31,32] and is astrain hardening process where further twinning isresisted as beds tilt during folding [29,30,33,34].Therefore, it typically preserves early horizontalcompression prior to folding as layer-parallelshortening [27,35]. In these instances, a regionalpaleostress direction can be statistically deter-mined for a sample from the array of paleostressdirections of an aggregate of twinned calcitegrains.
The primary objective of this paper is to deter-mine the original geometry of the CAA that bestsatis¢es paleostress data from deformed lime-stones, in addition to other constraints (paleo-magnetic and regional structural data). Themicrostructural analysis presented here independ-ently evaluates the kinematics of whole arcformation and provides information on thestate of regional stress during Variscan deforma-tion.
2. Geologic background and sampling
The CAA forms the eastern margin of the west-
ern Variscan orogen and is characterized by fore-land-directed concavity. Two fold sets have beenidenti¢ed: (1) early longitudinal folds (tangentialto the arc) overprinted by (2) a cross-cutting ra-dial fold set, forming regional (Type I) interfer-ence structures [36]. Tracing the longitudinalstructure of the Somiedo unit from north tosouth, ¢rst-order folds give way to an increasingnumber of thrusts (Fig. 1). The SZ is not onlyunique in structural style, but also shows amarked change in orientation to an overall east^west trend (with respect to the HZ; Fig. 1).
Paleozoic deposition through Carboniferoustimes resulted in a stable shelf sequence, generallythicker in the west (present-day coordinates) andtapering to the east. Although minor detachmentzones occur in several structurally weak unitsthroughout the lower Paleozoic sequence, the pri-mary basal detachment soles below the CambrianLancara Formation [1]. Two well-exposed Devon-ian limestone units, the Santa Lucia and Portillaformations, were sampled to determine paleo-stress/strain as recorded by the mechanical twin-ning of calcite. These units represent conditions ofa reef-rimmed carbonate platform allowing forthe deposition of bioclastic grainstones and pack-stones with limey mudstones present [37].
Paleotemperature estimates based on conodontalteration index (CAI) values indicate maximumtemperatures of 150^250³C in the Somiedo unit[38,39]. Values associated speci¢cally to the unitssampled in this paper, the Santa Lucia and Por-tilla formations, revealed temperatures in therange of 120^155³C and 70^95³C respectively[38]. This agrees with the observed twinning char-acter of our samples (Fig. 2), which suggests tem-peratures of 9 200³C [40^42].
A concern in deciphering regional paleostressconditions is to avoid e¡ects that may be associ-ated with localized deformation. Thus, weavoided sampling near major faults, areas ofhigh shear and fold hinges, following the recom-mendations of Friedman and Stearns [43]. In par-ticular we sampled the thickest available coarse-grained beds using a portable gasoline-poweredcoring drill. Rock cores were oriented using aBrunton compass mounted on an aluminumcore orienting device.
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3. Calcite twinning analysis
3.1. Data collection
Optical measurements de¢ne the crystallo-graphic orientation of the twinned calcite grain(Fig. 3) as well as the twin set(s) measured. Twin-ning occurs most readily on e-twin planes in anyof three orientations (twin sets) per given calcitecrystal. All samples that were analyzed containedcalcite grains that were not preferentially orien-tated crystallographically, thus ensuring that theresults are not a consequence of an intrinsic fabricin the sample. Measured samples range from bio-clastic grainstone to packstone containing crin-oids, corals, bryozoa, and bioturbation observedin an otherwise crystalline calcite matrix. Calcitegrains were observed with one, two or all threetwin sets present, but most commonly contain twosets. For each grain, only twin sets that arestraight and continuous within grains were mea-sured to obtain the most accurate results. Whennecessary, this resulted in measurement of onlyone (acceptable) twin set per given calcite grain.
3.2. Analytical techniques
The use of twinned calcite as a paleostress/strain indicator in both experimentally and natu-rally deformed limestones has been shown to yieldrobust data [7,8,29,34,43^45]. Several methods areavailable to quantify paleostress and paleostrainwith calcite twinning analysis [41,46]. To obtaincompression and tension axes for each sample, weuse the dynamic analysis of Turner [47]. For eachtwinned calcite grain, compression and tensionaxes (C and T, Fig. 3a) are situated 71.5 and18.5³ to the c-axis respectively, and at 45³ to thee-twin plane (Fig. 3a,b). Calcite crystals havethree potential e-twin planes (Fig. 3c), where twin-ning glide will occur preferentially along oneplane depending on the orientation of the remotestress. Initial data were cleaned (removing all re-sidual values (RV), see below) as suggested byGroshong et al. [48], such that populations of 25or more twin sets remain. These are statisticallysigni¢cant to characterize the true twinning strainfor a whole sample [48]. We have chosen the samenumber of twin sets as a minimum number for
Fig. 2. A representative photomicrograph showing twinned calcite grains characterized by multiple (usually two) twin sets andthin twins indicating low temperature conditions.
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computing principal stress axes as well. The re-sulting compression and tension axes for a sampleare calculated as point means assuming a Fish-erian distribution. The data are listed in Table 1.
We have applied Groshong's [49] technique todetermine distributions of strain within each sam-ple using the calcite strain gauge program ofEvans and Groshong [50]. Shear strain is deter-mined for individually measured twin sets, fromwhich a complete strain tensor for the whole sam-ple is statistically calculated and assigned by aleast squares solution (multiple linear regression)(see [49] for a full description). If limestone ishomogeneously deformed, twin sets should yielda single (positive) shear strain, which is termed the`expected value' (equivalent to PEV in [49]). How-ever, when a signi¢cant number of twinned grainsis unfavorably oriented, the sample may have ex-perienced multiple homogeneous or inhomogene-ous strain. This results in some proportion of twinsets with opposing shear sense, referred to here as`residual values' (RV). Thus, each grain records
an expected value (EV) (shear strain) if favorablyoriented, or an RV if not [49,50]. Assigning EVsand RVs depend on a twin set's orientation rela-tive to the statistically determined direction ofshear assigned for the bulk of the aggregate [49].
Experimental work of Friedman and Stearns[43] and Teufel [34] have shown that two discretedeformations may be identi¢ed using the dynamicanalysis technique of Turner [47] and the straingauge technique of Groshong [49]. Friedman andStearns [43] concluded that superposed states ofstress are distinguishable if compression axes ofthe dominant twin sets (EVs) are plotted sepa-rately from subordinate twin sets (RVs). Experi-ments by Teufel [34] indicate that larger errors areobserved both in strain magnitude and orienta-tion for samples recording two deformations.Also, a second deformation is most accuratelydetermined when oriented at a high angle to the¢rst ; however obliquely superposed deformationscan be clearly determined as well.
In presenting our data, a number of qualityconsiderations have been included in Table 1.The ¢rst indicator of reliability is a low percent-age of RV from the calcite strain gauge analysis,which re£ects a largely homogeneous positiveshear strain recorded by the sample. When a sam-ple contains a high percentage of RVs, it is splitinto two distinct populations of twin sets (onecomposed entirely of EVs and the other ofRVs), and each are analyzed as separate deforma-tions. If the separation yields low percentages(6 15% is used as a threshold following [34]) ofnewly calculated RVs, then both deformations arelikely to be robust. Additionally, because the ex-pected shear strain for the aggregate is calculatedby a multiple linear regression of the data, thestandard error is an additional measure of dataquality [34]. Lastly, because whole-sample princi-pal stress axes are calculated using a Fisherianstatistical point mean of individual twin set stressaxes, an K95 value is included (Table 1).
4. Results
Of the 33 sites measured from along the Somie-do unit, 26 record D1c1 paleostress orientations.
Fig. 3. (a) A calcite grain with a single e-twin and the com-pressive and tensile stress (C and T) oriented most favorablyto produce twinning (oriented 45³ to the e-twin plane). Thegeometric relation of e-twins to c-axis is ¢xed. (b) The samesituation as in a but now illustrated with an equal area pro-jection (lower hemisphere) to show the orientations of stressas related to the crystallography. (c) All three possible e-twinplanes and their poles are represented.
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Tab
le1
Cal
cite
twin
ning
resu
lts
for
all
mea
sure
dsa
mpl
es
Sam
ple
Raw
data
Cle
aned
data
Stra
inor
ient
atio
ns(b
rg.,p
lng.
)/E
long
atio
nSt
ress
Ori
enta
tion
s(b
rg.,p
lng.
)B
eddi
ng
N/R
S.E
.M.
N/R
S.E
.M.
e 1e 2
e 3c
3c
1K
95St
rike
and
dip
S3
S0
c1
c3c
0
(ori
gina
l)(t
reat
ed)
(T)
(C)
O:o
vert
urne
dD
1;D
2A
TC
D1
;D2
P3E
49/1
0.37
248
/00.
388
(020
,06)
/2.1
7(1
15,4
2)/3
0.50
(284
,47)
/31.
6731
3,52
204,
159.
618
0,75
w9
202,
318
26P
3R26
/10.
585
25/0
0.50
0(2
68,1
9)/1
.76
(167
,30)
/30.
05(0
26,5
3)/3
1.71
203,
1131
6,65
15.7
180,
75w
2528
8,3
0344
P11
E27
/03.
015
27/0
3.01
5(2
79,0
7)/3
.26
(184
,39)
/0.8
6(0
17,5
0)/3
4.12
235,
4023
,45
18.6
041,
73e
083,
25P
11R
31/4
0.82
827
/20.
759
(351
,56)
/4.0
4(2
28,2
0)/3
1.57
(128
,26)
/32.
4704
0,13
142,
4121
.304
1,73
e14
0,3
31P
1336
/50.
572
31/0
0.32
4(3
27,7
7)/2
.56
(210
,07)
/30.
42(1
18,1
1)/3
2.14
323,
7821
6,04
18.5
219,
65w
4822
2,05
46P
1640
/10.
804
39/0
0.70
7(0
62,0
4)/2
.61
(323
,63)
/30.
24(1
55,2
6)/3
2.37
239,
0414
7,16
9.8
063,
24s
392
148,
308
396
P17
40/4
2.14
936
/22.
190
(208
,11)
/5.8
3(1
12,2
9)/3
0.67
(316
,58)
/35.
1606
6,11
324,
4611
.908
6,70
n-O
369
154,
183
90P
23E
46/2
0.72
044
/40.
644
(226
,70
)/3.
04(3
27,0
4)/3
1.25
(058
,20)
/31.
7817
1,75
321,
1212
.513
4,84
s-O
3337
121,
0533
55P
23R
30/1
0.80
529
/30.
796
(269
,00)
/2.4
2(3
59,0
1)/0
.64
(151
,89)
/33.
0731
2,01
213,
8515
.313
4,84
s-O
321
221,
311
323
P45
335
/00.
602
35/0
0.60
2(2
57,0
3)/2
.31
(349
,32)
/1.1
9(1
63,5
8)/3
3.50
026,
2216
5,61
10.1
109,
55s
346
183,
103
61S0
240
/30.
731
37/0
0.63
7(1
20,1
9)/3
.05
(000
,55)
/30.
03(2
21,2
8)/3
3.02
084,
4024
1,48
10.9
077,
89n-
O3
7812
7,3
123
117
S07
36/1
0.61
435
/00.
627
(058
,08)
/3.0
2(3
25,2
1)/3
0.36
(169
,68)
/32.
6524
6,02
151,
5312
.307
6,46
s3
7915
7,08
387
S09
37/1
1.20
136
/01.
220
(150
,60)
/4.0
5(0
46,0
8)/3
1.05
(312
,29)
/33.
0011
1,58
302,
3213
187,
56w
3230
0,3
2356
S10
34/4
1.39
030
/01.
054
(276
,03)
/1.9
6(1
83,4
8)/0
.76
(009
,41)
/32.
7227
5,01
004,
6111
096,
85n-
O3
5918
3,24
361
S11
38/3
0.43
735
/00.
431
(056
,16)
/2.3
8(3
19,2
2)/0
.25
(179
,62)
/32.
6408
3,20
345,
2215
.110
2,47
n-O
353
219,
213
25S1
252
/10
0.45
042
/00.
311
(126
,46)
/1.9
9(2
87,4
2)/0
.38
(025
,10)
/32.
3713
6,62
044,
0611
.510
4,68
s07
9,57
S20E
36/0
0.74
736
/00.
747
(314
,08)
/3.8
2(2
22,1
8)/3
0.09
(067
,70)
/33.
7332
7,38
137,
5111
.631
1,72
n-O
360,
17S2
0R31
/50.
740
26/3
0.63
7(0
58,1
3)/2
.23
(317
,41)
/30.
54(1
62,4
6)/3
1.69
122,
4331
7,46
1831
1,72
n-O
085,
09S2
1457
/14
0.78
243
/10.
840
(223
,23)
/4.7
7(0
17,6
5)/3
1.07
(129
,09)
/33.
7022
5,26
020,
6311
.415
0,75
n3
522
3,06
321
S221
41/3
0.79
638
/10.
770
(351
,12)
/3.4
2(0
88,1
9)/3
1.04
(242
,56)
/32.
3834
3,07
076,
1611
123,
49n
332
258,
2114
S223
E46
/00.
624
46/0
0.62
4(0
03,3
2)/2
.05
(197
,57)
/0.6
7(0
97,0
6)/3
2.72
109,
5800
8,06
8.9
196,
80e
2520
1,09
25S2
23R
26/0
0.96
626
/00.
966
(091
,13)
/3.1
3(2
05,6
1)/0
.62
(354
,25)
/33.
7519
2,04
095,
5915
.619
6,80
e41
280,
2136
S244
46/7
0.67
139
/00.
656
(071
,31)
/2.7
1(3
38,0
5)/0
.77
(240
,59)
/33.
4709
2,24
225,
5810
.890
,75
s3
6515
2,3
093
92S2
7735
/61.
271
29/1
1.04
4(0
84,2
5)/4
.27
(323
,48)
/31.
34(1
90,3
1)/3
2.94
098,
3834
9,23
13.7
098,
49n
357
169,
233
75S3
4E56
/10.
659
55/1
0.64
4(2
30,3
7)/4
.25
(352
,35)
/30.
87(1
10,3
3)/3
3.38
202,
5204
6,35
9.5
206,
66e
3524
5,01
69S3
4R28
/20.
890
26/2
0.77
6(0
76,4
7)/4
.2(3
22,2
1)/1
.03
(216
,35)
/35.
2305
8,28
213,
6019
.520
6,66
e51
329,
324
85S3
6E47
/30.
920
44/0
0.86
8(2
32,5
0)/3
.64
(055
,40)
/0.7
7(3
24,0
1)/3
4.41
060,
5316
3,10
12.2
169,
47e-
O3
218
1,03
5S3
6R25
/00.
845
25/0
0.84
5(1
49,0
1)/4
.45
(058
,15)
/0.1
4(1
65,7
3)/3
4.58
178,
1807
2,44
18.7
169,
47e-
O14
264,
0320
S43E
34/0
0.91
234
/00.
912
(336
,53)
/2.8
1(0
82,1
1)/3
0.56
(180
,35)
/32.
2534
5,42
207,
3817
.423
0,74
s-O
5927
3,07
97S4
3R29
/11.
216
28/0
1.18
4(2
02,3
9)/2
.97
(094
,21)
/0.6
7(3
43,4
4)/3
3.64
216,
1832
9,51
18.3
230,
74s-
O33
0,3
54S4
435
/70.
710
28/1
0.58
0(1
52,0
1)/4
.13
(059
,83)
/0.4
2(2
42,0
6)/3
4.54
018,
0010
7,72
14.8
185,
87e-
O30
284,
1540
S45
42/5
0.79
537
/10.
715
(238
,31)
/3.2
8(3
57,3
7)/0
.04
(122
,37)
/33.
3124
2,33
080,
5612
.226
0,79
n-O
105
315,
0971
S48E
37/0
0.71
637
/00.
716
(026
,07)
/3.3
1(1
33,6
5)/0
.04
(293
,24)
/33.
3419
3,00
282,
6112
.222
3,88
w68
297,
323
53S4
8R35
/40.
929
31/0
0.87
1(2
86,2
0)/4
.03
(093
,70)
/30.
57(1
95,0
4)/3
3.46
255,
5405
1,32
21.5
223,
88w
5219
1,08
15S4
9E51
/00.
768
51/0
0.76
8(1
85,7
6)/2
.37
(304
,07)
/30.
16(0
35,1
2)/3
2.21
274,
5102
5,14
8.6
227,
48n
5620
2,06
26S4
9R28
/11.
394
27/0
1.29
4(0
09,0
1)/4
.43
(279
,09)
/30.
62(1
07,8
1)/3
3.85
038,
0929
7,55
14.1
227,
48n
7230
5,09
61S6
8E34
/00.
537
34/0
0.53
7(1
13,3
9)/2
.34
(311
,50)
/31.
01(2
11,0
8)/3
1.33
104,
3231
4,53
11.6
308,
75n
001,
09
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Of these 26 samples, 13 record a second twin-pro-ducing deformation D2c1 (Table 1; Fig. 4). Twoadditional samples (P13A, S79A) record only di-rections that agree best with D2c1 based on theorientation of the recorded stress relative to struc-tural trend. The remaining ¢ve samples are notinterpretable for one of two reasons. (1) Compres-sions are oriented at steep angles to bedding(v 25³), indicating twins that probably re£ect lo-cal perturbations or are syn- to post-folding (e.g.sites S12, S43R, Fig. 4). (2) Multiple deformationsmay be present but are not distinguishable in asample, perhaps due to local rotation and tiltingof folds during arc tightening (in that case com-pressions are intermediately oriented betweenstrike normal and strike parallel, e.g. sites P11,S20, S68, Fig. 4).
Samples recording two deformations typicallyindicate D1c1 and D2c1 to be oriented at highangles to each other. It has previously been estab-lished that longitudinal structures (tangential tothe arc) belong to an early deformation that hasbeen overprinted by a later, radial fold-producingevent [1]. This forms the basis by which we inter-pret D1c1 and D2c1 in samples recording twotwinning events. Maximum compressive stress isconsidered D1c1 when oriented (nearly) layer-parallel and at a high angle to strike of foldedbeds (does not fall under uninterpretable category1 above). This occurrence illustrates the character-istic behavior of twinned calcite to record layerparallel paleostresses and strains that are passivelyrotated with subsequent folding within the sameoverall stress ¢eld (compression directed at a highangle to the hinge line). This allows for distinctionof D1c1 compression oriented at a high angle tostrike and D2c1 sub-parallel to strike (does notfall under uninterpretable category 2 above). Oneexception, sample S43, illustrates that D1c1
(S43R) is uninterpretable (category 1, above)whereas D2c1 (S43E) is robust (Fig. 4)
Whether a sample records one or two deforma-tion events appears to be related to its location inthe region (Fig. 5). Samples that record twoevents are nearly ubiquitous in the HZ whereasa single deformation dominates samples of theSZ. In the NZ, ¢ve of seven samples recordD1c1 directions and four preserve D2c1 compres-T
able
1(c
onti
nued
)
Sam
ple
Raw
data
Cle
aned
data
Stra
inor
ient
atio
ns(b
rg.,p
lng.
)/E
long
atio
nSt
ress
Ori
enta
tion
s(b
rg.,p
lng.
)B
eddi
ng
N/R
S.E
.M.
N/R
S.E
.M.
e 1e 2
e 3c
3c
1K
95St
rike
and
dip
S3
S0
c1
c3c
0
(ori
gina
l)(t
reat
ed)
(T)
(C)
O:o
vert
urne
dD
1;D
2A
TC
D1
;D2
S68R
31/4
0.78
327
/00.
716
(336
,41
)/2.
46(1
57,4
8)/3
0.32
(067
,00)
/32.
1532
3,07
101,
3516
.530
8,75
n26
7,13
S69E
40/0
0.91
940
/00.
919
(015
,32)
/3.1
7(1
42,4
5)/0
.15
(266
,29)
/33.
3304
2,60
268,
2210
.208
9,70
n33
8210
9,08
3367
S69R
27/0
1.28
727
/01.
287
(233
,37)
/3.0
1(1
34,1
1)/0
.41
(030
,51)
/33.
4228
2,26
054,
5515
.808
9,70
n3
6620
7,02
337
S71
30/4
1.08
926
/10.
855
(043
,30)
/2.7
6(1
40,1
4)/0
.20
(251
,56)
/32.
9634
7,01
256,
5913
.307
8,64
n-O
377
202,
323
342
S72
30/4
0.93
226
/00.
879
(072
,15)
/3.7
2(3
32,3
1)/3
0.02
(184
,55)
/33.
7007
1,16
295,
7012
.509
7,73
n-O
358
206,
310
338
S75E
49/1
0.40
648
/00.
377
(142
,01)
/2.5
4(2
39,8
2)/1
.08
(052
,08)
/33.
6228
8,62
044,
129.
807
2,74
s33
9906
7,30
3310
9S7
5R27
/20.
939
25/1
0.81
9(0
54,0
9)/4
.13
(160
,59)
/2.4
1(3
20,3
0)/3
6.54
069,
1818
1,70
15.9
252,
74s
9733
6,03
92S7
7E45
/10.
588
44/0
0.54
5(1
59,7
2)/2
.34
(338
,17)
/30.
49(2
48,0
0)/3
1.86
124,
7829
8,13
9.3
140,
84s
3331
124,
2033
52S7
7R31
/40.
855
27/2
0.86
2(2
68,0
7)/2
.94
(360
,21)
/30.
34(1
59,6
8)/3
2.60
298,
2016
4,58
18.1
140,
84s
315
202,
307
342
S79
40/5
0.94
935
/20.
781
(280
,65)
/3.2
2(0
97,2
5)/3
0.46
(187
,10)
/32.
7730
4,72
137,
1811
.213
3,89
s33
3815
1,04
3325
Nis
the
num
ber
oftw
inse
tsan
alyz
edan
dR
isth
enu
mbe
rof
asso
ciat
edR
Vs.
Raw
data
wer
ecl
eane
dby
rem
ovin
gal
lR
Vs,
assu
gges
ted
byG
rosh
ong
[48]
.B
oth
raw
and
trea
ted
data
are
pres
ente
dw
ith
thei
rre
spec
tive
stan
dard
erro
rs(S
.E.M
.).
Dat
aar
ein
clud
edfo
rbo
thth
est
rain
gaug
ete
chni
que
ofG
rosh
ong
[49]
inco
lum
nse 1
,e 2
and
e 3as
wel
las
the
dyna
mic
anal
-ys
isof
Tur
ner
[47]
inco
lum
nsc
3(T
ensi
on),
c1
(Com
pres
sion
),an
dK
95.
Stri
kean
ddi
pan
dco
mpr
essi
ondi
rect
ion
(c1)
afte
rti
ltco
rrec
tion
(AT
C)
are
give
nw
ith
the
prop
erde
viat
ions
(S3
S0,c3c
0)
plot
ted
inF
ig.
8a,b
(D2c
1de
viat
ions
used
inF
ig.
8bar
ein
bold
face
).
EPSL 5559 2-8-00
J.M. Kollmeier et al. / Earth and Planetary Science Letters 181 (2000) 203^216 209
sion as determined by the relationship betweenstress orientations and strike. The results areshown in map view in Fig. 5.
5. Discussion
5.1. D1 folding and thrusting
All three structural zones of the Somiedo unitare unbent to an earliest geometry by rearrangingD1 stresses to an arbitrarily uniform (present-day)east^west reference orientation. What results is an
essentially linear, generally north^south trendingfold^thrust belt (Fig. 6). However, the SZ featuresa mild foreland-progressive counter-clockwise ro-tation in successively younger thrust sheets, whichculminates in a more southeasterly trend in thefrontal portion. This structural de£ection resultsperhaps from a greater foreland translation viathrusting relative to the HZ and NZ. Averagingthe resultant structural trends from the youngestsampled thrust sheets, we derive an orientation of27³ east of south as a best-¢t D1 structural trendfor all SZ samples.
Furthermore, the reconstruction in Fig. 6 is il-
Fig. 4. Equal-area lower-hemisphere plots of all sample results. Contoured compressive stress axes are shown as well as the prin-cipal strain axes calculated with the strain gauge of Groshong [49]. All data are represented in present-day ¢eld coordinates withbedding included. Tabulated D1c1 and D2c1 compressions illustrate the predominance of D2 associated with EVs and D1 associ-ated with RVs. Stress data are contoured with the ¢xed-circle counting method of Kamb, whereas maximum compressive stressdirections are calculated as point means assuming a Fisherian distribution.
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lustrated by a series of three equal-area lower-hemisphere projections of D1 maximum compres-sive stress axes (Fig. 7). Stress directions inpresent-day structures (Fig. 7a) are ¢rst untiltedto paleohorizontal (Fig. 7b), and then rotatedabout a vertical-axis to the original geometry pre-sented in Fig. 6 (Fig. 7c). Even with the scatterdue to assigning all SZ sites an average D1 trendof S 27 E (open symbols, Fig. 7c), the resultingalignment illustrates that uniform as well asnearly bedding-parallel compressions are valid as-
sumptions. In a very generalized fashion, Fig. 7illustrates an overall fold and strike test for theentire study area.
5.2. Arc rotations
When considering whether a curved orogenicbelt is either a primary feature (no rotation), oran `orocline' as put forth by Carey [22], plottingstructural trend versus pre-bending paleomagneticdeclination poses a powerful evaluation of rota-
Fig. 5. Map of the CAA adapted from 1:200 000 scale geological maps [53]. Black arrows indicate the orientation of (tilt-cor-rected) subhorizontal D1c1 and white bars represent later (in situ) D2c1 ; because D2c1 is generally strike parallel, in situ andtilt-corrected orientations are approximately the same. The three structural zones are delineated and labeled as NZ, HZ, and SZ.See Fig. 4 for a more detailed stress to strike relation as local structures may deviate from the generalized trends (e.g. S214).
EPSL 5559 2-8-00
J.M. Kollmeier et al. / Earth and Planetary Science Letters 181 (2000) 203^216 211
tion [9,20,51]. We use an analogous relationshipin Fig. 8, where the axes represent the deviationfrom a reference strike (abscissa) and deviationfrom a reference D1 maximum compressive stress(ordinate). This plot can then be interpreted interms of relative rotations, with the assumptionthat all stress was uniform within a single event.The likelihood of this assumption is later eval-uated by comparing the plots of both deformationepisodes (D1 and D2). Reference values (S0 and
Fig. 6. Arc geometry during D1c1 constrained by untighten-ing the arc to a reference east^west stress ¢eld. North^southtrending thrusts in the SZ have swept eastward in a forelandprogressive sequence as shown by arrows. Thin, short hori-zontal lines (doubly tipped with inward arrows) representcompressive stress and are related with the structural trend(thicker intersecting lines) for all 26 D1 compressions.
Fig. 7. Equal area lower hemisphere projections representingD1c1 orientations of all samples considered to unbend theCAA. Samples from the NZ and HZ are indicated with solidcircles and SZ samples are denoted with open circles. Orien-tations from in situ data (a) are untilted to paleohorizontal(b). Finally, the data are rotated to a reference north trend-ing structure for NZ and HZ samples, and an average struc-tural trend of S27E for all SZ samples (c). The scatter in theSZ data illustrates the variation in stress directions in succes-sive thrust sheets that is comparable to the NZ and HZ scat-ter combined.
EPSL 5559 2-8-00
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c0 in Fig. 8a,b) are chosen merely to produce abest-¢t line that passes through the origin whenconsidering all samples. Plotted points representthe deviation from these reference values (S03S,c03c), and data are presented with maximumcompressive stress axes untilted to paleohorizon-
tal (Table 1). Fig. 8a plots D1 paleostress orien-tations of individual sample sites from around thearc. Ideally, if a curved orogen is a primary fea-ture and a uniform stress ¢eld produced the orig-inal curvature, one would expect uniform stressorientations irrespective of the structural trend.This end-member condition would produce aline along the abscissa with zero slope (no devia-tion in c1 orientations). The other end-member isan initially linear belt, where during bending therecorded stress directions are rotated passivelywith the curved geometry; in this case the ex-pected slope of the best-¢t line is 1. Fig. 8a showsthat the results for the CAA correspond nearlyperfectly to the latter end-member model ; i.e.,curvature from an originally straight belt. Notethat this plot does not assess whether an origi-nally linear structure undergoes continuous or ep-isodic rotation, but merely indicates total rota-tion.
5.3. D2 folding and thrusting
In Fig. 8b the deviation from a reference D2maximum compressive stress is plotted. A slope ofV1 is again observed which indicates that D1 arcgeometry was nearly linear and that D2 was re-corded prior to arc bending. The agreement be-tween the rotations required for both D1 (Fig. 8a)and D2 (Fig. 8b) provides conclusive evidence oftightening with nearly all of the curvature beingsecondary and supports the assumption of uni-form compression for each event.
Fig. 8. (a) D1 compression deviations from the mean refer-ence stress (c0 = 244³) of all 26 samples tilt corrected to bed-ding horizontal plotted against strike deviations from thereference strike (S0 = 155³). (b) Likewise, D2 compression de-viations from the mean reference stress (c0 = 176³) of 13samples plotted against strike deviations from a referencestrike (S0 = 171³). In both cases, a linear regression is drawnwith associated correlation coe¤cients R2 and slope m (indi-cating the CAA was oroclinally bent from a nearly straightoriginal con¢guration, m =V1). (c) Paleomagnetic data [20]for the CAA showing deviations in (mean) declination andstrike for each of the three zones (NZ, HZ, and SZ), illus-trating rotation for approximately half of the CAA curva-ture.6
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5.4. Evolution of the CAA
In samples recording two deformations, partic-ularly those of the NZ and HZ, D2c1 is com-monly oriented sub-perpendicular to D1c1 (Figs.4 and 5). In the D1 fold geometry this results inD2c1 compressive stresses averaging to an essen-tially north^south orientation. Thus, analogous toD1 (Fig. 8a), D2c1 twinning occurs prior to thelater (D2) phase of macroscopic deformation ofthe belt (Fig. 8b). Overall, this suggests that D2produced clockwise bending in the NZ, radialfolding in the HZ, and tightening of thrusts inthe SZ. It is also observed that the SZ showsthe least amount of evidence of D2 as the samplesrecord only D1 maximum compressive stress di-rections in all but two samples.
Fig. 8c is a compilation of paleomagnetic datafor the CAA plotting the averages for each struc-tural zone [20]. The calculated slope is V0.5, in-dicating that the overall arc curvature was about50% at the time the pervasive syn-tectonic remag-netization was acquired. In comparison with thepaleomagnetic results, the calcite twinning datatherefore demonstrate that V50% of the curva-ture was attained prior to the remagnetization,but following D2 calcite twinning.
Fig. 9 presents our interpretation of the evolu-tion of the CAA in sequential block diagrams,beginning with £at lying shelf units recordingpre-folding twinning due to a uniform (present-day) east^west compression (D1c1 ; Fig. 9a). Dur-
ing D1, longitudinal structural trends form in anearly linear belt (Fig. 9b). D2c1 re£ects a changein stress ¢eld from east^west to north^south act-ing upon the previously formed D1 structures(Fig. 9c). Lastly, present-day structural geometryre£ects large scale folding (D2, manifested in ra-dial folds and regional curvature) in response tonorth^south compression (Fig. 9d). A tectonic ex-planation for these events would be an east^westcollision between the Ebro^Aquitaine massif andthe Laurussian margin (D1), followed by a great-er, perhaps protracted north-directed collision ofGondwana with Laurussia [52].
Acknowledgements
We thank J.C. Larrasoan¬a and J. Pares fortheir assistance in the ¢eld and J.H. Harris forhis advice in calcite twinning analysis. We alsothank M.A. Evans and R.H. Groshong for useof the CSG-22 program. Thoughtful reviews byD.A. Ferrill, B. Ward, and M. Burkhard im-proved the manuscript and are greatly appreci-ated. Support for this project was from NationalScience Foundation Grant EAR 9705755. In ad-dition, funding for the ¢eldwork came from aGeological Society of America research grantawarded to J.M. Kollmeier as well as the ScottTurner fund, Dept. of Geological Sciences, theUniversity of Michigan.[AC]
Fig. 9. Block diagrams summarizing the evolution of arc tightening beginning with east^west horizontal compression acting on astable margin in (a) and resulting in an arc geometry (b) after D1 folding and thrusting (as in Fig. 6). A later north^south hori-zontal compression (D2) is shown in (c) and leads to clockwise bending in the NZ, radial folding in the HZ, and tightening ofthrusts in the SZ of present-day arc geometry (d).
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