Alleghanian Paleostress Reconstruction in the Northern Appalachians: Intraplate Deformation Between Laurentia and Gondwana

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Alleghanian paleostress reconstruction in the northern Appalachians: Intraplate deformation between Laurentia and Gondwana

Stéphane Faure Institut National de la Recherche Scientifique, INRS-Géoressources, 2700 rue Einstein, Alain Tremblay } Ste-Foy, Québec G1V 4C7, Canada Université P. et M. Curie, Laboratoire de Tectonique Quantitative, Tour 26-25, E1, Jacques Angelier 4 place Jussieu, 75252 Paris, Cedex 05, France

ABSTRACT formation related to the indentation of Gondwana into Laurentia during the late Paleozoic Alleghanian orogeny. A numerical paleostress tensor analysis using striated fault planes has been conducted in the Quebec reentrant of the northern Appalachians INTRODUCTION to characterize the stress field of late Paleozoic deformations. Three directions of maximum compressional stress axes (σ1) have been found The development of the Appalachian orogen included several successive and correlated to (1) an early north-northwestÐsouth-southeast com- deformational events that were related to the formation of sedimentary pression, (2) a north-northeastÐsouth-southwest compression, and (3) a basins and to the accretion of oceanic and continental terranes against the late west-northwestÐeast-southeast compression. Fault populations as- eastern margin of Laurentia during Paleozoic time (Williams, 1979; Osberg sociated with these stress regimes are present in all tectonic zones of the et al., 1989). The present-day architecture of the Appalachian belt is attrib- Québec and northern New Brunswick Appalachians. Directions of σ1 uted to three major orogenies of early to late Paleozoic age (Williams, 1979; axes determined in the northern Appalachians resemble in orientation Hatcher, 1989): the Taconian, Acadian, and Alleghanian orogenies. and in relative chronology layer-parallel shortening fabrics and joint In Québec and northern New Brunswick (Fig. 1), regional deformation patterns found in the Appalachian foreland of the central Appalachians. of pre-Late Devonian rocks is primarily related to both the Taconian and The paleostress regimes are interpreted as the record of intraplate de- Acadian orogenies. Carboniferous rocks are only slightly deformed (St-

Figure 1. Tectonostratigraphic map of Québec and northern New Brunswick Ap- palachians. BBL: Baie VerteÐBrompton line; GF: La Guadeloupe fault; GPF: Grand Pa- bos fault; LL: Logan line; RBMF: Rocky Brook Millstream fault. Inset shows the loca- tion of major shear zones (black line) in northern and central Appalachians and the limit of Carboniferous basins (dashed line). CCF: Cobequid-Chedabucto fault; MB: Maritimes basin; NB: Narragansett basin; NF: Norumbega fault; NFL: Newfoundland; PEI: Prince Edward Island.

GSA Bulletin; November 1996; v. 108; no. 11; p. 1467Ð1480; 10 figures; 1 table.

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Julien and Hubert, 1975; Malo and Bourque, 1993) and little attention has Carboniferous rocks of the Maritimes basin form the youngest deposits been paid to post-Acadian deformation because it scarcely affects older of the Gaspé Peninsula and New Brunswick Appalachians (Fig. 1). In structures and the regional map pattern. Minor mesoscopic structures, such Gaspé, the Maritimes basin is made up of terrestrial redbeds that uncon- as brittle faults, veins, and joints affecting post-Devonian intrusions and formably overlie, or are in fault contact with, older rocks of the Gaspé belt Carboniferous rocks of the northern Appalachians, are mostly attributed to (Zaitlin and Rust, 1983). The Cretaceous Monteregian plutonic suite of undifferentiated post-Acadian deformation. Because these brittle structures southern Québec includes the youngest rocks of the study area (Foland et record the orientation of post-Acadian stress fields, their analysis is critical al., 1986). for an understanding of the Alleghanian evolution of intraplate deformation in the Appalachian orogen. Most of the northern Appalachians lie outside of PALEOSTRESS ANALYSIS the Alleghanian deformation front (see Williams and Hatcher, 1982), and not much is known about the significance and the extent of post-Acadian Paleostress Determinations: The Inverse Method brittle deformation in the Québec reentrant. In this paper we characterize the latest deformational stages of the north- For the purpose of this study, we collected fault-slip data from various ern Appalachian belt. We present orientation data on slickensided fibers and rock units of the Québec and northern New Brunswick Appalachians brittle faults and use them as indicators of both the orientation and the rela- (Fig. 2). Paleostress orientations are determined by using a computer-based tive magnitude of principal paleostress axes. These data provide a key to inversion technique developed and extensively discussed by Angelier understanding the major directions of maximum stress axes during the Alle- (1984, 1994). ghanian orogeny. A three-dimensional numerical analysis of fault popula- On the basis that the direction and sense of slip on a single fault plane tions is applied to various rock units of the Québec and northern New are those of the shear stress applied on this plane by the stress tensor (e.g., Brunswick Appalachians. Detailed data and results from Carboniferous Wallace, 1951), the inverse method aims at determining the paleostress rocks of the Gaspé Peninsula and from Late Devonian intrusions of southern tensor that best accounts for the directions and senses of slip on numerous Québec (Fig. 1) are presented as case examples. The complete data set is in- fault planes in a rock mass, as indicated by slickenside lineations. A best fit tegrated at the scale of the Québec reentrant in order to construct a model for between all slip data collected in the rock mass and an unknown common the late Paleozoic tectonic evolution of the northern Appalachians. stress tensor has to be found. A least square criterion is used and some residual misfits are commonly observed. The minimization function of the QUÉBEC REENTRANT OF THE NORTHERN APPALACHIANS method involves the angle between the observed slip (i.e., the slickenside lineation) and the theoretical shear stress computed from the best-fitting The Québec reentrant of Québec and New Brunswick is divided into stress tensor (see Angelier, 1979, for details). This angle also represents a three principal lithotectonic assemblages (Fig. 1): (1) Cambrian to Middle good misfit estimator, called ANG in Table 1. ANG values are used to Ordovician rocks belonging to the St. Lawrence lowlands and to the Hum- quantify the mechanical consistency of fault populations, and vary from 0¡ ber and Dunnage zones (Williams, 1979), (2) Upper Ordovician to Devon- (i.e., the computed shear stress is parallel to natural slip with the same ian rocks of the Gaspé belt (Bourque et al., 1995), and (3) Carboniferous sense of motion) to 180¡ (i.e., the computed shear stress and natural slip are cover rocks of the Maritimes basin (Bradley, 1982). parallel but in opposite directions). ANG values of 20¡ or less are required The St. Lawrence lowlands consist of autochthonous platform and fly- for reliable determinations. Faults with large ANG values must be carefully sch sequence lying unconformably on Proterozoic rocks of the Laurentian examined because they usually reveal polyphase tectonism, faulting inter- craton. This sequence is affected by reverse faults along Logan’s line actions, or mistakes in data collection. (Fig. 1). The Humber zone records a rift-drift evolution of the Laurentian Misfits in paleostress determinations can occur due to (1) instrumental continental margin related to the formation of the Iapetus ocean (Williams, and observation errors associated with data collection, (2) the number of 1979). These rocks were mainly deformed during the Taconian orogeny. data, (3) basic assumptions of the method itself, and (4) undetected poly- The Humber zone is divided into an external domain of thrust slices and phase evolution related to fault slips that did not occur during the tectonic nappes, and an internal domain of polydeformed and metamorphosed event under consideration (discussed herein). rocks (St-Julien and Hubert, 1975; Stanley and Ratcliffe, 1985; Tremblay Instrumental errors of field measurements are small (≈2¡). However, and Pinet, 1994). The Dunnage zone records the development and the sub- considering that exposed segments of a faults are commonly small relative sequent accretion of oceanic terranes to the Laurentian margin during and to its total surface, and that most faults and striae are far from being perfect after the Taconian orogeny (Williams, 1979; Tremblay, 1992; Tremblay et planes or straight lines, observation uncertainties related to data collection al., 1995). The Baie VerteÐBrompton Line (BBL, Fig. 1) represents a struc- are higher than instrumental errors. Repeated measurements on large fault tural boundary between the Humber and Dunnage zones and is interpreted surfaces (see Angelier, 1979) suggest that such errors range between 4¡ and as a major fault zone that was active during both the Taconian and Acadian 16¡ for each of the measured angles (strike, dip, and pitch); 6¡ is an aver- orogenies (Williams and St-Julien, 1982; Malo et al., 1992). age value for nearly planar fault surfaces. Silurian and Devonian rocks of the Gaspé belt are interpreted as succes- The quality of paleostress determination increases with the number of sor basin deposits (Bourque et al., 1995) that unconformably overlie older data collected at a site. Some results listed in Table 1 are obviously of lim- units of the Humber and Dunnage zones. The Gaspé belt and underlying ited intrinsic value because the number of data (less than 10) is scarcely pre-Silurian rocks were deformed during the Acadian orogeny (Malo and larger than the number of unknowns. However, these sites are not abundant Bourque, 1993; Tremblay and Pinet, 1994). In southern Québec, the Aca- (≈16% of all sites) and their results fit generally well with the general pic- dian deformation is dominated by dip-slip tectonics, whereas it is mostly ture of calculated paleostress trends from larger data sets. Much more con- characterized by wrench faulting and folding in the Gaspé Peninsula and fidence should be put in results obtained with larger numbers of data. Other adjacent New Brunswick (Malo et al., 1995; de Roo and van Staal, 1994). studies with large data sets, such as for the Hoover Dam site (Angelier et Syn- to post-Acadian intrusive rocks, attributed to collision-related mag- al., 1985), demonstrated that as a function of the number of data, the qual- matism (Simonetti and Doig, 1990), are mainly found in southern Québec ity of the paleostress determination increases rapidly for small quantities where they crosscut rock units of the Gaspé belt (Fig. 1). (less than 50) and does not increase much for larger ones.

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Figure 2. Site numbers and locations used for the paleostress analysis of this study.

The mechanical assumption of inversion methods that faults moved in- sources of misfit imply that average ANG values as large as 20¡ may be ex- dependently but consistently under a homogeneous stress regime (i.e., a pected. In practice, ANG values are smaller, within the range of instru- single stress tensor) during a single tectonic event is obviously an approx- mental and observation errors. In the study area, fault data were measured imation because, in reality (1) stress dispersion occurs locally; (2) preexist- in competent lithologies such as limestones, sandstones, and igneous ing weakness zones induce stress perturbations; and (3) faults interaction rocks. The regional metamorphism varies from subgreenschist to lower influences relative block displacements and stress distributions. Field ap- greenschist grade. Fault-slip data (2245 striated fault planes distributed plications, however, indicate that the misfit values (ANG) commonly re- over 150 sites) were collected from exposures in quarries and along road mains small (<10¡), suggesting that such sources of stress dispersion are cuts, the horizontal surface of each site studied being less than 0.5 km2. Site statistically minor (e.g., Angelier, 1979). Dupin et al. (1993) showed that locations are shown in Figure 2. misfits related to these sources of stress dispersion are commonly less than 4¡ for fault-slip datasets of variable orientations. Polyphase Deformation: Separation of Fault Data into Subsets With the above limitations, rather homogeneous paleostress fields can be thus reconstructed by the compilation of local determinations of stress In the Appalachians, most geologic formations underwent several phases tensors (see Angelier, 1994). The results are expressed in terms of reduced of deformation. Brittle tectonic features observed in a single outcrop thus stress tensors that give the orientation of the maximum (σ1), intermediate correspond to a certain number of distinct stress regimes that belong to one (σ2), and minimum (σ3) stress axes (pressure is noted positive), as well as or more tectonic event(s). Therefore, reliable determinations of paleostress the ratio of principal stress differences, Φ =(σ2Ðσ3)/(σ1Ðσ3). The ratio Φ regimes frequently require the separation of fault-slip data into subsets that (Angelier, 1975) varies from 0 (σ2 = σ3) to 1 (σ1 = σ2), values that corre- can be attributed to distinct tectonic regimes and orogenic events. spond respectively to stress ellipsoids symmetric around σ1 and σ3 axes In the field, the recognition of successive brittle deformation is sup- (see Table 1). The quality of results is expressed in Table 1 through the ported by the observation of crosscutting relationships between different number of data and the average ANG value for each site. The various generations of structures (faults, veins, and dikes) and by superposed sets

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TABLE 1. GEOGRAPHIC COORDINATES OF SAMPLING SITES AND ORIENTATION OF PALEOSTRESS AXES FOR EACH SITE IN THE QUÉBEC REENTRANT TABLE 1. (Continued) Site* Rock† Chr¤ Nb# σ1** σ3** Φ†† Ang¤¤ Latitude Longitude Site* Rock† Chr¤ Nb# σ1** σ3** Φ†† Ang¤¤ Latitude Longitude 1 S 35 336 08 076 54 0.17 11¡ 46¡08′N71¡58′W 72 L 14 193 03 284 33 0.01 12¡ 48¡52′N64¡30′W 2 S 9 169 20 061 40 0.26 13¡ 46¡12′N71¡59′W 73 L 1 14 198 14 051 74 0.11 12¡ 48¡50′N64¡32′W 3 S, Sh 21 343 01 250 66 0.39 13¡ 46¡11′N71¡52′W 2 9 112 04 020 29 0.07 9¡ 48¡50′N64¡32′W 4 S, Sh 23 337 10 085 61 0.13 10¡ 46¡00′N72¡20′W 74 S, L 42 010 06 236 82 0.20 15¡ 48¡32′N64¡14′W 5 V 1 26 339 00 249 04 0.12 17¡ 45¡55′N72¡23′W 74a S 13 194 08 089 62 0.52 9¡ 48¡33′N64¡13′W 2 13 021 03 291 02 0.39 14¡ 45¡55′N72¡23′W 75 L 19 015 05 115 64 0.24 16¡ 48¡34′N64¡21′W 6 V 1 28 337 04 068 20 0.13 12¡ 45¡54′N72¡29′W 76a S 8 299 25 031 03 0.63 10¡ 48¡23′N64¡41′W 2 7 022 02 241 88 0.36 14¡ 45¡54′N72¡29′W 76b S 31 337 08 072 32 0.09 15¡ 48¡22′N64¡39′W 8 S 10 158 11 045 64 0.15 15¡ 45¡49′N72¡32′W 77 L, S 26 157 03 067 04 0.06 13¡ 48¡12′N64¡56′W 11 L 23 348 12 098 59 0.04 14¡ 45¡35′N72¡52′W 78 L 6 017 01 286 25 0.60 20¡ 48¡03′N65¡23′W 12 L, Sh 16 170 12 076 15 0.32 14¡ 45¡32′N72¡55′W 79a L 15 343 06 252 12 0.02 14¡ 48¡08′N65¡49′W 13 S 12 156 08 264 66 0.18 15¡ 45¡21′N72¡47′W 79b S 12 165 11 355 79 0.49 4¡ 48¡14′N65¡55′W 14 L 18 171 12 005 78 0.50 14¡ 45¡06′N73¡02′W 80 S, L 13 202 08 105 39 0.31 11¡ 48¡31′N66¡03′W 15 S 10 355 09 086 10 0.46 11¡ 45¡43′N72¡32′W 81a S 1 15 336 05 236 64 0.24 12¡ 48¡07′N66¡11′W 16 L 21 348 08 079 10 0.43 16¡ 45¡41′N72¡40′W 2 7 201 16 106 15 0.11 8¡ 48¡07′N66¡11′W 18 L, V 7 346 08 255 08 0.31 6¡ 45¡38′N72¡34′W 81b V 22 332 04 204 84 0.21 15¡ 66¡18′N48¡08′W 19 S 7 350 07 082 19 0.45 8¡ 45¡18′N72¡53′W 82 V 1 5 030 02 294 75 0.45 9¡ 48¡03′N66¡42′W 21 H 1 32 350 11 233 67 0.23 19¡ 45¡17′N72¡19′W 2 20 090 07 339 71 0.45 11¡ 48¡03′N66¡42′W 2 24 287 06 017 03 0.30 10¡ 45¡17′N72¡19′W 83 L, Sh 12 332 02 080 83 0.24 17¡ 48¡02′N67¡02′W 23 V, S 22 150 00 060 85 0.33 13¡ 45¡12′N72¡05′W 84 S 8 348 25 094 31 0.11 10¡ 48¡33′N68¡08′W 24 I 10 297 08 053 72 0.06 15¡ 45¡03′N72¡05′W 88 S 18 327 01 237 17 0.45 7¡ 48¡18′N68¡22′W 25 I 1 14 348 07 079 09 0.19 18¡ 45¡01′N72¡10′W 89 S, Sh 9 355 03 262 51 0.52 7¡ 48¡06′N68¡31′W 2 18 291 13 058 69 0.29 17¡ 45¡01′N72¡10′W 90 L, S 11 165 03 074 09 0.37 12¡ 47¡28′N68¡59′W 26 I 13 195 02 285 03 0.40 8¡ 45¡02′N71¡50′W 91 S 10 164 11 017 77 0.22 18¡ 48¡07′N69¡12′W 27 S 12 175 07 316 82 0.20 17¡ 45¡23′N71¡57′W 92 S 8 344 02 254 10 0.41 22¡ 47¡52′N69¡31′W 28 V 25 351 03 088 67 0.29 11¡ 45¡20′N71¡57′W 93 S 10 026 01 294 63 0.23 12¡ 47¡28′N69¡53′W 29 S, I 8 189 08 083 64 0.36 17¡ 45¡08′N71¡48′W 95 I 1 12 150 04 047 74 0.35 14¡ 46¡17′N70¡37′W 30 H 1 14 341 03 072 17 0.24 17¡ 45¡37′N72¡07′W 2 10 113 09 023 02 0.04 10¡ 46¡17′N70¡37′W 2 20 018 05 274 71 0.38 15¡ 45¡37′N72¡07′W 96 V 8 339 17 090 50 0.28 12¡ 46¡43′N70¡53′W 3 18 289 11 198 07 0.26 14¡ 45¡37′N72¡07′W 97 S 7 170 17 358 73 0.55 4¡ 46¡53′N70¡36′W 31 H 19 345 07 216 79 0.41 13¡ 45¡42′N72¡02′W 128 L 6 193 00 283 30 0.49 6¡ 46¡58′N71¡02′W 32 L 5 168 15 050 60 0.47 14¡ 45¡38′N71¡37′W 129 L 8 011 06 113 63 0.20 9¡ 46¡43′N71¡35′W 33 I 8 341 08 083 56 0.25 20¡ 45¡45′N71¡13′W 130 I 1 10 346 17 097 49 0.18 12¡ 47¡30′N66¡57′W 34 I 29 346 14 089 43 0.28 13¡ 45¡43′N71¡19′W 2 18 108 11 231 70 0.20 18¡ 47¡30′N66¡57′W 36 V 13 339 09 089 67 0.01 17¡ 46¡05′N71¡08′W 131 I 6 341 13 249 08 0.50 6¡ 47¡30′N66¡38′W 37 I 1 32 160 06 057 65 0.04 12¡ 45¡44′N70¡56′W 132 I, S 1 17 338 07 081 61 0.24 23¡ 47¡35′N66¡34′W 2 12 296 03 042 79 0.09 8¡ 45¡44′N70¡56′W 2 5 210 16 302 08 0.56 19¡ 47¡35′N66¡34′W 38 H 23 175 09 302 75 0.37 10¡ 46¡13′N71¡07′W 133 I, S 19 194 10 048 78 0.17 14¡ 47¡34′N66¡35′W 39 H 18 350 11 122 74 0.63 10¡ 45¡59′N71¡14′W 134 V 15 164 07 053 71 0.23 19¡ 47¡32′N66¡30′W 40 H 26 161 01 055 88 0.20 17¡ 45¡50′N71¡42′W 135 S, Sh 9 337 01 247 31 0.31 16¡ 47¡43′N66¡21′W 41 H 41 350 10 243 58 0.21 19¡ 45¡49′N71¡50′W 136 V 24 167 00 077 38 0.03 12¡ 47¡39′N66¡23′W 42 H 1 21 016 02 118 81 0.37 13¡ 45¡59′N71¡23′W 137 V 1 30 183 01 090 65 0.37 18¡ 47¡35′N66¡19′W 2 16 100 15 003 26 0.56 12¡ 45¡59′N71¡23′W 2 17 297 02 077 87 0.25 17¡ 47¡35′N66¡19′W 43 I 61 357 07 137 81 0.23 16¡ 45¡25′N71¡27′W 139 I 1 24 161 03 251 12 0.23 21¡ 47¡35′N65¡57′W 44 S 31 353 08 119 77 0.40 17¡ 46¡44′N71¡17′W 2 18 028 02 289 80 0.36 12¡ 47¡35′N65¡57′W 45 V 6 345 05 255 01 0.03 10¡ 46¡08′N72¡18′W 140 L, S 7 030 01 299 44 0.30 8¡ 47¡41′N65¡52′W 47 L 12 202 04 293 22 0.18 4¡ 45¡18′N73¡19′W 141 V 1 26 343 13 194 75 0.40 14¡ 47¡40′N65¡41′W 50 L 8 350 00 080 13 0.41 9¡ 45¡22′N73¡39′W 2 18 022 14 281 38 0.24 16¡ 47¡40′N65¡41′W 52 L 16 203 02 293 06 0.16 8¡ 45¡41′N73¡37′W 142 S, V 5 144 13 237 14 0.43 19¡ 47¡49′N65¡47′W 57 H 1 34 157 06 049 72 0.18 13¡ 46¡15′N70¡49′W 143 V, S 14 151 14 056 18 0.28 20¡ 47¡46′N65¡52′W 2 14 104 22 002 26 0.46 11¡ 46¡15′N70¡49′W 144 V 12 012 03 282 09 0.19 14¡ 47¡49′N66¡05′W 58 S, L 1 50 164 04 064 68 0.20 13¡ 46¡07′N70¡40′W 146 V 12 029 07 135 65 0.32 8¡ 48¡19′N67¡04′W 2 9 287 15 192 17 0.23 16¡ 46¡07′N70¡40′W 147 V, L 1 24 158 01 068 29 0.03 14¡ 48¡50′N66¡33′W 59 I 25 336 04 184 86 0.39 10¡ 46¡33′N78¡13′W 2 15 210 02 093 86 0.32 16¡ 48¡50′N66¡33′W 60 S 30 347 04 078 21 0.21 21¡ 48¡46′N67¡35′W 148 L 9 017 00 107 05 0.29 5¡ 45¡35′N73¡10′W 61 S 7 159 02 065 63 0.16 20¡ 48¡52′N67¡22′W 150 L 11 158 11 249 05 0.60 11¡ 45¡21′N74¡52′W ′ ′ 62 S 15 352 14 084 10 0.55 9¡ 48¡57 N67¡07W *Site is site name. ′ ′ 63 S 6 005 11 100 23 0.70 10¡ 49¡03 N66¡52W †Rock types are S—sandstone, Sh—shale, L—limestone, V—volcanic rocks, I— ′ ′ 64 S 11 154 00 064 63 0.21 16¡ 49¡01 N66¡23W intrusive rocks, H—harzburgite. ′ ′ 65 V 15 164 06 074 04 0.27 12¡ 48¡57 N66¡07W ¤Relative chronology; 1 is older. ′ ′ 66 I 21 166 10 256 01 0.34 14¡ 48¡52 N66¡06W #Nb is number of faults used for each stress tensor analysis. ′ ′ 67 V 13 162 00 252 02 0.35 11¡ 48¡45 N66¡08W **Orientations of maximum (σ1) and minimum (σ3) compressional stress axes. ′ ′ 68 S, Sh 8 190 10 308 70 0.10 17¡ 49¡11 N66¡18W ††The ratio Φ = (σ2 Ð σ3)/(σ1 Ð σ3). ′ ′ 69 S 16 027 01 119 70 0.45 12¡ 49¡07 N65¡15W ¤¤Ang is average angle between observed and calculated slickenlines. 71 L, Sh 23 171 14 263 09 0.48 10¡ 48¡47′N64¡12′W

of slickenside lineations indicating successive slips on fault planes (Fig. 3). ANG values). In sites where such inhomogeneous datasets are found, an Such relative chronological data were systematically collected. They allow automatic separation of stress tensors and related subsets of fault-slip data the distinction of tectonic events based on a matrix method (Angelier, was performed following a procedure proposed by Angelier and Manous- 1994), considering also the mechanical consistency expected for each sis (1980), and discussed by Angelier (1984). This procedure is based on stress regime. iterative statistical analysis applied to stress-slip relationships combined Polyphase tectonism commonly results in apparently large misfits (large with stress tensor determination. The automatic determination of subsets is

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Figure 3. Field examples of crosscutting relationships used in this study. (a) An undeformed Monteregian sill crosscutting a sinistral strike-slip fault that is compatible with a west-northwestÐeast-southeastÐoriented compression (site 50). Vertical view. Pencil for scale. (b) Crosscutting slickenlines along a sin-

gle fault plane. Subhorizontal striations (L1) are crosscut by steeply plunging striations (L2) (site 25). Vertical view. Pencil for scale.

made on the basis of the smallest misfit, that is, each fault slip is attributed the relative chronology established on the basis of a few observations to all to the tensor that results in the best individual fit. Consequently, the sepa- data collected in that site. The succession of brittle events locally estab- ration process does not depend on the choice of any threshold value. Be- lished in various sites can then be correlated throughout large areas on the cause the calculated tensors and the composition of subsets (hence the mis- basis of consistencies in stress regimes and corresponding trends, and a re- fits) are all unknown and interdependent, the iterative approach is gional paleostress field can be proposed for each major tectonic event. indispensable. The calculation is performed following a general algorithm proposed by Diday (1971) and modified by Angelier and Manoussis Case Examples of Polyphase Tectonism from Post-Late (1980), all variables being left free until a stable situation is obtained so Devonian Rocks that no better fit can be obtained for the number of events considered. The consistency and quality of the subsets finally identified are estimated ex- The detailed geometry of fault populations and their corresponding stress actly in the same way as for monophase sets. In most cases, a given striated tensors for the Carboniferous rocks of the southern Gaspé Peninsula and for fault surface can be unambiguously associated with a specific stress state Late Devonian plutons of southern Québec are presented in Figures 5 and 6. because one of its final misfits is acceptable, whereas the others are too These two examples illustrate both the determination of paleostress axes from large. Ambiguous situations are revealed by the presence of two or several brittle structures and the age constraints on post-Acadian tectonic events. acceptable misfit values for a single datum. Field observations and stress On the Gaspé Peninsula (Fig. 5), the Maritimes basin crops out along the orientations consistent with the existence of computed stress tensors from northern shore of Chaleurs Bay and consists of conglomerates, sandstones, automated data subsets also play an essential role for checking the validity shales, and minor basalts and limestones (Zaitlin and Rust, 1983). The age of stress determinations. of these rocks is believed to be Visean (Rust, 1981). The strata are subhor- An example of the separation of stress regimes is shown in Figure 4. For izontal except at some sites adjacent to reactivated Acadian faults (Kirk- the entire dataset (Fig. 4A), three subsets were identified and, respectively, wood, 1989; Malo and Bourque, 1993). In the Maritimes basin, these Car- correspond to north-northwestÐsouth-southeastÐtrending compression with boniferous rocks constrain the age of the brittle structures. The site 14 measurements (Fig. 4B), north-northeastÐsouth-southwestÐtrending locations, geometry, and paleostress analysis of striated fault planes mea- compression with 20 measurements (Fig. 4C), and west-northwestÐeast- sured in these rocks are shown in Figure 5 (see also Table 1). Neoformed, southeastÐtrending compression with 18 measurements (Fig. 4D), each sub- conjugate faults occur along thin (<1 cm) and planar, calcite- or quartz- set corresponding to reliable angular misfits (ANG less than 20¡). Remain- filled fractures with slickensides and well-developed congruent steps. Fault ing faults (Fig. 4E) correspond to large ANG values and are incompatible lengths vary from one to hundreds of meters, and fault offsets range from with calculated stress regimes and do not reveal the existence of a fourth a few centimeters to tens of meters. Reverse dip-slip motions occur along consistent stress state. Such automatic processing of fault-slip data allows a conjugate, moderately dipping (<45¡) east-westÐstriking faults (sites 74a rigorous separation of consistent fault-slip subsets. and 79b). Oblique slip occurs along moderately to steeply dipping, north- The compilation of various sources of chronological information (age of northeastÐ and northwest-striking conjugate faults (site 81a), whereas sub- faulted rocks, syntectonic mineral development, crosscutting slickenlines, vertical strike-slip faults were found at site 76a. All sites show a single recognition of neoformed fracture patterns) enables us to establish the phase of compression, except for site 81a, in which two different directions chronology of brittle deformation events at several sites. The number of of compressional axes were recognized. chronological data is generally small in comparison to the number of brit- Three preferred orientations of σ1 were established in the Carboniferous tle measures from a particular site; it is therefore necessary to extrapolate rocks. Sites 79b and 81a-2 show a north-northwestÐsouth-southeastÐoriented

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Figure 4. Stereographic projections illus- trating the separation of fault-slip dataset into homogeneous subsets during this study. (A) Data set for site 30. (B) Subset indicating north-northwestÐsouth-southeast compression. (C) Subset indicating north-north- eastÐsouth-southwest compression. (D) Sub- set indicating west-northwestÐeast-southeast compression. (E) Remaining faults inconsis- tent with calculated stress regimes. Fault planes are shown as thin continuous lines, slickenside lineations are indicated by black dots with thin arrows (centripetal arrows for reverse slip, double arrows for strike slip).

maximum compressional stress with a subvertical σ3. Sites 74a and 81a-1 vary between 383 ± 3 Ma and 374 ± 2 Ma (Simonetti and Doig, 1990). record a common, north-northeastÐsouth-southwestÐoriented maximum They are interpreted as late to post-Acadian intrusions for the following compression, whereas a west-northwestÐeast-southeastÐoriented compres- reasons: (1) most plutons have a rounded shape, (2) pluons and their meta- sional regime, related to strike-slip faults, is observed at site 76a. The Φ ratio morphic aureoles are undeformed, and (3) the 375 ± 3 Ma Aylmer (site 34 varies from 0.11 to 0.63, with a mean value of 0.40. The relative chronology in Fig. 6) pluton crosscuts the La Guadeloupe fault (Labbé and St-Julien, between the three paleostress orientations has not been established. 1989). Because of their homogeneous rheological behavior, such plutons In southern Québec (Fig. 6), Late Devonian intrusions are hosted by sed- represent excellent targets to study paleostress evolution. imentary rocks of the Gaspé belt. U-Pb zircon ages of these granitic rocks A large number of millimeter-thick veins and centimeter-wide aplite

Figure 5. Stereographic projections of striated fault planes and corresponding paleostress axes for Carboniferous rocks (in gray) of southern Gaspé Peninsula. Acadian shear zones are shown as thick gray lines (GPF: Grand Pabos fault). Same symbols as in Figure 4. Numbers in the upper left of stereograms refer to site locations of Figure 2 and Table 1.

1472 Geological Society of America Bulletin, November 1996

Figure 6. Stereographic projections of striated fault planes and corresponding paleostress axes for the Late Devonian plutons (in gray) of the southern Québec Appalachians. BBL: Baie VerteÐBrompton line; GF: La Guadeloupe fault. Other symbols as in Figure 5.

dikes cut the intrusive rocks. Most of these filled fractures were formed dur- Post-Acadian Paleostress History of Pre-Late Devonian Rocks ing the cooling history of plutons, and were reactivated as strike-slip faults during subsequent deformation. Although the detailed geometry of faults in To determine the regional extent of compression deduced from analysis each pluton is different (due to different cooling histories), the orientations of structures in Carboniferous rocks and in Late Devonian plutons, we col- of σ1 axes determined from striated faults are generally similar. Sample lected fault data at many sites within the St. Lawrence lowlands, the Hum- sites 26, 33, and 34 record a single phase of compression, whereas two com- ber and Dunnage zones, and the Gaspé belt (Fig. 1). At a regional scale, pressive events were found in sites 24, 25, and 37 (Fig. 6). With the excep- calculated stress axes from these rocks are consistent with paleostress re- tion of site 26, all sample sites gave calculated σ1 trends between N340 and constructions carried out in post-Late Devonian rocks of the Québec Ap- N348. In addition, an west-northwestÐeast-southeastÐoriented compression palachians. All trends of computed paleostress axes, including those ob- is well documented at sites 24 (N297/08), 25 (N291/13), and 37 (N296/03). tained in the Chaleurs Bay area and in southeastern Québec (Figs. 5 and 6), Both north-northwestÐsouth-southeast and west-northwestÐeast-southeast are compiled in three different maps (Figs. 7, 8, and 9). Because the re- directions of compression are clearly defined. For all stress tensors deter- gional dataset is too large to be completely described in this paper, only mined in these plutons, the Φ ratio ranges from 0.04 to 0.41 with a mean representative fault-slip datasets are presented along with calculated pale- value of 0.20, suggesting that brittle deformation was compressional with ostress axes. similar σ2 and σ3 magnitudes compared to σ1. The relative chronology be- North-NorthwestÐSouth-Southeast Compression. A north-north- tween west-northwestÐeast-southeast and north-northwestÐsouth-southeast westÐsouth-southeastÐoriented compressional event was determined from compressions is documented at sites 25 and 37 on five different fault planes 73 sites (Fig. 7). It is characterized by a uniform trend over a large area and where steeply plunging slickenlines associated with the west-northwestÐ is the most significant event in terms of fault populations (78% of all data). east-southeast compression crosscut shallow-plunging slickenlines formed Approximately 50% of these calculated paleostress tensors indicate strike- during the north-northwestÐsouth-southeast compression (Fig. 3b). How- slip faulting, whereas all other sites, including all sites in serpentinites ever, the timing between the north-northwestÐsouth-southeast and the cropping out along the Baie VerteÐBrompton line, suggest a reverse-fault north-northeastÐsouth-southwest compression (only observed in site 26) re- regime. The trends of computed σ1 axes cluster around N343 (Fig. 7). Sites mains unknown. located in the western part of the Humber zone (Fig. 1) show a slight clock-

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Figure 7. Paleostress axis orientations for the north-northwestÐsouth-southeastÐdirected compressional event in the Québec reentrant. Rose diagram shows the orientation of maximum (σ1) and minimum (σ3) compressional stress axes. Site numbers are shown in Figure 2. Other symbols as in Figure 5. Labels as in Figure 1.

wise rotation of σ1 axes (Fig. 7), which may be attributed to the influence 147; Table 1). At four of these locations (5, 6, 30, and 141), well-exposed of the shape of Proterozoic cratonic margin. Several low-dipping faults at- crosscutting relationships of slickensides indicate that the north-north- tributed to thrusting in Taconian or Acadian time were reactivated during westÐsouth-southeast compression predated the north-northeastÐsouth- the north-northwestÐsouth-southeastÐtrending compression (Faure, 1995). southwest compression. At site 5, few moderately dipping, west-south- The values of the ratio Φ show a large dispersion (Table 1), which is not westÐeast-northeastÐtrending faults moved first as reverse-oblique, dextral surprising considering both the variety of lithologies (volcanic and intru- faults, then as sinistral strike-slip faults. On east-westÐtrending, moderately sive rocks, limestones, and sandstones) and of structural settings in the belt. south-dipping faults at site 30, several shallow east-southeastÐplunging North-NortheastÐSouth-Southwest Compression. Faults related to a slickensides lineations exhibiting dextral movement are crosscut by south- north-northeastÐsouth-southwestÐoriented compressional direction repre- southwest down-dip slickensides showing reverse movement. sent 14% of the dataset and occur at 33 sites (Fig. 8). This compressional West-NorthwestÐEast-Southeast Compression. In the Late Devonian direction is mainly observed in southern Québec and in the Gaspé Penin- plutons and Carboniferous rocks, the west-northwestÐeast-southeastÐori- sula, where the regional structural trend is northeast-southwest. In the cen- ented compressional direction was found to be younger than the north- tral part of the Québec Appalachians where regional structures are parallel northwestÐsouth-southeast compression (see text). However, in pre-Late to the north-northeastÐsouth-southwestÐoriented σ1 axes, only one site Devonian rocks, interpretation of the west-northwestÐeast-southeastÐori- records this compression. In Gaspé, several east-westÐoriented Acadian ented compression raises a problem because the associated σ1 axes are faults have been reactivated as reverse strike-slip faults. The orientation of nearly coaxial with the maximum principal stress axes of both the Tacon- σ1 axes range from N003 to N030 with a mean value of N019 (Fig. 8). Φ ian and the Acadian deformation (Faure, 1995; Faure et al., 1994). One or ratios average 0.22Ð0.39, depending on rock type (Table 1). The north- two generations of slickenlines with orientations consistent with a west- northwestÐsouth-southeast and north-northeastÐsouth-southwestÐoriented northwestÐeast-southeast compression were commonly found in Cambrian compressional axes coexist in eight sites (sites 5, 6, 30, 81a, 132, 139, 141, to Middle Devonian rocks and were shown to be older than both the north-

1474 Geological Society of America Bulletin, November 1996

Figure 8. Paleostress axis orientations for the north-northeastÐsouth-southwestÐdirected compressional event in the Québec reentrant. Rose diagram shows the orientation of maximum (σ1) and minimum (σ3) compressional stress axes. Site numbers are shown in Figure 2. Other symbols as in Figure 5. Labels as in Figure 1.

northwestÐsouth-southeast and the north-northeastÐsouth-southwest com- is 0.34 in the serpentinites of the Baie VerteÐBrompton line, and 0.29 in pressions (Paradis and Faure, 1994). For this reason, the extent of the post- other rock types (Table 1). Acadian west-northwestÐeast-southeast compression is less-clearly de- In summary, faults related to west-northwestÐeast-southeast compres- fined in pre-Devonian rocks than in the younger rock units (Fig. 9). sion appearing to be older than the north-northwestÐsouth-southeast and West-northwestÐeast-southeast compressional stresses of inferred post- north-northeastÐsouth-southwest compressions were not considered in this Acadian age were, however, found in eight sites (sites 21, 30, 42, 57, 58, paper because they are attributed to Acadian-Taconian deformation (Faure, 95, 130, 137; Figs. 2 and 9). In these sites, crosscutting slickenlines on sev- 1995). Faults related to west-northwestÐeast-southeast compression eral fault planes have shown the west-northwestÐeast-southeast compres- younger than the north-northwestÐsouth-southeast and north-northeastÐ sion to be systematically younger than the north-northwestÐsouth-south- south-southwest compressions determined above are correlated to the east and north-northeastÐsouth-southwest compressions. Furthermore, in west-northwestÐeast-southeast compression found in Late Devonian plu- southern Québec, fault planes related to the west-northwestÐeast-southeast tons and Carboniferous rocks. compression have been found in Late Devonian plutons, in the Dunnage zone and along the Baie Verte–Brompton line. In the Gaspé Peninsula and DISCUSSION northern New Brunswick, west-northwestÐeast-southeastÐoriented compressional axes are found along the Grand Pabos and the Rocky Brook Age Constraints Millstream faults (Figs. 1 and 9). The orientation of calculated σ1 axes ranges from N090 to N119; the mean value is N108. Strike-slip deforma- On the basis of crosscutting relationships between the different genera- tional regimes are mainly found in southern Québec. The average Φ ratio tions of slickensides, we propose a relative chronology of the three direc-

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Figure 9. Paleostress axis orientations for the east-southeastÐwest-northwestÐdirected compressional event in the Québec reentrant. Rose diagram shows the orientation of maximum (σ1) and minimum (σ3) compressional stress axes. Site numbers are shown in Figure 2. Other symbols as in Figure 5. Labels as in Figure 1.

tions of compressive paleostress axes; an earlier north-northwestÐsouth- found by McHone (1978) and Eby (1984). On the basis of 40Ar/39Ar ages southeastÐoriented compressional phase followed by a north-northeastÐ determined from biotite and amphibole, Foland et al. (1986) suggested, south-southwestÐoriented compression, and a late west-northwestÐeast- however, a restricted time of emplacement of 124 ± 1 Ma. An undeformed southeastÐoriented compressional direction. Such an interpretation implies Monteregian sill in site 50 crosscuts a sinistral strike-slip fault that is com- that, at any given time, stress fields are assumed to have been practically patible with the west-northwestÐeast-southeastÐoriented compression uniform across the studied area. Present-day stress fields in orogenic belts (Fig. 3a). This observation suggests that the compressional directions pre- vary significantly in time and spatial distribution, and such a uniform stress sented here predate the Cretaceous intrusions. Assuming that a crustal ex- distribution in the Québec Appalachians may appear to be surprising. How- tensional regime associated to the opening of the Atlantic ocean character- ever, the studied area represents a linear and small segment of the Appala- ized the eastern margin of North America during Late TriassicÐJurassic chian orogen that is entirely located within the Québec reentrant, and as time (McHone, 1978, 1988; de Boer et al., 1988; Klitgord et al., 1988), such, it is characterized by relatively homogeneous orogenic constraints. which is consistent with the presence of an early Middle Jurassic dike in In the study area, the lower age limit for the deformation related to these southern Québec (Roddick et al., 1992), one may conclude that the age of compressional axes is given by the age of rocks in which it occurred. Cal- compressional directions described above is restricted to a period of culated compressional paleostresses are clearly post-Acadian because they ≈100 m.y., between post-Viséan time (ca. 330 Ma) and the breakup of affect Late Devonian plutons of southern Québec and Carboniferous rocks Pangea in Late Triassic time (ca. 230 Ma). Such time interval overlaps the of the Gaspé Peninsula. In the Québec Appalachians, the youngest rocks age of the late Paleozoic Alleghanian orogeny that is well recorded in the comprise the Monteregian intrusions and related dike swarms (Fig. 1) southern Appalachians of North America (Hatcher, 1989), as well as in Eu- where K-Ar ages ranging from 96 Ma to 136 Ma (Cretaceous) have been rope and in northwestern Africa (Arthaud and Matte, 1977).

1476 Geological Society of America Bulletin, November 1996

Alleghanian Orogen in the Northern Appalachians Structures related to the formation of the Maritimes basin may be correlative to the north-northeast and north-northwestÐoriented compressional In the Appalachian belt, the Alleghanian orogeny is interpreted as the re- axes found in Québec and northern New Brunswick. In southern New sult of convergence and collision between Laurentia (North America) and Brunswick, contractional structures of Early Carboniferous age (Ruiten- Gondwana (Africa) in Late Carboniferous to Early Permian time (Hatcher berg and McCutcheon, 1982) provide evidence that compression was act- et al., 1989). In the Québec and northern New Brunswick Appalachians, no ive during basin formation. The calculated north-northwestÐoriented com- structures have been unequivocally assigned to an Alleghanian deformation. pressional direction, moreover, is consistent with dextral faulting along the The stress tensor analysis presented in this paper suggests, however, that Cobequid-Chedabucto fault zone. However, in Maritime Canada there ex- brittle faults in Carboniferous rocks and Late Devonian plutons, as well as ists no regional folding event compatible with the north-northwestÐsouth- many fault populations in lower and middle Paleozoic rocks, represent a dis- southeast and north-northeastÐsouth-southwestÐoriented compressional tant expression of the Alleghanian orogeny and not Mesozoic rifting. axes determined in this study, as well as no structural evidence for early Lower Carboniferous rocks of the Gaspé Peninsula belong to the Mar- sinistral faulting (Keppie, 1982; Mawer and White, 1986). This relation itimes basin (Fig. 1) which extends from the Chaleurs Bay in Québec to suggests that such compressional directions represent discrete deforma- Nova Scotia to the east and Newfoundland to the north (Williams, 1974). tional events in the development of the Maritimes basin. However, early The tectonic evolution of the Maritimes basin and related subbasins is sinistral movement along northeast-southwestÐoriented faults in southern complex, mostly because sedimentation and faulting were active from New England is consistent with the north-northwest and north-northeast Middle Devonian to Early Permian time (Bradley, 1982; Gibling et al., compressions. Late Carboniferous to Permian dextral strike-slip faults and 1987). Models involving a large amount of displacement along major fold trends of Maritime Canada, and the dextral transpressive regime along strike-slip faults and the development of pull-apart basins were proposed the New England coast, are mechanically and temporally consistent with by Webb (1969), Fralick and Schenk (1981), Bradley (1982), and Gibling the west-northwestÐeast-southeastÐoriented compressional event recog- et al. (1987). More recently, it has been suggested that the Maritimes basin nized during our study. This is supported by qualitative stress analyses evolved during a period of crustal extension and subsidence following a along the Cobequid-Chedabucto fault zone (Eisbacher, 1969) as well as by major phase of lithospheric thinning (Durling and Marillier, 1993; Lynch the analysis of joints and tensile strength orientations in southern New and Tremblay, 1994). In the New England Appalachians, post-Acadian Brunswick and Prince Edward Island (Lajtai and Stringer, 1981). basins are slightly younger than the Maritimes basin; they formed mostly from Westphalian A or B to Stephanian B or C time (320Ð290 Ma; Lyons Layer-Parallel Shortening Fabrics and Joint Patterns in the and Darrah, 1978; Skehan and Murray, 1979). The Narragansett basin is in- Appalachian Foreland terpreted to have formed as a pull-apart basin within a left-lateral strike-slip fault system (McMaster et al., 1980; Mosher, 1983; Skehan et al., 1986). Since the mid 1960s, many workers have used layer-parallel shortening The oldest Alleghanian compressional deformation is documented in fabrics (LPS fabrics) and joint patterns, mostly in the Appalachian foreland Early Carboniferous rocks of southern New Brunswick, which are affected (Fig. 1), to infer the regional shortening and paleostress directions related by gently plunging west-southwestÐeast-northeastÐtrending folds and by to the Alleghanian orogeny (Nickelsen, 1966; Geiser, 1974; Engelder and north-northwestÐsouth-southeastÐtrending extensional veins (Ruitenberg Geiser, 1980; Geiser and Engelder, 1983; Dean et al., 1988; Gray and Mi- and McCutcheon, 1982). Compressive deformation affecting the Maritimes tra, 1993, Evans, 1994). Stratigraphic relationships indicate that, in the basin began in post-Westphalian B time (310Ð305 Ma) and ended in Early foreland of the U.S. Appalachians, Alleghanian deformation is character- Permian time (270 Ma; Keppie, 1982). It is attributed to the accretion of the ized by thin-skinned thrusting and folding above major detachment faults Meguma zone to North America during a late stage of the Alleghanian (Hatcher et al., 1989). orogeny (Keppie, 1982; Nance, 1987). This regional deformation is charac- The analysis of joint distributions and LPS fabrics in the Appalachian terized by right-lateral, east-westÐoriented strike-slip faults, northeast-south- foreland sequence reveals two main directions of compression; an older westÐtrending folds and thrust faults, and by a pervasive greenschist-grade north-southÐoriented and a younger west-northwestÐeast-southeastÐori- metamorphism (Keppie, 1982). Tight folding and ductile shearing occur ented compression axis (Engelder and Geiser, 1980; Dean et al., 1988; mainly along the Cobequid-Chedabucto fault zone (Keppie, 1982; Mawer Gray and Mitra, 1993; Evans, 1994). In the northern part of the Appala- and White, 1986). The westernmost Alleghanian fold axes are found in Early chian Plateau (western Pennsylvania and southern New York), Alleghanian Permian rocks of Prince Edward Island (Frankel and Crowl, 1969). In structures are east trending, and LPS fabrics indicate compressional stress Maine, most Alleghanian deformation occurs along the Norumbega fault axes varying in orientation from N340 to N010 (Engelder and Geiser, zone, which is a major transpressive strike-slip fault (Ludman, 1986; Swan- 1980; Geiser and Engelder, 1983). In this area, two sets of joints oriented son, 1992). Late Carboniferous rocks of southern New England, however, north-northwestÐsouth-southeast and north-northeastÐsouth-southwest record a different stress regime and deformation was more intense than in the were found and attributed, respectively, to the Lackawana phase and to the Canadian Maritimes (Mosher, 1983; Wintsch and Sutter, 1986). An earlier main phase of the Alleghanian orogeny. To the south of the Appalachian episode of Alleghanian deformation occurred between 295 and 275 Ma, and Plateau, in central Pennsylvania, Gray and Mitra (1993) found similar is characterized by two synchronous generations of north-south and north- shortening directions and demonstrated that they resulted from a progres- east-southwestÐtrending folds. In this area, the regional Alleghanian meta- sive clockwise rotation of Alleghanian compressional stress axes from morphism reached upper amphibolite grade (Dallmeyer, 1982). This defor- N337 to N007. Similarly trending sets of joints have been found also in the mation was followed by sinistral strike-slip faulting along north-northeastÐ northern Michigan basin (Holst, 1982). In the central Appalachian fore- south-southwestÐtrending shear zones (Mosher and Berryhill, 1991), which land, major Alleghanian structures are oriented north-northeastÐsouth- were initiated by a north-southÐ oriented shortening direction (Goldstein, southwest, and both north-southÐoriented and west-northwestÐeast-south- 1994). During a later stage of the Alleghanian orogeny, dextral strike-slip eastÐoriented shortening directions have been found (Dean et al., 1988; faulting along older northeast-southwestÐtrending structures is related to Evans, 1994). Using the attitude of joints, veins, and striated fault planes, west-northwestÐeast-southeast compressional directions (Mosher and Berry- Evans (1994) documented a counterclockwise rotation of compressional hill, 1991; Getty and Gromet, 1988). axes from N340Ð360 to N270Ð295. Farther south in the central Appala-

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chian Plateau (Virginia), Dean et al. (1988) also found an earlier compres- took place in the southern Appalachians. The sedimentological record, sion axis oriented between N330 and N350 that is followed by a progres- metamorphic ages, and directions of paleostress axes reported in this paper sive counterclockwise rotation of the compressional axis to N275. are in better agreement with the latter interpretation. During much of the The orientation of horizontal maximum compressive strain axes based Carboniferous, the northern Appalachians were subjected to a phase of on mechanical twins of calcite crystals is also consistent with paleostress lithospheric thinning that caused the formation of major sedimentary studies in the Appalachian Plateau. Limestones of the Appalachian Plateau basins. In contrast, the central and southern Appalachians were character- and of the Hudson River Valley record north-southÐ and west-northwestÐ ized by a compressive phase during which clastic wedges were formed in east-southeastÐoriented compressive directions (Craddock et al., 1993; response to the advance and uplift of metamorphic terranes at the south- Sierra et al., 1993). In areas located up to 1700 km northwest of the Appa- eastern edge of the Laurentian plate (Secor et al., 1986). In the southern lachian foreland, Jackson et al. (1989), Craddock and van der Pluijm Appalachians, high-grade metamorphism occurred between 315 and (1989), and Craddock et al. (1993) found that most directions of compres- 285 Ma (Secor et al., 1986). Along the New York promontory, ductile de- sion are perpendicular to the Appalachian-Ouachita tectonic front. formation and metamorphism occurred later, mainly between 295 and All these results are consistent with the orientation and the relative 275 Ma (Reck and Mosher, 1988). Northward, the intensity of metamor- chronology of compressional directions that we found in the Québec reentrant. The early north-northwestÐsouth-southeast compressional axes in Québec and New Brunswick are correlative to the earliest tectonic event recorded in the Appalachian foreland. Our data also are consistent with the clockwise rotation of the compressional axes from the north-northwestÐ south-southeast to north-northeastÐsouth-southwest observed in the northern Appalachian foreland. The rotation of the stress axis toward a west- northwest–east-southeast–directed compression in the Québec reentrant and in central Appalachian foreland appears to be coeval with the dextral strike-slip system in the northern Appalachians and with the northwest- vergent thrusts in central and southern Appalachians. However, there are currently no clear and systematic variations in the distribution of stress among areas located to the northeast and the southwest of the New York promontory, suggesting that stress patterns are possibly independent of such small curvature (i.e., the New York promontory) in the collision zone.

MODEL FOR THE TECTONIC EVOLUTION OF THE ALLEGHANIAN OROGENY

The variation in the orientation of paleostress across the northern Ap- palachians can be integrated into a dynamic model. According to Lefort (1988), Vauchez et al. (1987), and Piqué and Skehan (1992), the structural evolution of the Alleghanian orogeny was mostly controlled by the preex- isting geometry of the collision zone between Laurentia and Gondwana, and they suggested that the Reguibat uplift of northwestern Africa acted as rigid indentor at the latitude of the New York promontory. Although such an indentor model is consistent with the actual trend of Alleghanian structures, we believe that the sequence in which the collision occurred can be refined with our paleostress tensor analysis. Piqué and Skehan (1992) stated that the Alleghanian deformation developed first in the northern Appalachians and assumed a convergence vector orientation close to east-west (present-day orientation). However, Secor et al. (1986), and Sacks and Secor (1990) suggested that the initial collision

Figure 10. Schematic representation of the collision between Gond- wana and Laurentia during the Alleghanian orogeny showing the inferred vector displacement of Gondwana (black arrow) and the direction of stress induced in Laurentia (dashed line). (a) Early stage of deformation in the southern Appalachians related to the collision of the Reguibat uplift (RU) with the New York promontory (NYP) under a north-northwestÐsouth-southeastÐdirected compression. (b) Pro- gressive indentation of the Reguibat uplift and Laurentia that caused a fanning of the stress field in Laurentian craton. (c) Final accretion between Gondwana and Laurentia and dextral strike-slip induced by east-southeastÐwest-northwest compression.

1478 Geological Society of America Bulletin, November 1996

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L., ed., Continental deformation: Oxford, United Kingdom, Pergamon Press, p. 53Ð100. Van Der Voo, 1981). In the first stage of collision between the Reguibat up- Angelier, J., and Manoussis, S., 1980, Classification automatique et distinction de phases superposées en tec- lift and the New York promontory, the stress was transferred to the North tonique cassante: Paris, Académie des Sciences Comptes Rendues, v. 290, p. 651Ð654. Angelier, J., Colletta, B., and Anderson, R. E., 1985, Neogene paleostress changes in the Basin and Range: A American plate throughout the Québec reentrant (Fig. 10a). Deformation case study at Hoover Dam, Nevada-Arizona: Geological Society of America Bulletin, v. 96, p. 347Ð361. 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C., 1982, Subsidence in late Paleozoic basins in the northern Appalachians: Tectonics, v. 1, p. 107Ð123. in part to the north-northwestÐsouth-southeastÐdirected compression. The Craddock, J. P., and van der Pluijm, B. A., 1989, Late Paleozoic deformation of the cratonic carbonate cover of progressive indentation of the Reguibat uplift to North America induced a eastern North America: Geology, v. 17, p. 416Ð419. Craddock, J. P., Jackson, M., van der Pluijm, B. A., and Versical, R. T., 1993, Regional shortening fabrics in east- progressive clockwise rotation of the maximum compressive stress from ern North America: Far-field stress transmission from the Appalachian-Ouachita orogenic belt: Tecton- ics, v. 12, p. 257Ð264. the north-northwestÐsouth-southeast to the north-northeastÐsouth-south- Dallmeyer, R. D., 1982, 40Ar/39Ar ages from the Narragansett Basin and southern Rhode Island basement ter- west, both in the Québec reentrant and in the northern Appalachian fore- rane: Their bearing on the extent and timing of the Alleghanian tectonothermal events in New England: Geological Society of America Bulletin, v. 93, p. 1118Ð1130. land (Fig. 10b). However, in the central and southern Appalachian foreland Dean, S. L., Kulander, B. R., and Skinner, J. M., 1988, Structural chronology of the Alleghanian orogeny in the direction of compressional axes rotated counterclockwise from north- southeastern West Virginia: Geological Society of America Bulletin, v. 100, p. 299Ð310. de Boer, J. Z., McHone, J. G., Puffer, J. H., Ragland, P. C., and Whittington, D., 1988, Mesozoic and Cenozoic northwestÐsouth-southeast to west-northwestÐeast-southeast. During a late magmatism, in Sheridan, R. E., and Grow, J. A., eds., The Atlantic continental margin: U.S.: Boulder, Col- stage of the Alleghanian orogeny, the collision of the African and North orado, Geological Society of America, Geology of North America, v. I-2, p. 217Ð241. de Roo, J., and van Staal, C. R., 1994, Transpression and extensional collapse: Steep belts and flat belts in the American plates was followed by a modification of the convergence direc- Appalachians central mobile belt, northern New Brunswick, Canada: Geological Society of America Bul- letin, v. 106, p. 541Ð552. tion (Fig. 10c). To accommodate the ongoing deformation, a drastic change Diday, E., 1971, Une nouvelle méthode de classification automatique et reconnaissance des formes: la méthode in stress trajectory toward a west-northwestÐeast-southeast orientation des nuées dynamiques: Revue de Statistiques Appliquées, v. 15, p. 283Ð300. Dupin, J.-M., Sassi, W., and Angelier, J., 1993, Homogeneous stress hypothesis and actual fault slip: A distinct gave rise to widespread dextral strike-slip fault zones that transected ear- element analysis: Journal of Structural Geology, v. 15, p. 1033Ð1043. lier Alleghanian structures along the eastern edge of the Appalachians Durling, P., and Marillier, F., 1993, Structural elements of the Magdalen basin, Gulf of St. Lawrence, from seis- mic reflection data, in Current research: Geological Survey of Canada Paper 93-1D, p. 147Ð154. (Mosher, 1983; Secor et al., 1986; Valentino et al., 1994). In the Québec Eby, G. N., 1984, Geochronology of the Monteregian Hills alkaline igneous province, Québec: Geology, v. 12, reentrant, the west-northwestÐeast-southeast compression is thus inter- p. 468Ð470. Eisbacher, G. H., 1969, Displacement and stress field along part of the Cobequid fault, Nova Scotia: Canadian preted to be the result of the final stage of collision between Laurentia and Journal of Earth Sciences, v. 6, p. 1095Ð1104. Engelder, T., and Geiser, P., 1980, On the use of regional joint sets as trajectories of paleostress fields during the de- Gondwana. velopment of the Appalachian Plateau, New York: Journal of Geophysical Research, v. 85B, p. 6319Ð6341. Evans, M. A., 1994, Joints and décollement zones in Middle Devonian shales: Evidence for multiple deformation events in the central Appalachian Plateau: Geological Society of America Bulletin, v. 106, p. 447Ð460. CONCLUSION Faure, S., 1995, Reconstitution des paléocontraintes tectoniques dans les Basses-Terres du Saint-Laurent et les Appalaches du Québec et du nord du Nouveau-Brunswick [Ph.D. thesis]: Ste-Foy, Université du Québec, INRS-Géoressources, 259 p. Faure, S., Tremblay, A., and Malo, M., 1994, Paleostress tensor analysis in the northern Appalachians: Geolog- This study documents the influence of the Alleghanian orogen north and ical Society of America Abstracts with Programs, v. 26, no. 3, p. 16Ð17. west of the main orogenic belt, in the Québec and northern New Bruns- Foland, K. A., Gilbert, L. A., Sebring, C. A., and Jiang-Feng, C., 1986, 40Ar/39Ar ages for plutons of the Mon- teregian Hills, Québec: Evidence for a single episode of Cretaceous magmatism: Geological Society of wick Appalachians. Ductile structures are absent, but conjugate sets of re- America Bulletin, v. 97, p. 966Ð974. verse and strike-slip brittle faults are the main tectonic features. The stress Fralick, P. W., and Schenk, P. E., 1981, Molasse deposition and basin evolution in a wrench tectonic setting: The late Paleozoic, eastern Cumberland basin, Maritime Canada, in Miall, A. D., ed., Sedimentation and tec- tensor analysis of these faults correlates well with regional shortening di- tonics in alluvial basins: Geological Association of Canada Special Paper 23, p. 77Ð97. rections reported for the Appalachian foreland. Three compressional stress Frankel, L., and Crowl, G. H., 1969, PermoÐCarboniferous stratigraphy and structure of central Prince Edward Island: Geological Survey of Canada Paper 69-17, 20 p. directions related to the Alleghanian orogen have been distinguished in the Geiser, P. A., 1974, Cleavage in some sedimentary rocks of the central Valley and Ridge Province, Maryland: Geological Society of America Bulletin, v. 85, p. 1399Ð1412. Québec reentrant: an early north-northwestÐsouth-southeast compression, Geiser, P., and Engelder, T., 1983, The distribution of layer parallel shortening fabrics in the Appalachian fore- which is the most significant in terms of numbers of faults, followed by a land of New York and Pennsylvania: Evidence for two non-coaxial phases of the Alleghanian orogeny, in Hatcher, R. D., Jr., Williams, H., and Zietz, I., eds., Contributions to the tectonics and geophysics of moun- progressive rotation to a north-northeastÐsouth-southwestÐdirected com- tain chains: Geological Society of America Memoir 158, p. 161Ð175. pression, and by a drastic change in stress orientation to a west-north- Getty, S. R., and Gromet, L. P., 1988, Alleghanian polyphase deformation of the Hope Valley shear zone, southeastern New England: Tectonics, v. 7, p. 1325Ð1338. westÐeast-southeast. Mesoscopic faults related to these three compres- Gibling, M. R., Boehner, R. C., and Rust, B. R., 1987, The Sydney basin of Atlantic Canada: An upper Paleo- sional events are interpreted as the intraplate effects of indentation during zoic strike-slip basin in a collisional setting, in Beaumont, C., and Tankard,A. J., eds., Sedimentary basins and basin-forming mechanisms: Canadian Society of Petroleum Geologists Memoir 12, p. 269Ð285. the collision of the irregular continental margin of Gondwana with the Goldstein, A. G., 1994, A shear zone origin for Alleghanian (Permian) multiple deformation in eastern Massa- chusetts: Tectonics, v. 13, p. 62Ð77. North American plate. Gray, M. B., and Mitra, G., 1993, Migration of deformation fronts during progressive deformation: Evidence from detailed structural studies in the Pennsylvania anthracite region, U.S.A.: Journal of Structural Geol- ogy, v. 15, p. 435Ð449. ACKNOWLEDGMENTS Hatcher, R. D., Jr., 1989, Tectonic synthesis of the U.S. Appalachians, in Hatcher, R. D., Jr., ed., The Appala- chian-Ouachita orogen in the United States: Boulder, Colorado, Geological Society of America, Geology of North America, v. F-2, p. 511Ð535. Hatcher, R. D., Jr., Thomas, W. A., Geiser, P. A., Snoke, A. W., Mosher, S., and Wiltschko, D. V., 1989, Al- This paper is part of Faure’s Ph.D. dissertation. The project was sup- leghanian orogen, in Hatcher, R. D., Jr., ed., The Appalachian-Ouachita orogen in the United States: Boul- ported by a doctoral fellowship of the FCAR (Fonds pour la Formation de der, Colorado, Geological Society of America, Geology of North America, v. F-2, p. 233Ð318. Holst, T. B., 1982, Regional jointing in the northern Michigan basin: Geology, v. 10, p. 273Ð277. Chercheurs et l’Aide à la Recherche) to Faure and by a Natural Sciences Jackson, M., Craddock, J. P., Ballard, M., Van der Voo, R., and McCabe, C., 1989, Anhysteretic remanent mag- Engineering Research Council of Canada operational grant (OGP 15029) netic anisotropy and calcite strains in Devonian carbonates from the Appalachian Plateau, New York: Tectonophysics, v. 161, p. 43Ð53. to Tremblay. We thank D. Kirkwood, G. Lynch, M. Malo, N. Pinet, and P. Keppie, J. D., 1982, The Minas geostructure, in St-Julien, P., and Béland J., eds., Major structural zones and Sacks for critical reviews, and T. Engelder, S. Mosher, B. van der Pluijm, faults of the northern Appalachians: Geological Association of Canada Special Paper 24, p. 263Ð280. Kirkwood, D., 1989, Géologie structurale de la région de Percé: Ministère Énergie et Ressources du Québec, R. Marret, and J. M. Bartley for formal reviews. ET 87-17, 42 p.

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R., 1982, Acadian and Hercynian structural evolution of southern New MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 28, 1995 Brunswick, in St-Julien, P., and Béland J., eds., Major structural zones and faults of the northern Ap- REVISED MANUSCRIPT RECEIVED MARCH 25, 1996 palachians: Geological Association of Canada Special Paper 24, p. 131Ð148. MANUSCRIPT ACCEPTED APRIL 22, 1996

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