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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 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 dur- ing 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 di- rections 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-northwest–south-southeast com- deformational events that were related to the formation of sedimentary pression, (2) a north-northeast–south-southwest compression, and (3) a basins and to the accretion of oceanic and continental terranes against the late west-northwest–east-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 Verte–Brompton 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. 1467–1480; 10 figures; 1 table. 1467 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/11/1467/3382436/i0016-7606-108-11-1467.pdf by guest on 23 September 2021 FAURE ET AL. 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 Verte–Brompton 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
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