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Downloaded from geology.gsapubs.org on March 19, 2010 Geology Stratigraphy and structure of the Laurentian rifted margin in the northern Appalachians: A low-angle detachment rift system John S. Allen, William A. Thomas and Denis Lavoie Geology 2009;37;335-338 doi: 10.1130/G25371A.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2009 Geological Society of America Downloaded from geology.gsapubs.org on March 19, 2010 Stratigraphy and structure of the Laurentian rifted margin in the northern Appalachians: A low-angle detachment rift system John S. Allen1, William A. Thomas1, Denis Lavoie2 1 Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA Geological Survey of Canada, Centre Géoscientifique de Québec, 490 de la Couronne, Quebec City, Quebec G1K 9A9, Canada 2 ABSTRACT The Neoproterozoic–Early Cambrian rifted margin of eastern Laurentia is framed by promontories and embayments defined by northeast-trending rifts offset by northwesttrending transforms. A regional first-order synthesis of the available stratigraphic data in the northern Appalachians reveals significant along-strike variations in the thickness, composition, age, and facies of synrift and early postrift stratigraphy. These variations are consistent with models for low-angle detachment rift systems and allow for resolution of regional structures specific to low-angle detachments, including upper-plate margins, lower-plate margins, and transform faults that bound zones of oppositely dipping low-angle detachments. et al., 1998). Early Cambrian postrift siliciclastic and carbonate deposits (Cheshire and Dunham Formations) above the synrift succession in the New England rift zone are regionally extensive and have a combined thickness of ~800 m, but thin abruptly northward across the international border (Osberg, 1969). In southeastern Quebec, synrift shelf clastic deposits (Pinnacle Formation) along the Quebec rift zone (Fig. 1) are thin (140–250 m) and conformably overlie bimodal volcanics of the ca. 554 Ma Tibbit Hill Formation (Marquis and Kumarapeli, 1993), suggesting that synrift deposition postdates synrift sedimentation in northern New England. Felsites in the Tibbit Hill Formation have trace and rare earth EXPLANATION SCALE 200 km 0 Rift N Transform fault Intracratonic basement fault Appalachian Y NA SILMI structural front UE EN G A B Thrust fault S RA G Baie VerteBrompton Line OT TAW GR AB A EN RI FT ZO NE ST. LAWRENCE PROMONTORY An tic os ti I sla nd NG G RA NG -M AY SE M NT O Y NC RE O FT RI PÉ NE AS O G Z SF AN PT -IL ES TR AN SF O RM 60°W RM O EC E EB ON U Q TZ F RI TR 70°W N UE RM FO NS RA D I T AN UO GL E SQ EN ZON W T NE RIF SI IS M QUEBEC EMBAYMENT 50°N E SA MP K YOR Y NEW ONTOR M PRO NORTHERN APPALACHAIN RIFTED MARGIN This article relies on a brief synthesis of significant synrift and early postrift stratigraphy along the Laurentian margin in the northern Appalachians. In palinspastically restoring Iapetan rift deposits in Appalachian thrust sheets, we assume thrust translation roughly orthogonal to regional northeast-trending foreland structures. On a regional scale, any obliquity in Paleozoic Appalachian thrust displacement is likely minimal and does not substantially affect our interpretations. Internal basement massifs are omitted because of uncertain palinspastic relationships with geo- graphically opposing segments of the Laurentian margin. Along the New England rift zone (Fig. 1), Iapetan synrift siliciclastic rocks (Pinnacle Formation) consist of alluvial-fan deposits that are geographically extensive and have a thickness of 2.0–3.5 km (Cherichetti et al., 1998). In northern Vermont, the Pinnacle Formation underlies and is interlayered with rift volcanics of the ca. 554 Ma Tibbit Hill Formation (Kumarapeli et al., 1989), indicating that synrift deposition in the New England rift zone began prior to the latest Neoproterozoic. Northward into southern Quebec, the Pinnacle Formation thins dramatically and is expressed as deltaic and coastal deposits (Marquis and Kumarapeli, 1993; Cherichetti LO INTRODUCTION Along-strike variations in synrift and early postrift stratigraphy are a powerful guide to the evolution of the upper crustal architecture of a continental rift. The exhumed stratigraphy exposed along deformed continental margins offers a rare glimpse into the mechanisms by which continents rift apart. Observations of the deformed Neoproterozoic–Cambrian synrift and postrift stratigraphy from northern Mexico to eastern Canada led to the proposal that promontories and embayments along the eastern Laurentian rifted margin are defined by northeast-trending rifts offset by northwesttrending transforms (Thomas, 1977), and that along-strike variations in the synrift and postrift stratigraphy reflect upper-plate, lower-plate, and transform segments of the margin in the southern Appalachians (Thomas, 1991, 1993; Thomas and Astini, 1999). This article synthesizes synrift and postrift stratigraphy and structures in New England and Canada and proposes a revised model for the northern Appalachians based on a low-angle detachment rift system. 45°N Figure 1. Interpreted Neoproterozoic–Early Cambrian continental margin defined by rift segments and transform faults (modified from Thomas, 1977). Map shows general outline of Paleozoic Appalachian orogenic belt and intracratonic basement fault systems. MP—Montmorency promontory; SILMI—Sept-Iles layered mafic intrusion. © 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY, April 2009 Geology, April 2009; v. 37; no. 4; p. 335–338; doi: 10.1130/G25371A.1; 3 figures. 335 Downloaded from geology.gsapubs.org on March 19, 2010 336 { 59°W 56°W SEG. 1 { HBA 49°N { SEG. 5 IER LONG { RANG SEG. 3 SEG. 4 CANAD A TRANS BAY FORM E INL { 51°N Belle Isle BELLE IS L TRANSF E ORM SEG. 2 BON N TRAN E BAY SFOR M SERP ENT TRAN INE LAK E SFOR M ST EE INL L MT IER N eratic beds (Saint-Damase Formation, formerly Cap Enragé Formation) increase in abundance and contain boulder-sized clasts of platform carbonate, rift basalt, and basement gneiss (Lavoie et al., 2003). The Sept-Iles transform offsets the rift by ~500 km from the Quebec embayment to the St. Lawrence promontory (Fig. 1). North of the transform on the Anticosti platform, thin (<855 m), autochthonous Early Ordovician shelf carbonates (Romaine Formation) unconformably overlie crystalline basement (Sanford, 1993). In contrast, south of the transform in the Appalachian allochthon on the Gaspé Peninsula are deep-water Middle Cambrian clastic deposits (Orignal Formation) (Lavoie et al., 2003). Northwest along strike of the transform is the large, ca. 565 Ma rift-related Sept-Iles layered mafic intrusion (Fig. 1) (Higgins and van Breeman, 1998). Geochemical and isotopic data indicate that magmas in the mafic intrusion were derived from an upper mantle source and that they did not interact with continental crust. A possible explanation is that fracture systems related to the Sept-Iles transform penetrated the entire crust and tapped the upper mantle, channeling magmas of the Sept-Iles layered mafic intrusion through the crust. The Long Range rift zone composes the length of the St. Lawrence promontory in western Newfoundland (Fig. 1), and can be divided into five segments on the basis of the distribution of synrift and postrift stratigraphy and basement massifs (Fig. 2). External basement massifs of Mesoproterozoic crystalline basement are located along segments 1, 3, and 5. Neoproterozoic–Early Cambrian synrift deposits along these segments consist exclusively of shelf deposits (Labrador Group). On Belle Isle (segment 1), shoreline outcrops of the Labrador Group contain a lower rift succession of faultbounded arkosic conglomerates (Bateau Formation) that vary locally in thickness (0–240 m) and are crosscut by Neoproterozoic basalt dikes (Lighthouse Cove Formation) (Williams, 1995). Unconformably overlying the lower rift succession are Early Cambrian sandstones (Bradore Formation) of the upper Labrador Group. Along segments 3 and 5, parautochthonous late synrift and early postrift Early Cambrian shallowmarine shelf facies consisting of siliciclastic deposits with minor carbonate (upper Labrador Group) unconformably overlie basement and are regionally thin (~240–740 m) (Knight, 1991; Williams 1995; Cooper et al., 2001). A Middle Cambrian–Middle Ordovician carbonate shelf (Port au Port Group through Table Head Group) oversteps the siliciclastic shelf and ranges from 1200 to 1500 m thick (Williams, 1995). Slope deposits in the Humber Arm allochthon (Fig. 2) along segments 3 and 5 consist of coarse Middle Cambrian passive-margin carbonate debris flows HAA element chemistries that resemble within-plate granites associated with thick continental crust (Kumarapeli et al., 1989). Rift-to-drift and early passive-margin deposits (Cheshire-Gilman and Dunham Formations) apparently do not exceed 600 m in total thickness and thin northward from the international border (Osberg, 1969; Marquis and Kumarapeli, 1993). Synrift slope deposits in southern Quebec (lower Armagh Formation and the Green Sandstone Unit) have an estimated thickness of 600 m, overlie rift basalts of the 550 ± 7 Ma Mt. St-Anselme Formation (Hodych and Cox, 2007), and contain late Early Cambrian macrofauna and acritarchs (Lavoie et al., 2003). Along the Gaspé rift zone (Fig. 1), Laurentian shelf deposits are buried beneath the Appalachian allochthon except along the St. Lawrence Estuary, where seismic profiles indicate platform strata beneath the estuary (Pinet et al., 2008). Appalachian thrust sheets on the Gaspé Peninsula are dominated by Laurentian slope deposits. There, synrift sediments (St-Roch Group) were deposited on a steep continental slope (Cousineau and Longuépée, 2003) and are interlayered with rift basalts of the Lac Matapédia suite, which were dated as 565 ± 6 Ma and 556 ± 7 Ma (Hodych and Cox, 2007). The latter age is statistically indistinguishable from volcanics in the Tibbit Hill Formation; however, the ca. 565 Ma age apparently is unique to the Gaspé rift zone, suggesting that synrift deposition in Gaspé began prior to synrift deposition in the Quebec rift zone. A separate suite of rift basalts (Shickshock Group) has geochemical signatures that indicate magmatic interaction with highly attenuated continental crust (Camire et al., 1995). The Saguenay-Montmorency transform (Fig. 1) is proposed herein to separate the Quebec and Gaspé rift zones. The northwestern extension of the transform is the Saguenay graben, which is a tensional structure that extends into the continent perpendicular to the strike of the margin and contains two known synrift igneous complexes (Kumarapeli, 1985). The intersection of the transform and the Quebec rift zone forms the second-order Montmorency promontory, which is recognized as an asymmetric structural and stratigraphic high with a steep northeast gradient within the Quebec embayment (e.g., Cousineau and Longuépée, 2003). Sedimentary deposits along the Montmorency promontory consist of thin (200–800 m), late-Early to Middle Cambrian glauconite-bearing sandstones (Anse Maranda Formation) interpreted to represent a narrow sediment-starved shelf (Longuépée and Cousineau, 2005). Southwest of the transform, the Anse Maranda Formation is overlain by sparse Middle to Late Cambrian conglomerates (Breakeyville and Lauzon Formations). Along strike to the northeast across the transform into the Gaspé rift zone, conglom- N EXPLANATION 0 50 KM Allochthonous slope deposits and ophiolite complexes Shelf carbonates Shelf siliciclastic rocks Internal massif of shelf and slope Crystalline basement Figure 2. The geology of western Newfoundland divided into five segments (Seg.) on the basis of synrift and passive-margin stratigraphy. Each segment is separated by abrupt along-strike transitions in shelf and slope stratigraphy interpreted to be at transforms (see text). HAA—Humber Arm allochthon; HBA—Hare Bay allochthon. (Cow Head Group) that grade upward into Middle Ordovician flysch (James and Stevens, 1986; Cawood and Botsford, 1991). Outcrops of Neoproterozoic–Early Cambrian synrift slope deposits are lacking in the Humber Arm allochthon along segments 3 and 5, suggesting little or no deposition on the Laurentian slope along these segments during the Early Cambrian (Lavoie et al., 2003). Along segment 4, the Early Cambrian shelf is partially hidden beneath the Humber Arm allochthon. Where it is exposed, the Early Cambrian siliciclastic shelf grades upward into Middle Cambrian phyllite and limestone conglomerate (Reluctant Head Formation) interpreted as a prograding carbonate ramp, which in turn grades upward into a Late Cambrian shallow-marine carbonate platform (upper Port au Port Group) (Knight and Boyce, 1991). The succession indicates a deeper water environment on the shelf along segment 4 resulting from prolonged subsidence during the Early and Middle Cambrian. In contrast to the succession in seg- GEOLOGY, April 2009 Downloaded from geology.gsapubs.org on March 19, 2010 SILMI ST. LAWRENCE PROMONTORY OTTAWA GRABEN RK Y YO R W NTO E N MO O PR NE E NG G ON RI RA L TR S AN FT RI PÉ NE S GA ZO ZO ES IL RM FO EC E EB ON U Q TZ F RI FT PT SE CY EN OR TM M ON OR -M F AY NS EN TRA GEOLOGY, April 2009 QUEBEC EMBAYMENT GU SA RIFTED MARGIN AS A LOW-ANGLE DETACHMENT RIFT SYSTEM The four-dimensional architecture of a rifted continental margin can be inferred from the synrift and postrift stratigraphy. In the northern Appalachians, the Iapetan rift succession displays conspicuous along-strike variations in thickness, age, and depositional environment that strikingly match the modeled stratigraphic characteristics for low-angle detachment continental rift systems. Models for low-angle detachment rifts include distinctive structural configurations and thermal patterns for continental extension facilitated by a shallow-dipping (<30°) listric fault system that separates the crust into conjugate lower- and upper-plate domains partitioned along strike by steep transform faults (e.g., Wernicke, 1985; Lister et al., 1986, 1991; Thomas and Astini, 1999). The lower plate (beneath the low-angle detachment) is characterized by highly attenuated continental crust that undergoes rapid thermal decay during rifting and thus undergoes rapid subsidence (Buck et al., 1988; Lister et al., 1991). This facilitates thick synrift and early postrift sedimentary accumulation on the lower plate within fault-rotated crustal blocks above the oceanward-dipping detachment. In contrast, the upper plate (above the low-angle detachment) is characterized by a narrow zone of transition from full thickness continental crust to oceanic crust. The proximity of full thickness continental crust on the upper plate to the active rift axis (Thomas and Astini, 1999, their Fig. 3) results in prolonged thermal expansion on the upper plate, which delays passive-margin thermal subsidence (Buck et al., 1988). Consequently, initial synrift deposits on the upper plate are younger than those on the conjugate lower plate, and are more limited in both thickness and distribution. Transform faults both offset individual rift segments and bound domains of oppositely dipping rift detachments (Lister et al., 1986). Thus, transforms facilitate abrupt SAGUENAY GRABEN M D OR SF LAN AN NG NE E O TR OI EW T Z N RIF QU IS SS MI ments 3 and 5, Neoproterozoic(?)–Early Cambrian synrift slope deposits (Curling Group) at the base of the Humber Arm allochthon in segment 4 consist of a coarse siliciclastic succession with a minimum measured thickness of 1840 m (Palmer et al., 2001). The top of the synrift slope succession is marked by massive conglomerates that contain large rounded blocks of shelf carbonate and crystalline basement. Sedimentary deposits in the Hare Bay allochthon (Fig. 2) along segment 2 are less well studied, but appear to include a slope succession that is similar to that in the Humber Arm allochthon. There, Early Cambrian slope deposits (Maiden Point Formation) are estimated as 2000 m thick and locally contain blocks of metamorphic and granitic basement (Williams, 1995). Figure 3. Schematic block diagram of eastern Laurentian rifted continental margin and intracratonic fault systems of northeastern North America (present coordinates) in context of low-angle detachment rift system. SILMI—Sept-Iles layered mafic intrusion. (<25 km) along-strike changes in the architecture of the low-angle detachment rift system, and thereby the synrift and postrift stratigraphy. Implicitly, low-angle detachment rifts are marked by abrupt along-strike changes in the synrift stratigraphy at transforms. Thick Neoproterozoic alluvial-fan deposits overlain by relatively thick early postrift shelf sandstones in the New England rift zone indicate rapid synrift subsidence consistent with a lower-plate rift setting (Fig. 3). Northward into the Quebec rift zone, the sharp contrast in thickness and facies of synrift and postrift deposits, the younger age of synrift deposits, and the geochemistry of felsic volcanic phases in the Tibbit Hill Formation all indicate an upper-plate rift setting (Fig. 3). The abrupt along-strike variation in synrift and postrift stratigraphy near the international border implies a transform fault (the Missisquoi transform) between the lowerplate New England rift zone and the upper-plate Quebec rift zone (Cherichetti et al., 1998). Cousineau and Longuépée (2003) originally suggested that the Gaspé rift zone developed as a lower-plate rift domain, and our model based on the data presented here supports their interpretation (Fig. 3). In the Gaspé rift zone, the stratigraphy is dominated by synrift and passive-margin slope sediments; this requires a broad attenuated margin to accommodate the volume of slope deposits. This is consistent with the geochemistry of rift volcanics that indicate that the Gaspé rift zone is underlain by highly attenuated continental crust. Furthermore, synrift deposition in Gaspé apparently commenced earlier than synrift sedimentation in southern Quebec. The Saguenay-Montmorency transform is the boundary between the upper-plate Quebec rift zone and the lower-plate Gaspé rift zone, and marks a fundamental along-strike change in the composition and facies of synrift and passive-margin deposits in the Quebec embayment (Cousineau and Longuépée, 2003). The increase in abundance of coarse conglomerates northeast of the transform is typically attributed to Cambrian reactivation of the Saguenay graben (e.g., Lavoie et al., 2003). Alternatively, coarse conglomerates with rift basalt and basement-derived clasts may have been supplied by erosion of the transform margin during Middle and Late Cambrian sea-level lowstands (e.g., Lavoie et al., 2003). On the St. Lawrence promontory, the stratigraphy on segments 1, 3, and 5 of the Long Range rift zone includes a thin Early Cambrian synrift clastic shelf succession overlain by a thin passive-margin succession and no observed Neoproterozoic–Early Cambrian synrift slope deposits. These observations are consistent with an upper-plate rift setting for these segments of the promontory (Fig. 3). In this context, the lack of Early Cambrian synrift slope deposits is attributed to thermal uplift of the upperplate margin, which would effectively limit synrift deposition. In contrast, segments 2 and 4 contain a thick synrift slope succession that includes coarse conglomerates with basementderived clasts, indicating rapid subsidence and erosion of the margin along these two segments during Iapetan rifting. Furthermore, shelf deposits along segment 4 indicate prolonged subsidence of the shelf that lasted through late Middle Cambrian time. These observations are consistent with a lower-plate setting for segments 2 and 4 of the Long Range rift zone (Fig. 3). The boundaries between each of the five segments on the St. Lawrence promontory are abrupt along-strike discontinuities in shelf 337 Downloaded from geology.gsapubs.org on March 19, 2010 and slope stratigraphy (Cawood and Botsford, 1991). These abrupt along-strike changes in the stratigraphy are interpreted to mark transforms that separate upper- and lower-plate margins along the St. Lawrence promontory. Cawood and Botsford (1991) originally recognized these transforms on the basis of along-strike discontinuity in the passive-margin and foreland-basin slope stratigraphy, and we adopt their nomenclature for these transform faults. CONCLUSIONS The Neoproterozoic–Early Cambrian stratigraphy of the eastern Laurentian margin in the northern Appalachians records protracted continental rifting. A synthesis of stratigraphic and structural observations along the rifted margin includes: (1) abrupt along-strike changes in thickness of synrift and early postrift stratigraphy (in most places by an order of magnitude); (2) along-strike facies transitions within the synrift stratigraphy; (3) along-strike diachroneity of synrift sedimentation; and (4) differential along-strike subsidence of the margin. These significant along-strike variations in synrift and postrift stratigraphy reflect along-strike partitioning of the rift into segments that differ fundamentally in tectonic framework, subsidence history, and sediment dispersal. Specifically, these characteristics conform to a low-angle detachment model for rifting continental crust, and they constrain the range of acceptable models for continental rifting. This new interpretation is consistent with the Iapetan rift along the entire length of the eastern Laurentian margin from Newfoundland to Mexico and provides a regional constraint on the breakup of Rodinia, as well as highlights stratigraphic constraints for models of continental rifting. ACKNOWLEDGMENTS Part of this research was supported by grants from the Geological Survey of Canada and the Geological Society of America. We thank Nicolas Pinet for a critical and thoughtful review of an early draft of the manuscript. 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Manuscript received 3 August 2008 Revised manuscript received 17 November 2008 Manuscript accepted 26 November 2008 Printed in USA GEOLOGY, April 2009