Early Mesozoic rift basins of eastern North America and their gravity anomalies: The role of detachments during extension (original) (raw)
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Speculations on the origin of the North American Midcontinent rift
Tectonophysics, 1992
The Midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere. Very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began. Volcanism continued, along with extension and deep rift subsidence, for the ensuing 15 m.y. Much of the basaltic magma, including some of the earliest flows, was formed by partial melting of isotopically primitive asthenosphere contained in the plume head. The intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head. As the plume head spread beneath the lithosphere, it stretched the overlying lithosphere radially away from the Lake Superior region, the triple junction of the rift system, and partially melted to form the great volume of basalt and related intrusive rocks of the region.
Precambrian plate tectonics: The midcontinent gravity high
Earth and Planetary Science Letters, 1973
Examination of the shape of the midcontinent gravity high of central North America has led to the hypothesis that the Keweenawan rift system that caused it is the result of plate tectonic interaction. A numerical test has been carried out on the width and postulated transform fault offsets of the gravity high. The exactness of fit to a plate tectonic geometry implies that the continental lithosphere behaved as rigid plates during the Late Precambrian, about 1.I by ago. This exactness of fit also suggests that the total amount of separation on the Keweenawan riffs is equal to the width of the gravity high. Gravity modelling studies bear out the plausibility of a major amount of rifting, up to 90 km under central Lake Superior. The midcontinent gravity high may represent an intermediate stage of continental rifting, since similar gravity highs and strong associated magnetic anomalies are found on the modern rifted margins of the Atlantic Ocean.
Anatomy and evolution of the Triassic-Jurassic Continental Rift System, eastern North America
Tectonics, 1993
Mesozoic rift basins in eastern North America formed during continental extension associated with the separation of North America and Africa. These basins locally overprint the Appalachian orogen and involve the extensional reactivation of Paleozoic faults. Half graben are thought to have formed where Mesozoic extension was subperpendicular to orogenic strike. Transtensional basins formed where the extension was more oblique. Segmented border fault systems and predominanfiy synthetic intrabasinal faults characterize the half graben. These basins resemble elongate synclines in longitudinal section; this geometry resulted from border-fault displacement that was greatest near the center of the fault and decreased toward both ends. Large-scale segmentation of some border fault systems resulted in the formation of multiple synclinal subbasins separated by transverse anticlines at segment boundaries, where fault displacement was less. As displacement increased on individual fault segments, the faults and associated basins grew in length, perhaps linking originally isolated basins. Smaller-scale fault segmentation resulted in the formation of relay ramps, rider blocks, and transverse folds. Some transverse synclines are located near the centers of fault segments, and related anticlines are located at segment boundaries. Adjacent half graben units within larger rift zones do not alternate polarity along strike and are generally not linked via accommodation zones, as in the East African rift system. Strike-slip-dominated basins are characterized by a network of strike-slip and normal faults, and are shallower and narrower than dip-slip-dominated basins. INTRODUCTION Fault-bounded sedimentary basins, typically half graben, are a fundamental manifestation of continental extension [e.g., Bally, 1982; Wernicke and Burchfiel, 1982; Anderson et al., 1983; Jackson and McKenzie, 1983; Gibbs, 1984; Rosendahl, 1987] and are also the prevalent architecture of transtensional regimes, at least within the Tanganyika-Rukwa-Malawi system in East Africa [Rosendahl et al., 1992; Scott et al., 1992]. Numerous basins formed along the margins of the incipient North Atlantic Ocean during the Mesozoic breakup of Pangea (Figure la) [Van Houten, 1977; Froelich and Olsen, 1984; Tankard and Balkwill, 1989]. In eastern North America, basins crop out over a distance of > 1700 km; other basins are concealed beneath coastal plain deposits and the continental shelf (Figure lb) (see recent summaries by Froelich and Robinson [ [ 1988]. The exposed basins are filled with thousands of meters of exclusively nonmarine strata [e.g., Smoot, 1985; Manspeizer et al., 1989; Olsen et al., 1989; Smoot, 1991] as well as tholeiitic lava flows and diabase intrusions [e.g., Froelich and Gottfried, 1988; Puffer and Philpotts, 1988; Manspeizer et al., 1989; Puffer and Ragland, 1992], most of which crystallized at-200 Ma [Sutter, 1988; Dunning and Hodych, 1990]. Biostratigraphic dating indicates that preserved basin strata range in age from Middle Triassic to Early Jurassic (Table 1) [Comet and Olsen, 1985; Olsen et al., 1989]. Rift basins in eastern North America locally overprint the Appalachian orogen and exhibit parallelism with Paleozoic contractional structures (Figure lb) [e.g., Lindholm, 1978; Swanson, 1986; Ratcliffe et al., 1986]. The exposed rift basins are situated landward of the hinge zone of the continental margin; this region experienced considerably less crustal thinning than did the seaward region [Klitgord et al., 1988]. Consequently, many of these landward basins were not deeply buried by postrift strata and therefore remain accessible today. This paper examines the similarities and differences in the architecture of exposed Mesozoic rift basins in eastern North America. Particular attention is given to the geometry of border fault systems and associated structures, especially along-strike variability. The relationship between structural geology and stratigraphy is used to infer the evolution of the rift basins. Finally, the nature of the linkage of basins within the rift complex is examined and compared to that of the wellstudied East African rift system. NOMENCLATURE The border fault system (BFS) of a half graben refers to the network of normal faults bounding the asymmetric basin (Figure 2a); movement on these faults was largely responsible for the formation of the basin. Because the exact slip direction on most of the boundary faults is difficult to determine and some boundary faults likely experienced significant strike slip, B FS is used in the general sense for the primary basinbounding fault system regardless of the nature of slip along it. Longitudinal structures or profiles are oriented parallel to the BFS; transverse structures or profiles are oriented perpendicular to the BFS (Figure 2c). Because the BFS commonly consists of multiple faults, the term segment refers to an individual fault within the BFS. As the geometry of the faults at depth is poorly constrained, segmentation is based exclusively on the map view trace of the BFS. Following the scheme used to define the segmentation of seismically active faults [e.g., Zhang et al., 1991 ], segment boundaries are marked by changes in strike as well as offsets or overlaps (Figures 2b and 2c). For overlapping faults, a common segment boundary may be placed in the center of the region of overlap. The relatively unfaulted blocks of rock located between overlapping fault segments are called ramps by Kelley [1979], fault bridges by Ramsay and Huber [1987] and relay ramps by Larsen [1988], Morley et al. [1990], and Peacock and Sanderson [1991]. Where such structures occur in extensional basins, synrift strata unconformably overlap prerift rocks on relay ramps (Figure 2c) [e.g., Larsen, 1988]. In regions of overlapping or subparallel faults, the bottom of the basin may step down along progressively more basinwardsituated faults. The blocks between the faults are termed riders by Gibbs [1984] (Figure 2a). Schlische: Triassic-Jurassic Continental Rift System 1027 Ramping margin refers to the relatively unfaulted basin margin that generally dips toward the BFS (Figure 2a). The intersection of this margin with the Earth's surface is the prerift-synrift contact. Prerift rocks form the "basement" to the basins; synrift rocks refer to the basin fill. Several basins are composed of smaller or structurally distinct subbasins. The Fundy basin (Figure 3a) is subdivided into the northeast trending Fundy and Chignecto subbasins and the east trending Minas subbasin. The Deerfield and Hartford subbasins form the Connecticut Valley basin (Figure 4a). The Deep River basin (Figure 6c) consists of the Durham, Sanford, and Wadesboro subbasins. 75 ø 45" North America 65 c 40 ø Atlantic Ocean MESOZOIC BASINS evolution, Philos. Trans. R. Soc. London, A305, 325-338, 1982.
Earth-Science Reviews, 2014
Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary and volcanic rocks in Utah, Nevada, and SE Idaho record a dynamically evolving landscape along the North American Cordillera margin, which included: (1) initial development of intracratonic basins with deposition of siliciclastic strata of the Uinta Mountain Group from~770 to 740 Ma; (2) early rifting and volcanism along a N-S (present day geographic coordinates) basin system with deposition of diamictite-bearing strata of the Perry Canyon and related formations from~720 to 660 Ma; (3) early, broad subsidence with deposition of mature siliciclastic strata of the lower Brigham and McCoy Creek groups from~660 to 580 Ma; (4) final rifting, volcanism, and transition to drift with deposition of variably immature siliciclastic strata of the Prospect Mountain and correlative formations from~570 to 520 Ma; and (5) regional subsidence along a passive margin with deposition of Middle Cambrian to Devonian carbonate-rich strata. The Uinta Mountain Group comprises fluvial to marine, feldspathic to quartzose sandstone, conglomerate, and mudstone, with detrital zircon (DZ) patterns recording a mix of local basement sources to the N and distal Laurentian sources to the SE. The lower Perry Canyon and related formations contain variably feldspathic sandstone, quartz-pebble diamictite deposited during an older glacial episode, and mudstone, with DZ patterns recording a mix of distal sources, local basement sources, and sediment recycling during early rifting. The upper Perry Canyon and related formations contain mafic volcanic rocks, polymict diamictite deposited during a younger glacial episode, volcaniclastic wacke, and mudstone, with DZ patterns recording local basement sources along an evolving rift margin and felsic volcanism from~700 to 670 Ma. Mafic volcanic rocks and trachyte to rhyolite clasts in diamictite have geochemical signatures typical of continental rifting. The lower Brigham and McCoy Creek groups contain mostly mature quartz arenite deposited in shallow marine environments, with DZ patterns recording distal Laurentian sources. The base of the Prospect Mountain and correlative formations is marked by an influx of feldspathic, coarsegrained sediment derived from local basement sources and~570-540 Ma basalt volcanism, which was followed by deposition of subfeldspathic strata with dominant 1.7-1.8 Ga DZ grains, recording sources from the SE rift margin and a marked decrease in distal sources during uplift of the Transcontinental Arch. Overlying carbonate-rich strata were deposited in shallow marine settings, with episodic influx of siliciclastic sediment derived from basement exposed during regressions. Stratigraphic thickness-age relations of Neoproterozoic to early Paleozoic strata are consistent with two episodes of rifting concentrated at ca. 700-670 Ma and 570-540 Ma along western Laurentia, leading to final development of a passive margin. Early rifting was incomplete with an estimated 25-40% extension of initially thick lithosphere that was weakened by igneous activity. Final rifting of previously thinned lithosphere involved an estimated 20-35% additional extension, renewed igneous activity, and thermal thinning of mantle lithosphere, with localized extension culminating in final separation along the continental margin. Stratigraphic, geochronologic, and available paleomagnetic data Earth-Science Reviews 136 (2014) 59-95 ⁎ Corresponding author.
Earth and planetary science letters, 1993
A finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the Early Tertiary North American Cordillera. Crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the Great Basin during extension. Marginal highlands, corresponding to the Sierra Nevada and Colorado Plateau, develop near the edges of the weakened region, where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain. The great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the Great Basin, favoring the formation of metamorphic core complexes during the early stages of extension. Ductile flow becomes inhibited as the crust thins and cools, possibly contributing to the change in the style of extension from low-angle detachment faulting to high-angle normal faulting during the Middle Miocene. As the lithosphere cools, the strength of the uppermost mantle increases. After about 10 m.y. the upper mantle becomes stronger beneath the Great Basin than beneath adjacent unextended regions. Strain then begins to migrate outward from the interior of the Great Basin onto the marginal highlands, and their interior edges begin to collapse and are incorporated into the widening Great Basin. After 40 m.y. of extension the highest rates of strain are localized at the edges of the Great Basin, in positions corresponding to the Walker Lane tectonic belt of western Nevada and the Colorado Plateau transition zone. The weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle. Portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions, creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies.
2020
Both magmatic and tectonic processes contribute to the formation of volcanic continental margins. Such margins are thought to undergo extension across a narrow zone of lithospheric thinning (~100 km). New observations based on existing and reprocessed data from the Eastern North American Margin contradict this hypothesis. With~64,000 km of 2-D seismic data tied to 40 wells combined with published refraction, deep reflection, receiver function, and onshore drilling efforts, we quantified along-strike variations in the distribution of rift structures, magmatism, crustal thickness, and early post-rift sedimentation under the shelf of Baltimore Canyon Trough (BCT), Long Island Platform, and Georges Bank Basin (GBB). Results indicate that BCT is narrow (80-120 km) with a sharp basement hinge and few rift basins. The seaward dipping reflectors (SDR) there extend~50 km seaward of the hinge line. In contrast, the GBB is wide (~200 km), has many syn-rift structures, and the SDR there extend~200 km seaward of the hinge line. Early post-rift depocenters at the GBB coincide with thinner crust suggesting "uniform" thinning of the entire lithosphere. Models for the formation of volcanic margins do not explain the wide structure of the GBB. We argue that crustal thinning of the BCT was closely associated with late syn-rift magmatism, whereas the broad thinning of the GBB segment predated magmatism. Correlation of these variations to crustal terranes of different compositions suggests that the inherited rheology determined the premagmatic response of the lithosphere to extension.