Eric Clausen argues that my paper, “Late Oligocene–early Miocene Grand Canyon: A Canadian connect... more Eric Clausen argues that my paper, “Late Oligocene–early Miocene Grand Canyon: A Canadian connection?” (GSA Today, Nov. 2013), does not assess alternative hypotheses for the origin of the upper Missouri River, especially his own proposal that the river was formed, not by a north-flowing pre-ice age stream, but rather by massive south-flowing meltwater outbursts from an Antarctic-sized North American ice sheet. Clausen contends that the meltwater then reversed its flow to fill a hypothetical hole left by the melting glacier. Clausen bases his proposal on interpretation of topographic maps of the region, as discussed in detail on his website (geomorphologyresearch.com). The sedimentological evidence that I cited for a north-flowing paleo-river system derives from fluvial deposits with excellent paleo-flow indicators, including channels, cobble imbrication, grain-size distribution, and the provenances of clasts and detrital zircons. The fluvial deposits are well dated to Oligocene and ...
ABSTRACT A robust Proterozoic plate reconstruction that joins the NE Siberian and SW North Americ... more ABSTRACT A robust Proterozoic plate reconstruction that joins the NE Siberian and SW North American cratons provides constraints on tectonic, sedimentary, and thermal evolution of lithospheric extension leading to continental break-up. The reconstruction blends clues from Siberia and North America to show that the lithospheric extension occurred in temporally isolated episodes spanning more than one billion years. The first episode culminated in opening of the intracratonic Mesoproterozoic Belt basin at about 1.5 Ga, with widespread anorogenic magmatism, dike swarms, and rifting. It may have responded to drift stagnation of the parent supercontinent with attendant entrapment of heat and thermal expansion of the sub-lithospheric mantle. The rifting and magmatism followed a continent-wide icosahedral pattern but did not open into sea-floor spreading. The second episode took place at about 780 Ma, much along the lines of the first rifts, culminating in the Gunbarrel magmatic event and Windermere rift sedimentation, as drift stagnation and thermal expansion re-opened the older icosahedral lithospheric fractures. The third episode began in latest Proterozoic to earliest Cambrian, and proceeded to sea-floor spreading and thermal subsidence of conjugate margins, with transform movement along some earlier rift lines. Transform segments of the rift system evolved into steep continental margins, as along the Rocky Mountain trench of Canada, whereas linking segments formed low-angle detachment surfaces with broad continental shelves on upper and lower plates, as in the Verkhoyansk Mountains of Siberia and and conjugate Great Basin of North America. The relative pole of rotation between the cratons appears to have shifted in Late Devonian leading to renewed extension and mafic magmatism in some areas and convergence in other areas of the developing conjugate margins. The overall history implies that initial rifting of sluggish supercontinents may be driven by thermal expansion of insulated asthenosphere, but that an advance to seafloor spreading awaits permissive plate-tectonic circuits that exploit previously opened lithospheric fractures.
Impacts oflarge meteorites or comets into Proterozoic continental crust may have opened basins as... more Impacts oflarge meteorites or comets into Proterozoic continental crust may have opened basins as much as hundreds of kilometers in diameter. We suggest that basalt magma generated through pressure-relief, partial melting flooded the craters to form lava lakes, which evolved into lopoliths capped with flood-basalt flows. Thermal-contraction subsidence of these heavy igneous rocks created basins that continued to subside under sedimentary loading. Meanwhile, continued partial melting in the upper mantle generated more basalt magma that entered the basin fill as either flows or diabase sills. Continuing igneous activity within the basin, instead of along a hotspot track, implies that the Proterozoic lithosphere was stationary. Sharply limited rifting associated with large Proterozoic basins similarly suggests a nearly stationary crust. Perhaps very large areas of relatively thin continental crust blocked sweeping plate movement. Thrusting associated with subduction within extremely large impact basins of generally oceanic character may have thickened the Proterozoic crust while diminishing its area. Relatively straight and long mobile belts appeared near the end of Proterozoic time, apparently marking the time when ocean basins became large enough to permit the transition to modern plate tectonic settings.
Special Paper 323: Accommodation zones and transfer zones; the regional segmentation of the Basin and Range Province, 1998
... 242 JW Sears and WJ Fritz Figure 1. Location map of the major tectonic features discussed in ... more ... 242 JW Sears and WJ Fritz Figure 1. Location map of the major tectonic features discussed in this chapter. Figure 2. Major extensional basins of southwestern Montana for area shown in Figure 1. The ranges are shown in the map; refer to text for spe-cific basin terminology. ...
Special Paper 383: Orogenic curvature: Integrating paleomagnetic and structural analyses, 2004
... Clasts of dark shaley siltstone, probably derived from the underlying Colorado Group, are com... more ... Clasts of dark shaley siltstone, probably derived from the underlying Colorado Group, are commonly deformed by duraclasts within these lower debris flows. ... 2) associated with the Lewis and Clark line pro-vided the source for large angular blocks of sandstone and vol-canic ...
Special Paper 430: Plates, Plumes and Planetary Processes, 2007
Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack len... more Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack length and therefore required the least work to nucleate and propagate new fractures across the supercontinent. The fracture arrangement met conditions imposed by Euler's rule for ordering polyhedrons on a spherical shell. Linear grabens accumulated Permian rift facies along 10,000 km of the fracture system in east Gondwana. Large igneous provinces erupted more than 100 Ma later along these fractures. This suggests that widening of existing fractures rather than impingement of deep-mantle plumes triggered outbreaks of flood basalt. The tensile stress field that initiated the fractures was symmetrical with Gondwana and exploited pre-existing lithospheric suture zones. The stress field was also symmetrical about the African geoid bulge in the Permian locus of Gondwana. Tensile hoop-stress along the Gondwana boundary initiated radial fractures that defined the lateral edges of Australia, India, Arabia, Libya, and northwest Africa. Fractures then evidently propagated inward across Gondwana, spontaneously bending at critical lengths congruent with the tessellation. Fractures later branched outward from the bends to create triple-rift junctions. Plate tectonic processes later exploited the icosahedral fractures to separate the Gondwana daughter continents.
Many Precambrian plate restorations place one of the rifted margins of the Siberian craton agains... more Many Precambrian plate restorations place one of the rifted margins of the Siberian craton against one of the rifted margins of Laurentia. Inherent in the proposed restorations is the implication that the conjugate margins share a similar geodynamic history of rifting and thermal subsidence until interrupted by other tectonic processes. We compare the stratigraphic and structural history of Verkhoyansk miogeocline
Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely app... more Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncatedicosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Longlived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes.
Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack len... more Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack length and therefore required the least work to nucleate and propagate new fractures across the supercontinent. The fracture arrangement met conditions imposed by Euler's rule for ordering polyhedrons on a spherical shell. Linear grabens accumulated Permian rift facies along 10,000 km of the fracture system in east Gondwana. Large igneous provinces erupted more than 100 Ma later along these fractures. This suggests that widening of existing fractures rather than impingement of deep-mantle plumes triggered outbreaks of flood basalt. The tensile stress field that initiated the fractures was symmetrical with Gondwana and exploited pre-existing lithospheric suture zones. The stress field was also symmetrical about the African geoid bulge in the Permian locus of Gondwana. Tensile hoop-stress along the Gondwana boundary initiated radial fractures that defined the lateral edges of Australia, India, Arabia, Libya, and northwest Africa. Fractures then evidently propagated inward across Gondwana, spontaneously bending at critical lengths congruent with the tessellation. Fractures later branched outward from the bends to create triple-rift junctions. Plate tectonic processes later exploited the icosahedral fractures to separate the Gondwana daughter continents.
... User Name Password Sign In. Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrus... more ... User Name Password Sign In. Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrust belt, United States and Canada. ... Abstract. The Mesoproterozoic Belt-Purcell Basin of the United States–Canadian Rocky Mountains formed in a complex intracontinental-rift system. ...
Paleogeographic reconstruction of the northern Siberian craton against southwestern Laurentia, in... more Paleogeographic reconstruction of the northern Siberian craton against southwestern Laurentia, in accordance with paleomagnetic data, basement piercing points, and the best fit of the rift margins, aligns Mesoproterozoic dike and sill swarms in northeast Siberia with correlative ones in Montana and Wyoming. It also juxtaposes the Mesoproterozoic Belt-Purcell basin of west Laurentia against the Mesoproterozoic Udzha basin of the northern Siberian craton. We review the veracity of this hypothetical Belt-Purcell-Udzha basin as a test of our Siberia-west Laurentia reconstruction. Various elements of the structural framework of the basins are closely aligned in the reconstruction. The Udzha trough is aligned with the St. Mary-Moyie fault zone and Vulcan basement structure of southwestern Canada. The Khastakh trough is aligned with the Helena embayment of central Montana. Parts of the Belt-Purcell Supergroup appear to correlate with parts of the Riphean section in northeastern Siberia. However, the Siberian section appears to be much thinner than the Belt-Purcell section, and precise correlation is not possible with present stratigraphic and geochronological data. The reconstruction leads to the prediction that the Udzha rift channeled sediment from a cratonic pediment into the delta and alluvial fan complex in the deep Belt-Purcell rift-basin. Distributary channels may have shifted within the Udzha basin to feed shifting depocenters in the Belt-Purcell basin. Details of age and tectonic evolution for the Udzha basin are less clear than for the Belt-Purcell basin, but we outline specific geological relationships predicted by the reconstruction model, and suggest tests for future research.
The Siberian-West Laurentian connection is a postulated reconstruction of a Proterozoic craton th... more The Siberian-West Laurentian connection is a postulated reconstruction of a Proterozoic craton that formed between 1800 Ma and 1500 Ma. Correlation of geological trends and of rift facies between the Siberian platform and western Laurentia indicates that, although the two cratons underwent significant intracontinental rifting in Mesoproterozoic and Neoproterozoic times, they did not undergo complete separation until early Cambrian, ca. 530 Ma. Although this interpretation does not support the hypothetical Columbia supercontinent model of Rogers (2000) and Rogers and Santosh (2002), some elements of it are compatible with that model.
The allochthonous Cassiar platform, in north-central British Columbia, is a cratonal fragment of ... more The allochthonous Cassiar platform, in north-central British Columbia, is a cratonal fragment of ancestral North America juxtaposed against autochthonous North American crust along the Tintina Northern Rocky Mountain trench fault. The Cassiar platform records a Neoproterozoic to early Paleozoic rift to passive-margin history that includes Lower Cambrian archeocyathan-bearing limestones of the Rosella Formation in the Cassiar Mountains. This study indicates
Eric Clausen argues that my paper, “Late Oligocene–early Miocene Grand Canyon: A Canadian connect... more Eric Clausen argues that my paper, “Late Oligocene–early Miocene Grand Canyon: A Canadian connection?” (GSA Today, Nov. 2013), does not assess alternative hypotheses for the origin of the upper Missouri River, especially his own proposal that the river was formed, not by a north-flowing pre-ice age stream, but rather by massive south-flowing meltwater outbursts from an Antarctic-sized North American ice sheet. Clausen contends that the meltwater then reversed its flow to fill a hypothetical hole left by the melting glacier. Clausen bases his proposal on interpretation of topographic maps of the region, as discussed in detail on his website (geomorphologyresearch.com). The sedimentological evidence that I cited for a north-flowing paleo-river system derives from fluvial deposits with excellent paleo-flow indicators, including channels, cobble imbrication, grain-size distribution, and the provenances of clasts and detrital zircons. The fluvial deposits are well dated to Oligocene and ...
ABSTRACT A robust Proterozoic plate reconstruction that joins the NE Siberian and SW North Americ... more ABSTRACT A robust Proterozoic plate reconstruction that joins the NE Siberian and SW North American cratons provides constraints on tectonic, sedimentary, and thermal evolution of lithospheric extension leading to continental break-up. The reconstruction blends clues from Siberia and North America to show that the lithospheric extension occurred in temporally isolated episodes spanning more than one billion years. The first episode culminated in opening of the intracratonic Mesoproterozoic Belt basin at about 1.5 Ga, with widespread anorogenic magmatism, dike swarms, and rifting. It may have responded to drift stagnation of the parent supercontinent with attendant entrapment of heat and thermal expansion of the sub-lithospheric mantle. The rifting and magmatism followed a continent-wide icosahedral pattern but did not open into sea-floor spreading. The second episode took place at about 780 Ma, much along the lines of the first rifts, culminating in the Gunbarrel magmatic event and Windermere rift sedimentation, as drift stagnation and thermal expansion re-opened the older icosahedral lithospheric fractures. The third episode began in latest Proterozoic to earliest Cambrian, and proceeded to sea-floor spreading and thermal subsidence of conjugate margins, with transform movement along some earlier rift lines. Transform segments of the rift system evolved into steep continental margins, as along the Rocky Mountain trench of Canada, whereas linking segments formed low-angle detachment surfaces with broad continental shelves on upper and lower plates, as in the Verkhoyansk Mountains of Siberia and and conjugate Great Basin of North America. The relative pole of rotation between the cratons appears to have shifted in Late Devonian leading to renewed extension and mafic magmatism in some areas and convergence in other areas of the developing conjugate margins. The overall history implies that initial rifting of sluggish supercontinents may be driven by thermal expansion of insulated asthenosphere, but that an advance to seafloor spreading awaits permissive plate-tectonic circuits that exploit previously opened lithospheric fractures.
Impacts oflarge meteorites or comets into Proterozoic continental crust may have opened basins as... more Impacts oflarge meteorites or comets into Proterozoic continental crust may have opened basins as much as hundreds of kilometers in diameter. We suggest that basalt magma generated through pressure-relief, partial melting flooded the craters to form lava lakes, which evolved into lopoliths capped with flood-basalt flows. Thermal-contraction subsidence of these heavy igneous rocks created basins that continued to subside under sedimentary loading. Meanwhile, continued partial melting in the upper mantle generated more basalt magma that entered the basin fill as either flows or diabase sills. Continuing igneous activity within the basin, instead of along a hotspot track, implies that the Proterozoic lithosphere was stationary. Sharply limited rifting associated with large Proterozoic basins similarly suggests a nearly stationary crust. Perhaps very large areas of relatively thin continental crust blocked sweeping plate movement. Thrusting associated with subduction within extremely large impact basins of generally oceanic character may have thickened the Proterozoic crust while diminishing its area. Relatively straight and long mobile belts appeared near the end of Proterozoic time, apparently marking the time when ocean basins became large enough to permit the transition to modern plate tectonic settings.
Special Paper 323: Accommodation zones and transfer zones; the regional segmentation of the Basin and Range Province, 1998
... 242 JW Sears and WJ Fritz Figure 1. Location map of the major tectonic features discussed in ... more ... 242 JW Sears and WJ Fritz Figure 1. Location map of the major tectonic features discussed in this chapter. Figure 2. Major extensional basins of southwestern Montana for area shown in Figure 1. The ranges are shown in the map; refer to text for spe-cific basin terminology. ...
Special Paper 383: Orogenic curvature: Integrating paleomagnetic and structural analyses, 2004
... Clasts of dark shaley siltstone, probably derived from the underlying Colorado Group, are com... more ... Clasts of dark shaley siltstone, probably derived from the underlying Colorado Group, are commonly deformed by duraclasts within these lower debris flows. ... 2) associated with the Lewis and Clark line pro-vided the source for large angular blocks of sandstone and vol-canic ...
Special Paper 430: Plates, Plumes and Planetary Processes, 2007
Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack len... more Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack length and therefore required the least work to nucleate and propagate new fractures across the supercontinent. The fracture arrangement met conditions imposed by Euler's rule for ordering polyhedrons on a spherical shell. Linear grabens accumulated Permian rift facies along 10,000 km of the fracture system in east Gondwana. Large igneous provinces erupted more than 100 Ma later along these fractures. This suggests that widening of existing fractures rather than impingement of deep-mantle plumes triggered outbreaks of flood basalt. The tensile stress field that initiated the fractures was symmetrical with Gondwana and exploited pre-existing lithospheric suture zones. The stress field was also symmetrical about the African geoid bulge in the Permian locus of Gondwana. Tensile hoop-stress along the Gondwana boundary initiated radial fractures that defined the lateral edges of Australia, India, Arabia, Libya, and northwest Africa. Fractures then evidently propagated inward across Gondwana, spontaneously bending at critical lengths congruent with the tessellation. Fractures later branched outward from the bends to create triple-rift junctions. Plate tectonic processes later exploited the icosahedral fractures to separate the Gondwana daughter continents.
Many Precambrian plate restorations place one of the rifted margins of the Siberian craton agains... more Many Precambrian plate restorations place one of the rifted margins of the Siberian craton against one of the rifted margins of Laurentia. Inherent in the proposed restorations is the implication that the conjugate margins share a similar geodynamic history of rifting and thermal subsidence until interrupted by other tectonic processes. We compare the stratigraphic and structural history of Verkhoyansk miogeocline
Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely app... more Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncatedicosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Longlived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes.
Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack len... more Gondwana broke apart along a truncated-icosahedral fracture system that minimized total crack length and therefore required the least work to nucleate and propagate new fractures across the supercontinent. The fracture arrangement met conditions imposed by Euler's rule for ordering polyhedrons on a spherical shell. Linear grabens accumulated Permian rift facies along 10,000 km of the fracture system in east Gondwana. Large igneous provinces erupted more than 100 Ma later along these fractures. This suggests that widening of existing fractures rather than impingement of deep-mantle plumes triggered outbreaks of flood basalt. The tensile stress field that initiated the fractures was symmetrical with Gondwana and exploited pre-existing lithospheric suture zones. The stress field was also symmetrical about the African geoid bulge in the Permian locus of Gondwana. Tensile hoop-stress along the Gondwana boundary initiated radial fractures that defined the lateral edges of Australia, India, Arabia, Libya, and northwest Africa. Fractures then evidently propagated inward across Gondwana, spontaneously bending at critical lengths congruent with the tessellation. Fractures later branched outward from the bends to create triple-rift junctions. Plate tectonic processes later exploited the icosahedral fractures to separate the Gondwana daughter continents.
... User Name Password Sign In. Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrus... more ... User Name Password Sign In. Belt-Purcell Basin: Keystone of the Rocky Mountain fold-and-thrust belt, United States and Canada. ... Abstract. The Mesoproterozoic Belt-Purcell Basin of the United States–Canadian Rocky Mountains formed in a complex intracontinental-rift system. ...
Paleogeographic reconstruction of the northern Siberian craton against southwestern Laurentia, in... more Paleogeographic reconstruction of the northern Siberian craton against southwestern Laurentia, in accordance with paleomagnetic data, basement piercing points, and the best fit of the rift margins, aligns Mesoproterozoic dike and sill swarms in northeast Siberia with correlative ones in Montana and Wyoming. It also juxtaposes the Mesoproterozoic Belt-Purcell basin of west Laurentia against the Mesoproterozoic Udzha basin of the northern Siberian craton. We review the veracity of this hypothetical Belt-Purcell-Udzha basin as a test of our Siberia-west Laurentia reconstruction. Various elements of the structural framework of the basins are closely aligned in the reconstruction. The Udzha trough is aligned with the St. Mary-Moyie fault zone and Vulcan basement structure of southwestern Canada. The Khastakh trough is aligned with the Helena embayment of central Montana. Parts of the Belt-Purcell Supergroup appear to correlate with parts of the Riphean section in northeastern Siberia. However, the Siberian section appears to be much thinner than the Belt-Purcell section, and precise correlation is not possible with present stratigraphic and geochronological data. The reconstruction leads to the prediction that the Udzha rift channeled sediment from a cratonic pediment into the delta and alluvial fan complex in the deep Belt-Purcell rift-basin. Distributary channels may have shifted within the Udzha basin to feed shifting depocenters in the Belt-Purcell basin. Details of age and tectonic evolution for the Udzha basin are less clear than for the Belt-Purcell basin, but we outline specific geological relationships predicted by the reconstruction model, and suggest tests for future research.
The Siberian-West Laurentian connection is a postulated reconstruction of a Proterozoic craton th... more The Siberian-West Laurentian connection is a postulated reconstruction of a Proterozoic craton that formed between 1800 Ma and 1500 Ma. Correlation of geological trends and of rift facies between the Siberian platform and western Laurentia indicates that, although the two cratons underwent significant intracontinental rifting in Mesoproterozoic and Neoproterozoic times, they did not undergo complete separation until early Cambrian, ca. 530 Ma. Although this interpretation does not support the hypothetical Columbia supercontinent model of Rogers (2000) and Rogers and Santosh (2002), some elements of it are compatible with that model.
The allochthonous Cassiar platform, in north-central British Columbia, is a cratonal fragment of ... more The allochthonous Cassiar platform, in north-central British Columbia, is a cratonal fragment of ancestral North America juxtaposed against autochthonous North American crust along the Tintina Northern Rocky Mountain trench fault. The Cassiar platform records a Neoproterozoic to early Paleozoic rift to passive-margin history that includes Lower Cambrian archeocyathan-bearing limestones of the Rosella Formation in the Cassiar Mountains. This study indicates
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