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The Proterozoic connection between northeastern Siberia and western Laurentia that we proposed in 1978 is strongly supported by several new lines of evidence. New age data and refined structural trends in predrift basement rocks improve the resolution of the fit between the cratons. The mouth of the large river that is inferred to have provided the point source for the lower part of the Mesoproterozoic Belt-Purcell Supergroup in western Laurentia aligns with the Mesoproterozoic Udzha trough of Siberia. The elbow bend in the Udzha trough bypasses the Archean Wyoming Province to link the Belt-Purcell basin with Paleoproterozoic regions in southwest Laurentia having appropriate Nd crustal-residence ages and zircon crystallization ages to have provided sources for much of the sediment. The Grenville and Granite-Rhyolite provinces of southwest Laurentia provide sources for detrital zircons and felsic volcanic fragments in the east-derived Mesoproterozoic Mayamkan Formation of Siberia. The ages of mafic sills in the Sette-Daban region of Siberia overlap those in southwest Laurentia. Ediacara occur in off-shelf environments on both margins. The two margins have very similar latest Neoproterozoic-earliest Cambrian rift-drift signatures, including a breakup unconformity and Tommotian sheff assemblages that record the onset of thermally driven subsidence. Two possible submarine volcanoes with archeocyathan caps may confirm the establishment of Early Cambrian seafioor spreading. The Siberian-west Laurentian connection provides better correlations among prerift terranes than does the southwest United States-East Antarctic connection (SWEAT), and is more compatible with the overall geologic history of Laurentia and Gondwana. Keywords: Siberia, Laurentia, Gondwana, Belt-Purcell, Rodinia, tectonics, rifting, reconstruction, paleogeography.

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 Neo-proterozoic 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 that an extensive oolitic shoal developed toward the western edge of this carbonate platform during the deposition of the Nevadella zone, parallel to the western limit of thick continental crust (initial-Sr 0.706 isopleth). Paleogeographic studies from other archeocyathan-bearing units in the Cordillera indicate that a semicontinuous oolitic shoal was along the western margin of the continental shelf from Alaska to Mexico. There is a distinctive gap in the passive-margin record from southeastern Washington to southern Idaho. Paleogeographic constraints from the Rosella Formation and published paleomagnetic data from the overlying Sylvester allochthon suggest that this miogeoclinal slice was originally deposited near present-day Idaho and was transported northward, along poorly constrained dextral strike-slip faults.

Deposits of a Neogene paleovalley provide a geologic datum in southwestern Montana that brackets the age of faulting associated with the Yellowstone hotspot. Ages of paleovalley deposits on top of the modern Blacktail, Ruby, Tendoy, and Centennial ranges and in adjacent valleys range from 16 to 2 Ma. At least 2 km of offset has occurred since emplacement of the Timber Hill basalt (6.0 + or - 0.1 Ma); much may have occurred after deposition of the 2.0 Ma Huckleberry Ridge Tuff, which is now found in isolated outcrops as far north as the Ruby Range. Gravel clasts within the paleovalley have a northern provenance, whereas volcanic and volcaniclastic rocks were derived from the Snake River Plain to the south. We propose that uplift associated with passage of the hotspot caused the drainage reversal.

Traditional stratigraphic nomenclature shows that conglomerate with distinctive red quartzite roundstones flowed southward from southwestern Montana and that basalt spread northward from the Snake River Plain to backfill the Sixmile Creek which is considered to be paleovalley. However, informal sequence stratigraphy reveals that the sub-Sixmile Creek erosion surface is a regional or interregional characteristic and not a paleovalley.

The boundaries of the Laurentian craton conform to edges of two adjacent faces on a truncated icosahedral projection of Earth. The congruence of the geology and the icosahedral tessellation has tectonic significance. It apparently originated as a large-scale fracture pattern during the tectonically quiet early Mesoproterozoic, 100-200 m.y. after Paleoproterozoic consolidation of a parent supercontinent. Homogeneous stretching of the young supercontinent across a geoid high above an insulated and thermally expanded mantle may have induced the fractures. Neoproterozoic and Cambrian rift zones exploited the icosahedral fractures during breakout of Laurentia, and miogeoclines accumulated along the margins. The Grenville and Appalachian orogenies appear to have shortened the tessellation. Because of its high geometric symmetry, the rift tessellation may provide a reference frame for restoration of an early Mesoproterozoic supercontinent. Keywords: tectonics, Laurentia, Precambrian, miogeoclines, Mesoproterozoic, rifting.

Paleozoic rotation of the Siberian craton along transform faults on the western Laurussian margin is a hypothesis that incorporates geologic and paleomagnetic evidence. The hypothesis could ultimately explain the origins of allochthonous Paleozoic terranes in the western North American Cordillera and northeast Russia, and supports a Precambrian connection of the Siberian craton to southwest North America. The hypothesized rotation may have opened a marginal sea leading to deposition of nearly identical early Paleozoic shelf sequences on the conjugate Cordilleran and Verkhoyansk margins. The model proposes that tectonic shearing transferred a series of terranes from Laurussia to the Siberian plate. The transforms evolved into convergent boundaries in the middle and late Paleozoic. The displaced terranes were ultimately accreted into the Cordillera and orogenic belts in northeast Russia. [ABSTRACT FROM AUTHOR]