James W. Sears

New look at the Siberian connection: No SWEAT

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 shelf assemblages that record the onset of thermally driven subsidence. Two possible submarine volcanoes with archeocyathan caps may confirm the establishment of Early Cambrian seafloor 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.

Tightening the Siberian connection to western Laurentia

Newly available geological and geophysical data tighten the Proterozoic connection between the rifted margins of the northern Siberian craton and western Laurentia, and permit a Siberia-Laurentia-Australia troika, with northern Australia connected to the southern margin of the Siberian craton. The continental assembly is linked by a prominent 2.0–1.8 Ga collisional belt and the ca. 1.3–1.0 Ga Grenville orogen. The reconstruction also aligns a 1.5–1.45 Ga dike/sill swarm that extends from the Wyoming province, through the Belt-Purcell basin, into the northern Siberian craton. Separation of Australia and Siberia may have occurred by the early Neoproterozoic, but separation of Siberia and Laurentia may not have been completed until the Early Cambrian. Continental extension began along the zigzag Siberia-Laurentia rift zone in the early Mesoproterozoic. Rift-related igneous and sedimentary assemblages dated at ca. 1.5, 1.38, and 1.2 to 1.0 Ga correlate across the reconstructed Siberia-Laurentia rift zone. Renewed rifting in the Neoproterozoic and Vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders. Correlative archeocyathan reefs, endemic olenellid trilobite fauna, and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the Atdabanian stage. Black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the Botomian Sinsk event.

Tectonic evolution of lower Proterozoic rocks, Uinta Mountains, Utah and Colorado

Rocks of the early Proterozoic orogenic system that fringes the Archean-aged Wyoming province are exposed along the Uinta fault in the northeastern Uinta Mountains of Utah and Colorado. Exposed here are the Red Creek Quartzite, an early Proterozoic-type miogeoclinal metasedimentary sequence more than 4 km thick, and an underlying, newly recognized, Archean gneiss complex more than 2.7 b.y. old. During the Hudsonian orogenic period, the miogeoclinal sequence was emplaced northward over the Archean complex by tectonic translation along a thick mylonite zone in the waning phases of upper amphibolite metamorphism. The orogen was disrupted by east-trending block faults with several kilometres displacement during initiation of the Uinta aulacogen and was buried by more than 7 km of middle Proterozoic sediments of the Uinta Mountain Group. The middle Proterozoic block faults were reactivated with reversed sense of displacement during the Laramide uplift of the Uinta Mountain block.

Tectonics of the Yellowstone hotspot wake in, southwestern Montana

Deposits off 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 ±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.

Linking the Mesoproterozoic Belt-Purcell and Udzha basins across the west Laurentia–Siberia connection

Abstract 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.

Direction and shear sense during suturing of the Seven Devils-Wallowa terrane against North America in western Idaho

A northeast-dipping 1.5-km-thick mylonite near Dworshak Dam marks the suture zone between Precambrian North America and the Seven Devils-Wallowa terrane in western Idaho. The mylonite formed under amphibolite facies conditions from quartz diorite containing apparently synplutonic mafic and synkinematic pegmatite dikes of the Kamiah plutonic complex. Mylonitic lineations and fold axes have a mean plunge of 48° toward 056°, nearly down the dip of the mylonitic foliation. Shear sense, given by offset of late-stage crosscutting pegmatites, is consistently top-to-the-southwest, reverse-slip, parallel to the mylonitic lineation. Folds that formed by progressive folding of the mylonitic foliation approach sheath-fold geometry. Axial planes and fold limbs are nearly parallel to the mylonitic foliation. Mafic dikes that are apparently synplutonic in the undeformed quartz diorite immediately south of the mylonite zone and north of Kamiah have variable dips and azimuths. In the shear zone, however, these dikes lie nearly in the mylonitic foliation. Transposition of the dikes into near concordance with the foliation by simple shear requires high values of shear strain and suggests that cumulative top-to-the-southwest, reverse-slip displacement across the mylonite zone is at least 27 km, and likely more than 80 km. This displacement involves underthrusting of the Kamiah plutonic complex, emplaced within the Seven Devils-Wallowa terrane, beneath North America during Late Cretaceous docking with continental North America.

Cassiar platform, north-central British Columbia: A miogeoclinal fragment from Idaho

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 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.

Transforming Siberia along the Laurussian margin

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.

Icosahedral fracture tessellation of early Mesoproterozoic Laurentia

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.