Karl E. Karlstrom

The cheyenne belt: analysis of a proterozoic suture in Southern Wyoming

Abstract The Cheyenne belt of southeastern Wyoming is a major shear zone which separates Archean rocks of the Wyoming province to the north from 1800-1600 Ma old eugeoclinal gneisses to the south. Miogeoclinal rocks (2500-2000 Ma old) unconformably overlie Archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift-formed continental margin. Intrusive tholeiitic sills and dikes are interpreted as rift-related intrusions and a date of 2000 Ma on a felsic differentiate of these intrusions gives the approximate age of rifting. There are no known post-2000 Ma felsic intrusions north of the Cheyenne belt. Volcanogenic gneisses and abundant syntectonic calc-alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks. The volcanics are a bimodal basalt-rhyolite assemblage. Plutons include large gabbroic complexes and quartz diorite (1780 Ma), syntectonic granitoids (1730-1630 Ma) and post-tectonic anorthosite and granite (1400 Ma). There is no evidence for Archean crust south of the Cheyenne belt. Structural data (thrusts in the miogeoclinal rocks, vertical stretching lineations, and the same fold geometries north and south of the shear zone) suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane (island arc) over the northern terrane (craton and miogeocline), probably as a continuation of the south-dipping subduction which generated calc-alkaline plutons of the southern terrane. A metamorphic discontinuity across the shear zone, with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south, also suggests thrusting of the southern terrane (deeper crustal levels) over the northern terrane (shallower levels). The Cheyenne belt may be a deeply-eroded master decollement, perhaps analogous to a ramp in the master decollement in the southern Appalachians. This interpretation of the Cheyenne belt as a Proterozoic suture zone provides an explanation for the geologic, geochronologic, geophysical, metallogenic, and metamorphic discontinuities across the shear zone.

Persistent influence of Proterozoic accretionary boundaries in the tectonic evolution of southwestern North America Interaction of cratonic grain and mantle modification events

Northeast-striking tectonic provinces and boundaries were established during 1.8–1.6-Ga assembly of juvenile continental lithosphere in the southwestern United States. This continental grain repeatedly has influenced subsequent intracratonic tectonism and magmatism. After 200 m.y. of stability, cratonic lithosphere was affected by regional, ∼1.4-Ga, dominantly granitic magmatism and associated tectonism that reactivated older northeast-striking shear zones in the Proterozoic accreted terranes, but not the Archean lithosphere. In contrast, 1.1-Ga, dominantly mafic magmatism and rifting did not reactivate northeast-striking zones, but occurred along new north–south fracture zones (e.g., Rocky Mountain trend) that reflect cracking of Laurentian lithosphere at a high angle to the Grenville collision. By 500 Ma, rifting had thinned the crust and mantle in the western United States creating the north–south Cordilleran miogeocline. East of the Cordilleran hingeline, isopachs in Lower Paleozoic sedimentary rocks follow northeast-trending structures (Cheyenne belt, Transcontinental arch, and Yavapai–Mazatzal province boundary), suggesting that older boundaries influenced isostatic response of the craton during thermal subsidence of the margin. Ancestral Rockies and Laramide uplifts and basins did not strongly reactivate northeast-striking boundaries. However, Ancestral Rockies structures end at the Archean–Proterozoic boundary, and Laramide magmatism (Colorado mineral belt) and metallogenic provinces follow northeast-striking Proterozoic boundaries, both suggesting deep-seated lithospheric influences on tectonism.nnPresent mantle structure and topography in the Rocky Mountain region continue to record an interaction between older crustal structures and younger mantle reorganization. Zones of partially molten mantle underlie northeast-striking Proterozoic boundaries (e.g., Snake River Plain, Saint George lineament, and Jemez lineament) and the north-striking Rio Grande rift, and are inferred to record replacement of lithosphere by asthenosphere preferentially along Archean–Proterozoic, Mojave–Yavapai, Yavapai–Mazatzal, and 1.1-Ga lithospheric anisotropies. Highest topography coincides with areas of low-velocity mantle, suggesting an importance of mantle buoyancy in the isostatic balance. Changes in topographic character across ancient crustal boundaries suggests a continued influence of crustal structures in differential uplift and denudation.nnInheritance of the Proterozoic northeast grain involves two basic factors: (1) “volumetric” inheritance, in which density and fertility of lithospheric blocks of differing compositions influence isostatic and magmatic response to tectonism; and (2) “interface” inheritance, in which mechanical boundaries are zones of weakness and mass transport. “Volumetric” inheritance is suggested by the distinctive isotopic signatures of different provinces and by the observation that Archean lithosphere has been consistently less fertile for magmas than Proterozoic lithosphere, due to thicker, colder mantle, and compositional differences. We infer that distinct mantle lithospheres have been attached to their respective crustal provinces (at scales of 100 km) since accretion. “Interface” inheritance controls include mechanical reactivation of northeast-striking province boundaries and shear zones as magma conduits, zones of renewed shearing, and zones accommodating differential uplift.

Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States

Growth of Proterozoic continental lithosphere in the southwestern United States involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca. 1.74, 1.70, and 1.65-1.60 Ga. Prograde metamorphism accompanied orogenic assembly, and peak metamorphic conditions outlasted deformation. Regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m.y. after assembly. Regions of low metamorphic grade were not uplifted substantially after assembly. We suggest that (1) relatively thin lithospheric fragments were assembled into isostatically stable, normal thickness continental lithosphere; (2) assembly did not erase lithospheric-scale heterogeneities; (3) the present juxtaposition of different crustal levels reflects differential uplift related to 1.4-1.1 Ga tectonomagmatic activity; and (4) the boundaries between different lithospheric blocks were repeatedly reactivated from Precambrian through Tertiary time.

Tectonic significance of an Early Proterozoic two-province boundary in central Arizona

A compilation of U-Pb zircon dates for lower Proterozoic rocks in central Arizona shows that, although rocks tend to be older in the northwest (1800−1696 m.y.) than the southeast (1738−1630 m.y.), there is no single boundary separating distinct geochronologic provinces in Arizona. Instead, the distribution of isotopic ages reflects the presence of two major tectonic provinces separated by a regionally subhorizontal boundary or boundaries. The northwestern part of central Arizona contains the Yavapai Series (1800−1755 m.y.) and calc-alkaline batholiths (1750−1696 m.y.), both believed to represent oceanic island-arc materials. The southeastern part of central Arizona is dominated by the Alder, Red Rock, and Mazatzal Groups and related hypabyssal intrusions (1710−1692 m.y.), with voluminous rhyolitic ash-flow tuffs and quartz arenite believed to record a relatively stable continental tectonic setting. Two working hypotheses emerge to explain the juxtaposition of representatives of these two tectonic provinces over a 100-km-wide zone in central Arizona. One interpretation (model 1) suggests that rocks of the southeast province were deposited with angular unconformity on newly accreted continental crust composed of northwest province rocks. A second interpretation (model 2) suggests that the two areas represent allochthonous terranes that evolved separately and were juxtaposed by large subhorizontal movements on thrusts and strike-slip faults. An important new constraint is that the 1699-m.y.-old strongly peraluminous Crazy Basin Quartz Monzonite was emplaced in the northwest province during ductile deformation at depths greater than 8 km at the same time that rhyolitic ash-flow tuffs and quartz arenite were being deposited in the southeast province. For model 1, this implies a rapid change of tectonic regimes about 1700 Ma, from convergence to uplift, erosion, sedimentation, and possibly extension. For model 2, the differences in crustal level, structural style, and petrologic affinity between ∼1700-m.y.-old rocks in both provinces are believed to result from juxtaposition of different crustal blocks after 1700 Ma.

Stratigraphy and depositional setting of the Proterozoic Snowy Pass Supergroup, southeastern Wyoming: Record of an early Proterozoic Atlantic-type cratonic margin

Metasedimentary rocks in the Medicine Bow Mountains and Sierra Madre are divided into four groups. The > 3-km-thick Phantom Lake Metamorphic Suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late Archean granites. The > 2.5-km-thick Deep Lake Group unconformably overlies the Phantom Lake Suite and late Archean granites and contains fluvial sediments, including radioactive quartz-pebble conglomerates, and glaciomarine deposits. Both successions are intruded by large sills of tholeiitic gabbro. The 4.5-km-thick lower Libby Creek Group is inferred to be in thrust-fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta. This succession is intruded by the 2,000-m.y.-old Gaps Intrusion and comagmatic tholeiitic to weakly alkalic dikes. The 3-km-thick upper Libby Creek Group is bounded by a thrust fault below and by the Cheyenne Belt above and contains carbonates and marine slates. The early Proterozoic Deep Lake, lower Libby Creek, and upper Libby Creek Groups collectively are named the Snowy Pass Supergroup. Lithologies and stratification sequence in the well-preserved Medicine Bow Mountain section suggest transgressive, miogeoclinal sedimentation during the early Proterozoic. Paleocurrent data indicate that fluvial, then deltaic, sedimentation of the Deep Lake and lower Libby Creek Groups took place on a southwest-dipping paleoslope, parallel to the inferred south cratonic boundary of the Wyoming Province. This and a few west-directed paleocurrents suggest a continental or microcontinental block to the south, bounding sedimentation in a northeast-elongated basin. A rift setting for deposition of these units explains the transgressional character of the sediments, the deltaic sedimentation with paleocurrents parallel to the cratonic boundary, and the 120° bend in the Cheyenne Belt between the Medicine Bow Mountains and the Sierra Madre. The upper Libby Creek Group is interpreted to represent open marine conditions following separation of the two continental blocks. Tholeiitic sills in the Deep Lake Group and tholeiitic to weakly alkalic dikes in the Libby Creek Group are thought to be related to basaltic igneous activity associated with compound early Proterozoic rifting between 2,300 and 2,000 m.y. ago.

Gravity profiles across the Cheyenne Belt, a precambrian crustal suture in southeastern Wyoming

Abstract Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm 3 . Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.

Proterozoic Farwell Mountain–Lester Mountain suture zone, northern Colorado: Subduction flip and progressive assembly of arcs

This paper considers the amalgamation of arc and oceanic terranes to be the main mechanism of ca. 1.8-1.6 Ga continental crustal growth in southwestern Laurentia. On the basis of geologic and seismic reflection data and teleseismic images, we propose the Farwell Mountain-Lester Mountain suture zone as the northern-most Paleoproterozoic arc-arc suture. North-dipping (Farwell Mountain) seismic reflections project from 18 km depths to the surface and are interpreted to represent conjugate thrusting as the 1.79-1.77 Ga Green Mountain arc was partially underthrust beneath the Archean craton. We speculate that a north-dipping high-velocity mantle slab in the teleseismic image is a continuation of this thrust zone. South-dipping (Lester Mountain) reflections project from 22 km depths to the surface and are interpreted to be a thrust zone between the Green Mountain arc and the 1.76-1.72 Ga Rawah block. Surface features of the Farwell Mountain-Lester Mountain suture zone are (1) marble, chert, rock with sillimanite pods, ultramafic rocks, sulfide deposits, and pillow basalts, which we interpret to be a dismembered accretionary complex; (2) an axial-planar fabric to north-verging isoclinal folds (F 2 ), which we interpret to be part of a north-vergent fold-and-thrust system; and (3) a metamorphic break between ∼500 °C rocks to the north and ∼610 °C rocks to the south, which we attribute to reactivation of the zone. Movement on the Farwell Mountain backthrust is inferred to relate to Cheyenne belt suturing at 1.78-1.75 Ga. We correlate suturing in the Farwell Mountain-Lester Mountain suture zone to S 1 /D 1 (1.746-1.74 Ga) in the Soda Creek-Fish Creek shear zone. We attribute the complexity of this broad suture zone to initial conjugate thrusting, plus overprinting and steepening of accretionary structures by subsequent tectonism.

The Chaparral shear zone: Deformation partitioning and heterogeneous bulk crustal shortening during Proterozoic orogeny in central Arizona

The Proterozoic Chaparral shear zone of central Arizona is one of a network of subvertical, northeast-striking shear zones that divide the Proterozoic orogenic belt of Arizona into tectonic blocks. The zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation. Stretching lineations plunge shallowly northeast, and displacement across the zone was dominantly right-lateral. Several lines of evidence constrain displacement across the zone to be greater than 5 km, but probably less than tens of kilometers: (1) stratigraphic and plutonic rocks can be correlated across the zone, (2) structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar, and (3) integration of shear strain (estimated by deflection of earlier fabric) suggests greater than 5 km of strike slip. More important than scale of movement across the zone, structural studies have clarified several aspects of the assembly history of the orogen. Early north- to northwest-striking S 1 foliation, on both sides of the Chaparral shear zone, records one or more deformational events that took place between 1.75 and 1.7 Ga. Early fabrics record northeast-southwest or east-west shortening and west-verging thrusting that we interpret to have formed during development of a primitive arc complex. In contrast, intense deformation in the Chaparral shear zone took place during regional northwest-southeast shortening, D 2 , that produced the dominant subvertical northeast-striking foliation and the present block architecture of the orogen. D 2 is interpreted to have been in part synchronous with emplacement of 1.70 Ga granitoids and to record assembly of volcanic belts to North America. Of general interest is the character of D 2 deformation partitioning in and across the shear zone. Two types of high-strain domains were generated during shortening. One type, represented by rocks southeast of the shear zone, involved extreme shortening and transposition by folding. The other type, represented by the shear zone, involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones. Overprinting relationships suggest that the second type may have nucleated on the first, implying an important component of deformation partitioning in time, as well as space. Mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing. Conjugate shear bands suggest a shallow southeast-plunging σ 1 late during strike-slip deformation. This is consistent with regional D 2 shortening but not with the incremental shortening direction expected during right-lateral simple shear. This supports a regional kinematic model in which the Chaparral shear zone (right-lateral) and a temporally related left-lateral shear zone were regional-scale conjugate shear zones that accommodated heterogeneous D 2 northwest-southeast orogenic shortening via escape-block tectonics. Thus, orogen-parallel strike-slip displacements in the Chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction.

Inter-Wedging Nature of the Cheyenne Belt – Archean-Proterozoic Suture Defined by Seismic Reflection Data

New seismic reflection data from the CD-ROM project (Continental Dynamics of the Rocky Mountains) show that the Archean-Proterozoic suture in Wyoming, the Cheyenne belt, consists of a crustal-scale, conjugate thrust wedge, where Archean and Proterozoic crust were thrust into each other. Moderately S-dipping reflections extend to depths of 15-18 km and surface at the mapped shear zones of the Cheyenne belt; these terminate close to a north-dipping reflection that extends to about 24 km depth and converges at the surface at the Farwell-Lester Mountain (FLM) area with several strong, south-dipping reflections. The FLM zone is marked by dismembered ophiolites suggestive of an ocean basin and may represent a back-arc basin subsequently closed by continued south-dipping subduction, resulting in a cryptic suture. Arcuate criss-crossing reflections in Archean basement to the north of the Cheyenne belt are related to folding, inferred conjugate thrusting and an antiformal duplex stack. This stack formed during Paleoproterozoic suturing in supracrustal rocks comprising at least the upper 24 km of the crust. Our analysis of wide-angle seismic data does not reveal a 7 km/s lower crustal layer that could be interpreted as underplate in southern Wyoming or northern Colorado. A suture, marked by complex interwedging of crustal blocks, was probably steepened by continued convergence to the south.

Geometry of Proterozoic sutures in the central Rocky Mountains from seismic reflection data: Cheyenne belt and Farwell Mountain structures

[1]xa0Seismic reflection data show ∼40° S-dipping reflectors that extend from the Cheyenne belt (CB-Archean-Proterozoic suture) to 20 km depths, and profound differences in crustal reflectivity across the suture zone, which is itself imaged as an interwedging boundary. Archean crust has a Moho reflection (at 40 km) and abundant upper crustal reflectivity; Proterozoic crust has no Moho reflection and is distinctly less reflective. Reflective zones south of the Cheyenne belt include 40° S- and N-dipping reflections that extend from the Farwell Mountain zone to 20 km depths; these are interpreted to be a complex cryptic suture zone between the 1.78–1.76 Ga Green Mountain block and 1.75–1.72 Ga Rawah block.