Robert S. Houston

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.

Proterozoic accretionary tectonics at the southern margin of the Archean Wyoming craton

The Proterozoic Cheyenne belt, at the southern margin of the Archean Wyoming craton, consists of strongly deformed lithotectonic blocks bound by major mylonite zones. From north to south and structurally lowest to highest, these blocks include (1) Archean crystalline basement and associated early Proterozoic miogeoclinal rocks; (2) an amphibolite-grade orthogneiss terrane of unknown age containing probable rift-related mafic intrusive rocks; (3) a 1750–1790 Ma, marginal basin(?) succession consisting of upper amphibolite-grade pelitic and volcano-genie schist, associated peraluminous granite, and minor ultramafic rocks; and (4) a 1750–1790 Ma intermediate to mafic plutonic-metamorphic complex interpreted as the deep roots of an island-arc system. The earliest deformation, D1, produced a synmetamorphic, penetrative, horizontal transposition foliation and associated recumbent folds. Tectonic blocks were juxtaposed along major mylonite zones during D2. Macroscopic anil microscopic structures associated with D2 indicate northward thrusting along low-angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the Wyoming craton at about 1750 Ma. Thrusting occurred at minimum temperatures of 475 °C and produced an inverted metamorphic gradient in pelitic rocks north of the belt. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of dextral strike slip (D3) between 1750 and 1400 Ma. Cataclasis (D4) and brecciation associated with the Laramide orogeny (D5) locally overprint earlier structures. Oblique convergence between an island arc and the Archean continental nucleus may explain accretion, crustal shortening, and subsequent strike slip. The Phanerozoic accretionary events of eastern and western North America provide an analog for this model. Presence of similar lithologies and shear zones south of the Cheyenne belt suggest that the southern margin of the Wyoming craton may have been a long-lived zone of crustal accretion.

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.

COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 2: Precambrian basement structure and its influence on Laramide deformation

The influence of Precambrian basement structures on subsequent deformation is of considerable relevance to studies of continental evolution. COCORP deep seismic-reflection profiles were recently recorded in southeastern Wyoming, where several major, but temporally separated, tectonic elements of the western United States are superimposed. Of these, a fundamental boundary between Archean and Proterozoic basements and the eastern front of Laramide deformation were the principal targets of the reflection survey. The former may represent an ancient Proterozoic plate boundary; the latter is a prominent physiographic feature that signifies crustal deformation far within the North American craton, more than 1,500 km from the nearest coeval plate margin. The major crustal feature controlling a lateral, north-south variation in Laramide tectonic style appears to be the Archean-Proterozoic crustal boundary, known in the nearby Medicine. Bow Mountains as the Mullen Creek-Nash Fork shear zone. COCORP data in the Laramie Mountains and the Laramie Basin suggest that this shear zone dips ∼ 55° to the southeast. Northwest of the shear zone, the seismic basement is also characterized by southeast-dipping events, suggesting that the early Proterozoic tectonics that produced the shear zone were distributed over a wide region. Complex reflections down to 15-km depth or more under the Laramie Basin may correspond to structures or erosional truncations in metasediments overlying the Archean basement complex. Deep crustal events (between 35 and 40 km) north of the shear zone are short and discontinuous, in contrast with flat, laterally continuous reflections south of the shear zone at about 48-km depth which are interpreted as the crust-mantle transition. Thus, COCORP data and published results of regional refraction and gravity surveys suggest that the crust is significantly thinner in the Archean basement terrane northwest of the shear zone than it is in the Proterozoic province to the southeast. Differences in crustal thickness may be partly responsible for the difference between Laramide structures in Wyoming and Colorado, and a thin crust may also have facilitated Laramide deformation farther east in the Black Hills, located north of the shear zone.

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.

Precambrian Geochronology of the Medicine Bow Mountains, Southeastern Wyoming

The Medicine Bow Mountains of Wyoming are located in a transitional zone between the 2.5 b.y. old Superior Geochronologic Province to the north and the 1.3 to 1.7 b.y. old Central U.S. Geochronologic Province to the south. The mountains are crossed by a major northeast-trending shear zone, northwest of which is a quartzofeldspathic-gneiss complex that is intruded by the Baggot Rocks granite, by metagabbro, and by pegmatite. The complex is overlain nonconformably by more than 35,000 feet of miogeosynclinal metasedimentary rocks. Rb-Sr whole-rock isochrons indicate that the quartzofelds-pathic gneiss is 2410 ± 50 m.y. old, and the Baggot Rocks granite is 2340 ± 50 m.y. old. The metasedimentary rocks are at least 1550 ± 50 m.y. old, as indicated by a biotite Rb-Sr date, and are younger than 2410 ± 50 m.y. Whole-rock isochrons of 1550 ± 425 m.y. and 1650 ± 60 m.y. for two formations in the metasedimentary sequence may represent times of metamorphism but define at least minimum dates for sedimentation. Pegmatite intruding the gneiss complex is at least 1600 m.y. old; an anomalous initial Sr 87 /Sr 86 (0.745) permits the interpretation that the primary age is closer to 2400 m.y. Gneiss southeast of the shear zone is more calcic and mafic than that to the northwest, and it is intruded by metagabbro, pegmatite, foliated granite, and massive granite thought to be related to the Sherman Granite. The gneiss is unsuitable for direct dating because of low Rb/Sr ratios. Pegmatite yields mineral isochrons ranging from 1455 ± 40 to 1570 ± 40 m.y., and the Sherman-type granite yields a whole-rock isochron of 1335 ± 30 m.y. Analyzed whole-rock samples of foliated granite do not form a single isochron but show no evidence of being older than about 1715 m.y. No evidence has been found to indicate an age of 2.5 b.y. for rocks southeast of the major shear zone in the Medicine Bow Mountains, nor has any been found in nearby areas southeast of the projections of the shear zone. Rubidium-strontium mineral isochrons and individual mineral dates indicate one or more episodes of metamorphism in the Medicine Bow area between 1600 and 1455 ± 40 m.y. ago. We suggest that miogeosynclinal sedimentary rocks in the northern Medicine Bow Mountains, roughly correlative with Animikie and Huronian strata of Minnesota, Michigan, and Ontario, were deposited on the southern edge of a craton consisting of rocks mainly ≥ 2400 m.y. Gneiss of the southern Medicine Bow Mountains and gneiss and graywacke of the Front Range may be the more metamorphosed, eugeosynclinal counterparts of these sedimentary rocks which have been metamorphosed and transformed during the 1600 to 1455 m.y. old, or older, orogeny.

Kinematic history of the Cheyenne belt, Medicine Bow Mountains, southeastern Wyoming

The Proterozoic Cheyenne belt marks the southern boundary of the Archean Wyoming province. Within the Medicine Bow Mountains, it consists of strongly deformed, lithologically distinct blocks bounded by mylonite zones. Detailed field and petrographic investigations of the boundary, augmented by study of microscopic kinematic indicators, allow us to refine existing models for tectonic development of the belt. Northward thrusting under amphibolite facies conditions along low-angle mylonite zones emplaced successively deeper crustal blocks over supracrustal rocks of the Wyoming province. Mylonite zones were subsequently steepened and reactivated under greenschist-facies conditions during a period of distributive dextral strike slip. We suggest oblique convergence between an island arc and the Wyoming craton as a possible mechanism for both deformational events. Accretionary events in eastern and western North America, which involved initial convergence followed by strike slip, provide a Phanerozoic analog for our model.

Structural analysis of a ductile-brittle Precambrian shear zone in the Sierra Madre, Wyoming: western extension of the Cheyenne belt?

Abstract Existing plate tectonic models for the Cheyenne belt, a Proterozoic suture at the southern margin of the Archean Wyoming craton, are based largely on data from the Medicine Bow Mountains, southern Wyoming. This paper tests the hypothesis that a major structural and geochronological discontinuity in the Sierra Madre of southern Wyoming is the western extension of the Cheyenne belt. The Sierra Madre shear zone consists of a cataclastic western segment and a mylonitic eastern segment that comprises several mylonite zones. The eastern segment records early ductile thrusting (ca. 1750 Ma) and later dextral strike-slip faulting as revealed by macrostructural, microstructural, and quartz fabric analysis. Structures associated with these events are kinematically compatible and coeval with structures within the suture zone in the Medicine Bow Mountains. The correlation of shear zones in the two ranges is supported further by lithologic similarity of distinctive rocks north of and within the shear zones and the presence of unusual quartz fabrics documenting 〈 c 〉 slip in synkinematic granites in both ranges. Isolated exposures of mylonite along the dominantly cataclastic western segment suggest that the mylonitic shear zone was formerly more extensive. The western segment of the Sierra Madre shear zone is a single zone of intense cataclasis that truncated and dismembered the earlier ductile shear zone. Kinematic analysis within the cataclastic zone suggests that it formed during north-directed thrusting. East-striking, south-dipping thrust faults and associated overturned-to-the-north recumbent folds are developed in supracrustal rocks directly north of the cataclastic fault zone. This spatial association of north-vergent structures and their apparent kinematic compatibility suggests that they are genetically related. Two major northwest-striking, cataclastic, dextral strike-slip in the eastern Sierra Madre appear to merge with the cataclastic thrust fault. These cataclastic faults are interpreted as parts of a north-vergent, thrust-tear system that cut across the original suture zone as rocks in the upper plate of the thrust were transported to the north. It is not known whether this deformational event occurred in response to continued, post-suture convergence to the south or if it represents a separate and distinct Precambrian event. The prominent bend in the Sierra Madre shear zone may be an oroclinal flexure produced as a consequence of northward translation along the cataclastic thrust system.

Amphibolitization of calc-silicate metasedimentary rocks

Diopside granofels layers are associated with other metasedimentary rocks and gneisses near McMurdo Sound, Antarctica. Disrupted diopside granofels layers form tectonic inclusions which may have reaction rims of amphibolite. This amphibolite may either be a typical hornblende-plagioclase amphiholite or a hornblende-quartz amphibolite according to the initial composition of the diopside granofels. Chemical analyses of the reaction rims show that some of the amphibolites have major element compositions near tholeiitic basalt but that a sedimentary origin could be recognized for others that are high in silica. During amphibolitization, the diopside granofels loses Ca and gains Fe and Mg by mutual exchange of material with the surrounding rock over short distances. The metasomatic amphibolites fall on the “igneous differentiation trend” in a Niggli mg-c plot. At least small volumes of metasedimentary rock can attain the composition of basalt by amphibolitization in a metasomatic reaction rim.

Modeling of aeromagnetic data from the Precambrian Lake Owens mafic complex, Wyoming

Aeromagnetic anomalies overlying the Pre-cambrian Lake Owens layered mafic complex and adjacent bodies in the Medicine Bow Mountains of southeast Wyoming help constrain their geometric and magnetic properties. Modeling of these anomalies shows that the magnetic part of the Lake Owens Complex corresponds well with its surface outcrop and is only about 1 km thick. The remanent magnetization is approximately as strong as the induced magnetization. The total magnetization of the complex is 5.8 x 10-3 emu/cc. Rocks this strongly magnetized, were they located in the deep crust, could account for long-wavelength magnetic anomalies. The anomaly also shows the presence of a small, strongly magnetic body to the northeast of the Lake Owens Complex. This is probably a small, separate mafic body or a piece of the complex broken and disrupted during tectonic activity and/or during later granite intrusion. The Mullin Creek layered mafic complex, to the west of the Lake Owens Complex, has weaker magnetization dominated by induced magnetization or a viscous remanent overprint in the direction of the present Earths magnetic field. Its surface outcrop corresponds poorly with boundaries of magnetic units. A magnetic unit between the two mafic complexes may be a quartz diorite or a separate, shallow mafic body.