Rocky Mountain Geology 33.2

Introduction to special issues: Lithospheric structure and evolution of the Rocky

K. E. Karlstrom

Keywords: Introduction to special issues: Lithospheric structure and evolution of the Rocky

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

K. E. Karlstrom and E. D. Humphreys

Keywords: Rocky Mountains, lithosphere, inheritance, Proterozoic, accretion.
Crustal and uppermost mantle structure along the Deep Probe seismic profile tc "Crustal and uppermost mantle structure along the Deep Probe seismic profile"

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.

Present 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.

Inheritance 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.

Crustal and uppermost mantle structure along the Deep Probe seismic profile

C. M. Snelson, T. J. Henstock, G. R. Keller, K. C. Miller, and A. Levander

Keywords: Lithospheric structure, Rocky Mountains, seismic refraction, gravity.

The Rocky Mountain region has undergone a complex tectonic history that includes Proterozoic accretion to form the North American craton, late Paleozoic deformation, Cretaceous to early Tertiary shortening, and Oligocene to Recent extension. Understanding the effects of these events on lithospheric structure was the primary goal of the Deep Probe seismic experiment. This is a lithospheric-scale study of the Rocky Mountain region that attempted to image crust and upper mantle structures up to 500 km depth to provide insights on the effect of various tectonic events on today’s continental structure. To accomplish this goal, instruments were deployed along a 2400-km-long transect from New Mexico to Canada to record explosions 10 times more powerful than those employed in conventional crustal studies. The Deep Probe results provide new constraints on the location and geometry of the Archean–Proterozoic boundary near the Colorado–Wyoming border, as well as new information on crustal thickness, and uppermost mantle velocities along the profile. Geophysical modeling of the profile used well log and geologic data to evaluate the composition and structure of the uppermost crust. Seismic refraction and reflection, gravity, and receiver function studies were employed to constrain properties of the lower crust and upper mantle structure. The final model shows that seismic velocities along the Deep Probe profile range from 3.5 km/s in the basins to over 8.2 km/s in the upper mantle. At the southern end of the profile, the model indicates a crustal thickness of about 35 km beneath the Basin and Range province. The crust gradually thickens to about 40 to 45 km going north along the profile into the Colorado Plateau. An area of 50 km-thick crust under northwestern Colorado may reflect Proterozoic tectonism related to the suture zone between the Archean and Proterozoic terranes. Northwestward thinning of the crust to about 40 km under southern Wyoming is interpreted as evidence for a relict (2.0 Ga) passive continental margin. The crust in the Archean Wyoming province thickens to over 50 km going north, and then thins again under southern Canada. This thickening is due to a lowermost crustal layer that is about 20 km thick and is confined to the Archean Wyoming province. This lower crustal layer has velocities ranging from 7.05 to 7.3 km/s, which corresponds to a mafic composition. Thus, this layer is interpreted as mafic material that was probably underplated during the Archean. The uppermost mantle of the Archean Wyoming province has lower velocities (~8.1 km/s) on average than typical cratonal areas, which is consistent with it being located in and adjacent to the North American Cordillera, which has undergone significant recent tectonism.

Deep structure beneath the Southern Rocky Mountains from the Rocky Mountain Front Broadband Seismic Experiment

A. Lerner-Lam, A. Sheehan, S. Grand, E. Humphreys, K. Dueker, E. Hessler, H. Guo, D. K. Lee, and M. Savage

Keywords: Seismology, structure of lithosphere and crust, Rocky Mountains.

The deployment of a two-dimensional array of broadband seismometers during the PASSCAL Rocky Mountain Front experiment produced a teleseismic and regional event data set which provides constraints on both the laterally averaged and laterally varying structure of the crust and upper mantle beneath the Southern Rocky Mountains and Great Plains. Results from a spectrum of seismological imaging and inversion techniques indicate that the western edge of the North American tectosphere has been relocated from its position beneath the Paleozoic passive margin to a position several hundred kilometers east of the Colorado Front Range. The mantle and crustal structure presently beneath the Colorado Rockies suggest a laterally variable dynamic state, which, on average, must provide partial support for the high topography.

Geophysical studies of crustal structure in the Rocky Mountain region: A review

G. R. Keller, C. M. Snelson, A. F. Sheehan, and K. G. Dueker

Keywords: Crustal structure, Rocky Mountains, Rio Grande rift, geophysics.

The Rocky Mountains have fascinated the geological community for over 100 years, but crustal-scale geophysical studies are relatively rare in this region. However, a knowledge of crustal structure is essential if we are to fully understand the region’s tectonic history. Thus, we have compiled and synthesized existing information on crustal structure in order to provide as complete a picture as possible at this time. We have focused on Wyoming, Colorado, and New Mexico where there are enough data to make useful correlations with geologic features. The Rocky Mountain region includes the crest of a broad uplift on which the Southern Rocky Mountains are located. In turn, the Southern Rocky Mountains are bisected by the Rio Grande rift. In the Rio Grande rift, distinct crustal thinning (at least 5 km relative to adjacent areas) can be documented from Albuquerque, New Mexico southward. The area of thinned crust widens southward, as does the physiographic expression of the rift. The thinnest crust documented to date (about 28 km) is found west of El Paso, Texas. In contrast with East Africa, the crustal thinning is gradual from the rift valley to the shoulders, perhaps reflecting the back-arc thermal regime that existed prior to rifting. The thickest crust in the region (about 53 km) appears to be associated with both the Southern Rockies in Colorado and the topographically lower Great Plains in Colorado and New Mexico. This lack of correlation between topography and crustal thickness implies that the mantle is playing a major role in the attainment of isostatic balance in this area. Magmatic modification of the crust during rifting appears to have been minor. However, the modification due to the voluminous mid-Tertiary magmatism in the Datil–Mogollon volcanic field (southwestern New Mexico) and San Juan volcanic field (southwestern Colorado) is substantial. In the Datil–Mogollon field, a batholith that accounts for about one fifth of the crustal thickness has been detected in the upper crust, and a feature of similar dimensions is indicated in the San Juan region. There is evidence of the crust thinning northward from Colorado into Wyoming, which could be a relic of Archean rifting of the southern margin of the Wyoming craton.

Large-scale geomorphology and fission-track thermochronology in topographic and exhumation reconstructions of the Southern Rocky Mountains

F. J. Pazzaglia and S. A. Kelley

Keywords: Rocky Mountains, Laramide orogeny, landform evolution, apatite fission-track, thermochronology, geomorphology, exhumation.

Long-term landscape evolution is the integrative sum of constructive rock-uplift processes and destructive erosional processes. For the Southern Rocky Mountains, it has been proposed that Phanerozoic rock uplift and erosion have been influenced by crustal structure that was inherited from the time of assembly of the continent in the Proterozoic. This paper compares large-scale geomorphology and long-term rock exhumation histories in three distinct crustal terranes to assess possible differences in Laramide and post-Laramide deformation of the Southern Rocky Mountains. We analyze modern topographic data and fission-track thermochronology for several portions of the Southern Rockies with distinctly different post-Laramide geologic histories and estimate the amount of Laramide and post-Laramide rock uplift. The areas of investigation include: (1) the Colorado Front Range, an area of regional elevated heat flow in the Yavapai province; (2) the New Mexico Taos Range, a region of localized high heat flow throughout the late Tertiary and Quaternary along the Rio Grande rift in the Mazatzal province; and (3) the New Mexico Sierra Nacimiento, a region of localized high heat flow in the Quaternary associated with the Jemez Mountains, also in the Mazatzal province. Both the Taos Range and Sierra Nacimiento lie in proximity to the Jemez lineament, which is thought to be a significant Proterozoic structural feature that has influenced Quaternary volcanism along its trend.

We utilize and test a dimensionless topographic index called the ZR ratio, which is the ratio between mean elevation and mean relief, as a quantitative measure of the differences in orogen-scale topography. Digital topography (DEM) and GIS software (ARC/INFO) allow for the rapid determination of this ratio at various length scales. Our results show that the Taos Range has the lowest ZR ratio (most rugged topography), and the Sierra Nacimiento has the highest ratio (least rugged topography). Fission-track thermochronology results show that a partial annealing zone (PAZ) is preserved in the Front Range and the northern portion of Sierra Nacimiento (the region incidentally not on the high-heat flow Jemez lineament); both the Taos Range and the southern portion of Sierra Nacimiento have had the PAZ removed by erosion. Reconstructed amounts and timing of rock denudation based on the fission-track data are consistent with the ZR ratios in that the most rugged topography exhibits the greatest and most recent denudation, whereas the least rugged topography experienced far less denudation, most of which occurred immediately after the Laramide orogeny. <BR><BR>Fission-track thermochronology and geomorphic results support an overall model of significant crustal thickening during the Laramide orogeny, followed by relatively low and differential rates of rock uplift, erosional exhumation, and isostatic rebound. Specific regions of greater post-Laramide rock exhumation and rugged topography are highly correlated with regions of known localized high heat flow and late Cenozoic volcanism, a finding consistent with the hypothesis that at least locally, the high mean elevation of the Southern Rockies is supported by a buoyant mantle. Both the style of Laramide deformation and subsequent magmatic input from a low-velocity, buoyant mantle may have been influenced by older crustal structure. All three factors, Proterozoic crustal structure, Laramide deformation, and post-Laramide uplift, appear to play an important role in the ability of streams to integrate through the Rocky Mountain foreland. As rates of erosion are strongly tied to local relief of well-drained landscapes, stream integration at the large scale may explain the correlation between long-term denudation rates and modern topographic ruggedness (ZR ratio), and be the limiting factor in the processes driving rock uplift in the post-Laramide Southern Rockies.

Medicine Bow orogeny: Timing of deformation and model of crustal structure produced during continent–arc collision, ca. 1.78 Ga, southeastern Wyoming

K. R. Chamberlain

Keywords: Medicine Bow orogeny, Precambrian, plate tectonics, Wyoming province, Cheyenne belt, crustal structure, continent–arc collision.

A four-dimensional model for the evolution of the late-Paleoproterozoic Cheyenne belt arc–continent suture is presented based on available geologic mapping, structural analysis, geophysical constraints, geobarometry, geochronology, and isotopic data. All of the data are consistent with a southeast-dipping suture and deformation that lasted from 1.78 Ga to at least 1.76 Ga, and possibly as late as 1.74 Ga. Archean crustal components and/or detritus were subducted at least 30 to 70 km south of the trace of the suture. There is considerable variation in crustal structure and tectonic evolution along strike of the Cheyenne belt. Thick-skinned, intra-cratonic uplift along the Laramie Peak shear zone and synorogenic emplacement of the Horse Creek anorthosite complex occurred in the east, whereas low-grade metamorphism and late cataclastic thrust faulting occurred in the west. Some of this lateral variation may reflect the influence of preexisting crustal features such as high-angle normal faults and crustal heterogeneities related to ca. 2.0-Ga rifting, whereas other differences may reflect variations in collision geometry. The lithospheric architecture of this arc–continent suture created compositional and structural anisotropies that influenced later deformation and magmatism, such as the generation and emplacement of the 1.43-Ga Laramie anorthosite complex and location of -Paleozoic diamond-bearing diatremes.

In this paper, the term “Medicine Bow orogeny” is proposed to describe the ca. 1.78-Ga arc–continent collision that formed the Cheyenne belt suture, and the subsequent structural evolution of the orogenic zone, which may have continued to ca. 1.74 Ga. The orogenic belt trends from southeastern Wyoming to northeastern Nevada, a distance of ~1900 km. This term is invoked to differentiate the tectonic history along the Cheyenne belt from both the Yavapai orogeny to the south, and Central Plains orogeny to the east, because these orogenies include younger rocks and younger deformation. The arc terrane involved in the Medicine Bow -orogeny is probably 50 to 100 km wide, and it makes up the basement of northern Colorado and continues at least as far south as the Soda Creek–Fish Creek shear zone in north-central Colorado. Accretion during the Medicine Bow orogeny represents a substantial addition to the North American continent.

Geophysical constraints on the deep structure of the Cheyenne belt, southeastern Wyoming

S. B. Smithson and N. K. Boyd

Keywords: Precambrian suture, crustal structure, Northern and Central Rockies, Precambrian geophysics, seismic reflection, gravity, Laramie orogeny.

The Cheyenne belt (CB), a Precambrian suture separating the Archean Wyoming province from Proterozoic island-arc rocks to the south, is marked at the surface by a mylonite zone up to 7 km thick in the Sierra Madre and Medicine Bow Mountains. Geophysical data used to determine deep structure of the CB and surrounding crust include old crustal-refraction profiles, crustal-reflection profiles, several wide-angle seismic profiles, and Bouguer gravity anomalies. Laramide frontal faults that dip 30° to 45° westward to depths of 15 to 20 km offset Precambrian structures in seismic-reflection profiles. As determined from refraction profiles, crustal thickness changes from about 38 km in the northern Laramie Mountains to over 50 km in central Colorado, and Bouguer gravity anomalies show a corresponding decrease. The CB is imaged with moderate apparent dips on three seismic-reflection sections; true dip is about 60° southeast, and maximum depth in seismic sections is 9 to 14 km. Bouguer gravity anomalies are marked by a decrease to the south and an increased gradient across the CB. This is interpreted as a ramp in the Moho near the CB and a gently thickening crust southward toward Colorado. If so, this Moho ramp near the CB probably has persisted since Proterozoic suturing. Long--wavelength, Laramide crustal thickening and an overprinted, late-Laramide, crustal–upper mantle thermal event is evident in Colorado.

Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity

A. Lester and G. L. Farmer

Keywords: continental lithosphere, xenoliths, lower crust, granulites, kimberlite, Cheyenne belt.

Lower-crustal and upper-mantle xenoliths entrained in Phanerozoic igneous rocks in the northern Southern Rocky Mountains provide the only direct means of studying the petrologic and geochemical nature of the deep-continental lithosphere in this region. Petrographic and geochemical features of peridotite xenoliths from the State Line kimberlite district, near the Cheyenne belt, suggest that during Devonian kimberlite emplacement, infertile, cryptically metasomatized, Archean mantle dipped south beneath a distinctly more fertile, and veined, lithospheric mantle associated with the Proterozoic continental crust of the Colorado province. Lower-crustal xenoliths recovered from the State Line kimberlites and late Cenozoic ultrapotassic igneous rocks at Leucite Hills (north of the Cheyenne belt) are dominated by mafic granulites. This indicates that both the Archean and Proterozoic lower-continental crust was, and may remain today, mafic in composition.

Existing major-element data support a tholeiitic basalt precursor for the mafic granulites associated with the Colorado province. Unfortunately, despite its importance for constraining models of lithospheric structure in the northern Rocky Mountains, detailed information regarding the age, trace element and isotopic compositions, or physical attributes of the mafic xenoliths is presently not available. In order to evaluate the age and composition of continental lithosphere in this region and to assess the possible role of mafic lower-crustal rock types as sources for Laramide magmatism we outline a plan (as part of the Rocky Mountain Continental Dynamics project) to obtain geochemical and geochronological information from xenoliths recovered at the State Line kimberlite district and Leucite Hills.