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The Hanna Basin of south-central Wyoming has been considered anomalous among other Laramide depocenters of the Rocky Mountain region because of its combination of small size and great thickness of synorogenic strata. Most prior interpretations of the Hanna Basin have assumed a history of subsidence and sedimentary infilling discrete from surrounding basins. In contrast, we summarize new geological and paleontological information from the northeastern corner of the modern Hanna Basin suggesting that, prior to late Paleocene time, the Hanna Basin and nearby Carbon, Pass Creek, Laramie, and Shirley Basins were unified and depositionally continuous with the much larger Green River Basin to the west. Only late in the local expression of the Laramide orogeny (late Paleocene and early Eocene) did this "greater Green River Basin" become subdivided through completion of development of intrabasinal, basement-involved thrust faulting and associated anticlines. We view the present Hanna Basin as only a small, structurally defined remnant of an enormous, ponded basin that extended eastward during most of Paleocene time from the Wyoming-Idaho-Utah thrust belt to the newly uplifted Laramie Mountains. That basin was bounded on the south by the Uinta Mountains and combined Sierra Madre-Medicine Bow Mountains and on the north by the Gros Ventre Range, Wind River Mountains, and Sweetwater arch. When the original configuration of the unified greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines its various margins, this basin at least rivaled dimensions of the Powder River Basin of Wyoming and Montana; the greater Green River Basin greatly exceeded the volume of sedimentary accumulation within the Powder River Basin. Rates of Paleogene basinal erosion associated with structural subdivision of eastern components of the greater Green River Basin were prodigious. Within context of a measured... [ABSTRACT FROM AUTHOR]

The Salina area of central Utah contains an unusually complete Tertiary stratigraphic record, affording unique opportunities to study details of the Tertiary geologic history. Stratigraphic and structural studies reveal Neogene ages for tectonically significant parts of all major structures such as the 80-km-long Sanpete-Sevier Valley anticline (SSVA) and its flanking structures, the Wasatch monocline and Sevier Valley syncline. Newly identified critical components of the Neogene history in the SSVA include: (1) a protracted Eocene through early Miocene period of quasi-continuous sedimentation and relative structural quiescence that buried an early phase of the anticline; (2) east-trending folds in rocks of the anticlinal core; (3) widespread plutons in the southern part of the core; (4) strike-slip faults; (5) major thinning of strata (vertical structural convergence) between uplifted Jurassic rocks of the anticlinal core and downdropped superjacent Tertiary rocks; and (6) a widespread zone of altered rock at the contact between core rocks and overlying Tertiary rocks. We interpret the thinning and widespread alteration (5 and 6) to be linked through protracted processes of extensional faulting (possibly including detachment faulting), fluid flow, massive dissolution, and dissolution collapse at the contact between the Jurassic core rocks and overlying Tertiary rocks. We suggest that the SSVA, one of several diapir-like elongate uplifts in central Utah, is somewhat analogous to flank ridges that develop at landslide margins by outward and upward flow of mechanically weak clay-rich rock or gouge. By analogy, the SSVA results from outward and upward movement of the relatively incompetent Arapien Shale from beneath the adjacent Sevier Valley. If the structures formed during these newly recognized components of Neogene history are palinspastically restored, much of the evidence for contractional deformation at shallow structural levels is eliminated; this affects... [ABSTRACT FROM AUTHOR]

Seismic-reflection and borehole data along with crustal-scale refraction/reflection data provide new evidence for the Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and Huntington, Ruby, and Lamoille valleys. Analyses of these data suggest: (1) along the western flank of the Ruby Mountains an early stage of upper-crustal extension provided accommodation space for deposition of apparently synextensional strata; (2) the oldest sedimentary rocks in the developing basin along the western flank of the Ruby Mountains are middle Eocene in age, suggesting that active upper-crustal extension and early basin formation in northeastern Nevada began at least by that time; (3) Ruby Valley is bounded by high-angle, east-dipping normal faults on the west and a relatively low-angle, west-dipping normal fault on the east; (4) crustal thicknesses beneath the eastern flank of the Ruby Mountains do not reflect local topographic relief and estimated amounts of extension; and (5) adjacent to the range, maximum thicknesses of basin-fill sedimentary rocks do not directly reflect maximum amounts of exhumation of the Ruby Mountains. Together, these observations suggest that either preexisting crustal roots (subsequently dissipated), or middle- or lower-crustal flow prior to and during extension, were involved in evolution of the core complex. [ABSTRACT FROM AUTHOR]

We present an interpretation of seismic-reflection data from the Playas and Hachita valleys of southwestern New Mexico which places new constraints on the geometry of structures responsible for Laramide shortening, and subsequent Basin and Range extensional structures. With seismic interpretation tied to well data and gravity modeling, we address aspects of the debate that still surrounds the question of whether Laramide shortening was accommodated by "thin-skinned" thrusting along a regional décollement or by basement-involved block uplifts. Our interpretation suggests that shortening was at least in part accommodated with low-angle, basement-involved thrusts. Subsequent extension occurred on range-bounding normal faults that become listric near 10-km depth. The relative lack of recent basin fill in the Playas Valley, and particularly the Hachita Valley, suggests either that uplift of the mountains along normal faults has been relatively recent or that extension has only just compensated for earlier shortening, and has yet to create deeply subsided basins. [ABSTRACT FROM AUTHOR]

A small, but perfectly exposed outcrop of Mesoproterozoic rocks in the western Llano uplift displays lithologic and structural relations from which a sequence of geologic events has been determined. Detailed geologic mapping and petrologic studies combined with new U-Pb ages of key units are used to constrain the tectonic evolution of this little-studied region and provide a basis for comparison with the previously studied southeastern uplift. The outcrop consists of layered mafic and felsic gneiss cross-cut by three generations of granitic dikes and sills. The sequence of events as deduced from cross-cutting relations and U-Pb geochronology is as follows: (1) generation of compositional layering (S[sub0]) and parallel biotite foliation (S[sub1]) in the gneisses at 1256 Ma; (2) emplacement of the first set of granitic sills and dikes at 1253 Ma; (3) transposition, isoclinal folding, amphibolite facies metamorphism, and fabric formation (S[sub2]) between 1253 Ma and 1126 Ma; (4) emplacement of pegmatitic granitic dikes at 1126 Ma; (5) open folding and boudinage between 1126 Ma and 1076 Ma; and (6) emplacement of aplitic granitic dikes at 1076 Ma. Four U-Pb ages of titanite, two each from the gneiss and pegmatitic granite, are concordant at ∼ 1114 Ma indicating that the region had been sufficiently hot to reset titanite ages in the gneiss and that the region had cooled below the titanite blocking temperature by this time. The tectonic evolution of this outcrop is remarkably similar to that of other areas of the Llano uplift, suggesting that the entire uplift experienced a similar evolution. [ABSTRACT FROM AUTHOR]

Recent detailed work in key regions along two north-south transects in northern New Mexico highlights continued controversy about Proterozoic tectonic evolution. Ductile deformation features (folds, ductile thrusts, and associated foliations and lineations) are grouped into three deformation generations. D[sub1] includes cryptic bedding-parallel foliation and fold nappes. D[sub2] involves north-verging, km-scale inclined folds, the main shortening foliation, and D[sub2], structures that further attenuate or reactivate F[sub2] folds. D[sub3] involves east-west-trending open folds and domes and associated crenulation cleavage. Although others can dominate locally, S[sub2] is the dominant regional foliation that could possibly be imaged seismically. Map relationships around ca. 1.65- and ca. 1.42-Ga plutons and porphyroblast-matrix studies of dated minerals show that D[sub3] occurred at ca. 1.42. The age of D[sub2] is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic studies also indicate multiple metamorphic events, M[sub1]-M[sub3], that may relate to the deformational events. New geochronology indicates that most metamorphic minerals grew (or were reset) at ca. 1.47-1.35 Ga. U-Pb dates on metamorphic zircon, monazite, titanite, staurolite, garnet, and tourmaline suggest regional metamorphism to 550-700°C at 1.47-1.42 Ga. Metamorphic aureoles are present around plutons, but the highest grades of metamorphism are in areas with no exposed 1.42-Ga plutons. Metamorphism is interpreted to record a regional mantle-driven thermal event, the latter parts of which correspond to a time of pluton emplacement. [sup40]Ar/[sup39]Ar dates record post-1.42-Ga cooling: the highest grade rocks yield the youngest cooling ages, indicating slow cooling and gradual unroofing of the 1.42-Ga thermal profile following 1.42-Ga metamorphism. Our preferred model is that macroscopic geometries (D[sub1]-D[sub2]) were established by 1.65 Ga, and that regional amphibolite-grade... [ABSTRACT FROM AUTHOR]

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 YavapaiMazatzal 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... [ABSTRACT FROM AUTHOR]