Ernest C. Hauser

The Moho in the northern Basin and Range province, Nevada, along the COCORP 40°N seismic-reflection transect

COCORP seismic-reflection profiles across Nevada at about 40°N image a prominent, essentially continuous band of reflectors at a two-way traveltime of 9 to 11 s. The approximate correspondence of this reflection time with estimates of the two-way traveltime to the Moho in this area provided by seismic-refraction data suggests that the prominent reflections are from the Moho. The relief on these reflectors (the “reflection Moho”) beneath Nevada, across a latitudinal transect of

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

Crustal structure of eastern Nevada from COCORP deep seismic reflection data

The western Nevada segments of the COCORP 40°N deep seismic-reflection survey of the North American Cordillera reveal the geometry of structures of Cenozoic and possibly earlier ages to travel-times of > 10 s, corresponding to depths of >30 km. The most striking feature of the data is a band of prominent reflections, typically at traveltimes of 9.5 to 10.5 s, that are present discontinuously across the entire data set. Few reflections are observed from beneath the base of this reflective zone, which is interpreted as the crust-mantle transition. This “reflection Mono” is inferred to be continuous across the survey area, varying gradually in depth but without resolvable offsets. It appears to have taken its present form or position during basin-range crustal extension. The middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west. These reflections may represent intrusions or shear zones related to basin-range or pre-basin-range extension, but some are likely to be inherited from earlier compressional deformation. Reflections from the upper crust are interpreted as images of basin-fill strata, basin-range normal faults, and Mesozoic and Paleozoic thrusts related to back-arc thrusting and accretion of oceanic and arc-related rocks.

COCORP Arizona transect: Strong crustal reflections and offset Moho beneath the transition zone

The Consortium for Continental Reflection Profiling (COCORP) transect across the southwest margin of the Colorado Plateau in Arizona reveals prominent subhorizontal zones of reflections, here termed the Bagdad reflection sequence (BRS), which dominate the upper and middle crust in the transition zone (TZ) between the Colorado Plateau and the Basin and Range province. A Cenozoic origin for the BRS is considered most likely; it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions, possibly emplaced along preexisting zones of weakness. Moho reflections observed beneath the TZ and the Basin and Range province contrast with the nonreflective Moho beneath the Colorado Plateau, suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the Moho. The Moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place; this is the best example of offset Moho yet found by COCORP and contrasts sharply with COCORP observations of a smooth Moho elsewhere. This offset is probably a late tectonic expression of crustal extension and thinning across the TZ.

Major Proterozoic basement features of the eastern midcontinent of North America revealed by recent COCORP profiling

COCORP profiling in the eastern midcontinent of North America has (1) traced an extensive sequence of Precambrian layered rocks beneath southern Illinois, Indiana, and western Ohio; (2) detected a broad zone of east-dipping basement reflectors associated with the Grenville front beneath western Ohio; and (3) discovered a wide region of west -dipping reflectors penetrating most of the crust beneath eastern Ohio. The Precambrian layered assemblage may be as much as 11 km thick beneath southern Illinois, extends at least 170 km in an east-west direction, and contains several strong reflectors that have a lateral continuity of 80 km or more. Industry seismic data indicate that the layering is extensive in a north-south direction as well. Possible explanations for the layering include the silicic igneous rocks of the ca. 1.48 Ga eastern granite-rhyolite province, which are penetrated by basement drill holes throughout the region, perhaps intermixed or underlain by mafic igneous or sedimentary rocks. The 40-50-km-wide zone of strong, east-dipping (25°-30°) reflectors beneath west-central Ohio corresponds to the position of the Grenville front as determined from potential field and drill-hole data. These dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones (mylonites) like those exposed at the Grenville front in Canada and imaged on the GLIMPCE seismic reflection lines in Lake Huron. Both the COCORP and GLIMPCE lines show a remarkably similar reflection geometry, despite the more than 500 km separating the two profiles. Easternmost Ohio appears to be underlain by pronounced west-dipping (<40°) reflectors in the middle and lower crust, which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width. Analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone. The west-dipping reflectors may be correlative with similar reflectors imaged on another COCORP survey in northern Alabama. The correlations suggested by these new results, though tentative, imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks.

Death Valley bright spot: A midcrustal magma body in the southern Great Basin, California?

A previously unrecognized midcrustal magma body may have been detected by COCORP deep seismic reflection profiles in the Death Valley region of the southern Great Basin. High-amplitude, relatively broad-band reflections at 6 s (15 km) are attributed to partially molten material within a subhorizontal intrusion. This “bright spot” extends laterally at least 15 km beneath central Death Valley. A moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000-yr-old basaltic cinder cone. The fault zone is inferred to have been a magma conduit during the formation of the cinder cone. Vertical variations in crustal reflection character suggest that the Death Valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and/or intruded lower crust. The Death Valley bright spot is similar to reflections recorded by COCORP in 1977 in the Rio Grande rift, where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20-km depth.

Tectonic heredity and the layered lower crust in the Basin and Range Province, western United States

Summary COCORP deep seismic reflection data were collected in a transect at 40°N latitude across the entire Basin and Range extensional province and its boundaries, the Sierra Nevada Mountains and the Colorado Plateau. In general, the data display little of the province-wide symmetry of the modern Basin and Range, and instead can be more directly related to the pre-17 Ma asymmetric orogens of the western United States, particularly in the eastern part (112–115°W) of the region. No one model of intracontinental extension is applicable to the entire province: it may be that pre-existing structure of the continental crust predisposes it to a particular mode of extension unless thermally activated processes render the crust mechanically isotropic. Such processes (e.g. enhanced ductility, magmatism, etc.) may be responsible for the development of a highly layered lower crust and pronounced reflection Moho; features not commonly seen in reflection profiles from cratonic regions.

Crustal structure of north-central Nevada: Results from COCORP deep seismic profiling

Deep seismic-reflection data which were collected by COCORP in central Nevada at ∼40°N latitude reveal a horizontally laminated, reflective lower crust and an upper crust containing prominent moderately dipping reflections from Cenozoic lava flows, basin fill, normal faults, older thrust: faults, and Paleozoic sediments. Prominent Moho reflections are seen at depths of 31–35 km, in agreement with refraction-based Moho depths for this area. The strongly layered character of the lower crust is best developed near the mid-Miocene Northern Nevada rift, suggesting that Cenozoic magmatism and ductile extension have played an important role in the development of this fabric. East-dipping reflections beneath southern Grass Valley in Lander County may locally disrupt an otherwise continuous “reflection Moho.”

Tectonic evolution of the west Spitsbergen fold belt

Abstract The west Spitsbergen fold belt has a complex tectonic history which is recorded in a thick, nearly complete upper Proterozoic-Phanerozoic layered sequence. Work since 1977 near Bellsund allows recognition of the main deformational events in that segment of the fold belt. The strata are grouped informally into the metamorphosed Hecla Hoek (HH) sequence (Proterozoic) and the Van Keulenfjorden (VK) sequence (Carboniferous-Cretaceous), separated by a pronounced unconformity. The HH is divided into the Antoniabreen succession, the Chamberlindalen succession, and the Kapp Lyell tillite; the first two consist of diverse clastic and carbonate rocks, along with some volcanic rocks. The VK is a platform sequence of shallow marine and terrestrial sedimentary rocks. These layered rocks are intruded by a few Mesozoic dolerite sills and dikes. Subhorizontal Paleogene sedimentary rocks are preserved in a small graben. Both layered sequences have undergone strong deformation. Structures (mainly Caledonian age) in the HH can be grouped as phase 1 (small isoclinal folds, subhorizontal axial planar foliation, ridge-groove lineation in the foliation, and large recumbent folds), phase 2 (tight to isoclinal folds, axial planar foliation), and younger (weak folds and foliations, kink bands, crinkles). Structures (mainly Tertiary age) in the VK include faults (thrust, reverse, down-to-the-east bedding-plane normal, other normal); folds (symmetric, asymmetric, overturned, recumbent, isoclinal); and foliation (in some tightly folded Triassic shales). Six deformational events can be identified here, each described below in terms of age, intensity, and kinematic pattern: • D1 - Vendian or early Paleozoic; very strong; NNW-SSE shortening?, NNW transport? • D2 - Early Paleozoic (pre-Carboniferous); strong; NE-SW shortening, NE vergence. • D3 - Middle (?) Carboniferous; moderate; unclear (limited exposure). • D4 - Early Cretaceous (?); weak; extension, direction unclear. • D5 - Early (?) Tertiary; very strong locally; uplift in W, ENE-WSW shortening, ENE translation. • D6 - Middle (?) Tertiary; moderate; NE-SW extension.

Grenville foreland thrust belt hidden beneath the eastern U.S. midcontinent

Grenville foreland thrust structures are observed beneath the eastern U.S. midcontinent on COCORP (Consortium for Continental Reflection Profiling) line OH-1 and a short seismic line in southwest Ohio. These structures represent the first evidence for a significant Grenville foreland thrust belt preserved in eastern North America. On the COCORP lines, the structures include a thrust ramp that truncates layered rocks of the footwall as well as a complex hanging-wall ramp anticline and an associated asymmetric syncline. The Grenville front tectonic zone appears to truncate these foreland structures, indicating a later, second phase, expressed as a deeply penetrating, out-of-sequence thrust zone associated with the main uplift of the Grenville province on the east. A short, shallow seismic line in southwestern Ohio reveals an east-dipping sequence of prominently layered rocks that may lie above a footwall ramp to a deeper Grenville thrust fault. A drill hole into the less reflective top of this dipping sequence encountered unmetamorphosed sedimentary rocks like those increasingly reported from other drill holes in southwestern Ohio and adjacent states. Although possibly part of a late Precambrian (Keweenawan?) rift, these clastic sedimentary rocks may instead preserve evidence of a heretofore unrecognized Grenville foreland basin in eastern North America. Alternatively, these Precambrian sedimentary rocks together with an underlying, but yet undrilled, strongly layered sequence may correlate with similarly layered rocks observed on COCORP and industrial seismic lines within the Middle Proterozoic granite-rhyolite province to the west in Indiana and Illinois and indicate that unmetamorphosed sedimentary material is an important constituent of the granite-rhyolite province. The presence of a Grenville foreland thrust belt beneath the eastern U.S. midcontinent, in contrast with its absence in Canada, may suggest a deeper level of erosion of the Grenville province and its foreland in Canada or reflect important lateral differences in the geology of the foreland.