COCORP Deep Reflection Research Articles

A COCORP deep reflection profile across the buried reelfoot rift, south-central United States

A COCORP deep reflection survey across the northern Mississippi Embayment reveals essential features of the late Precambrian (?)/early Paleozoic Reelfoot rift. Stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness, the base ranging from 2.0 to a maximum of 3.5 s twt ( ~ 4 to 8 km depth). In contrast, the base of Phanerozoic the sedimentary section immediately east of the rift is at 1.4 s (~2.8 km depth). Correlation with a basement-penetrating well indicates that the base of the Upper Cretaceous/Holocene Mississippi Embayment Supergroup, base of the Cambrian-Ordovician Knox Group, and the lower Upper Cambrian(?) Bonneterre Formation are basin-wide seismic stratigraphic markers. Clastic sedimentary strata of the Lamotte Formation, which immediately overlie Precambrian basement, show clear angular discordance with the overlying subhorizontal Bonneterre Formation indicating that the base of the Bonneterre marks the transition from active extension to passive subsidence within the Reelfoot rift. Palinspastic restoration of the basement suggests relatively modest extension associated with rift formation ( ~ 17%). The Blytheville Arch, an axial antiformal feature, as well as a number of lesser structures indicative of multiple episodes of fault reactivation, are also evident on the COCORP profile. The crystalline basement beneath the Reelfoot rift is relatively devoid of reflections down to ~ 5.0 s (12–15 km depth), beyond which it passes gradationally into a complexly reflective/diffractive middle and lower crust. The lower crust, however, does not exhibit the distinctly laminated character commonly observed in Mesozoic/Cenozoic extensional provinces. The crust-mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12–13 s ( - 40 km depth), followed by an unreflective zone, which in turn is followed by a distinct horizontal reflection at 14.5 s. The latter is not observed on existing profiles outside the rift. At present it is not possible to determine strictly whether the deep horizon represents Moho or alternatively a discrete feature (sill?) within the uppermost mantle; however, its spatial association with the Reelfoot rift suggests that the two are genetically related. Regional relations, together with the existence of dipping fabric in the deep crust beneath the rift, suggest the possibility that the Reelfoot rift formed along the southern extension of the circa 1 Ga Grenville Front.

Integration of COCORP deep reflection and magnetic anomaly analysis in the southeastern United States: Implications for origin of the Brunswick and East Coast magnetic anomalies: Alternative interpretation and reply

Integration of COCORP deep reflection and magnetic anomaly analysis in the southeastern United States: Implications for origin of the Brunswick and East Coast magnetic anomalies: Alternative interpretation and reply

COCORP deep reflections: Moho at 50 km (16 S) beneath the Colorado Plateau

COCORP deep reflection data on the Colorado Plateau reveal complex reflections to about 16 s two‐way travel time (50+ km), beyond which there is an abrupt decrease of reflectivity. This boundary is interpreted to represent the Moho, an inference consistent with the existing but limited refraction data sets when they are reconciled with the recent independent determination of an 8.10–8.15 km/s Pn velocity beneath the Colorado Plateau. This ∼50 km depth to Moho and 8.10–8.15 km/s Pn velocity for the Colorado Plateau closely resembles that of the High Plains just east of the Rocky Mountains. The absence of continuous strong reflections at the Moho suggests that the Moho beneath the Colorado Plateau is a transition, not a velocity step function on the scale of the reflection experiment. The reflection boundary may represent the base of complex and discontinuous reflectors of the crust and crust/mantle transition, below which the mantle may be largely peridotite with a velocity of 8.1 km/s. These results strongly reinforce the suggestion that the Moho on deep reflection data in relatively stable continental regions is represented by an abrupt decrease in reflectivity, below which the mantle is relatively seismically transparent.

Integration of COCORP deep reflection and magnetic anomaly analysis in the southeastern United States: Implications for origin of the Brunswick and East Coast magnetic anomalies

Integration of magnetic anomaly analysis with COCORP deep reflection data from the southeastern United States provides three new constraints on the interpretation of the Brunswick and East Coast magnetic anomalies, as well as on the reflection data. These are as follows. (1) The source of the Brunswick anomaly lies within the deep crust. This anomaly is not caused by a Mesozoic rift basin, as proposed by some workers. (2) A simple, seaward-dipping, high- susceptibility slab model can explain both the Brunswick and East Coast magnetic anomalies. The along-strike change in character of the two anomalies results largely from a change in azimuth of the source body. (3) Beneath the southeastern United States, this source body dips south, lies immediately on the south flank of the prominent southward-dipping reflective zone revealed on COCORP surveys, and was previously associated with the Alleghanian suture between North America and Africa. These results imply that a dipping, highly magnetized zone in the upper plate of the Alleghanian suture is responsible for both the Brunswick and East Coast magnetic anomalies. The high- susceptibility material responsible for these anomalies might be mafic lower continental or oceanic crust thrust upward during Alleghanian continental collision, or mafic igneous material intruded into the upper plate of the suture zone during subsequent Mesozoic rifting, or both. The latter hypothesis implies that the Alleghanian suture acted, as a zone of weakness (a repository ?) which was reactivated to control the site of ultimate Atlantic rifting and possibly initial sea-floor spreading.

Structure of the Laramide Wind River Uplift, Wyoming, from Cocorp deep reflection data and from gravity data

The question of the structure of the Wind River uplift, a Laramide foreland structure in Wyoming, has been answered by Consortium for Continental Reflection Profiling (Cocorp) deep crustal reflection profiling and by gravity interpretation. Wyoming uplifts are asymmetrical anticlines whose steep limb is commonly cut by a reverse fault or thrust. If the thrust continues into the crust with low dip, the uplift is caused by horizontal movements; and if the fault steepens into a high‐angle reverse fault in depth, the uplifts are caused by vertical movement. The Wind River uplift is the largest of the Laramide structures in Wyoming, and the thrust fault has 14 km of vertical separation and 26 km of horizontal separation. About 168 km of crustal reflection profiling has been completed by Cocorp across the uplift. The Wind River thrust fault generates a continuous reflection from near the surface to a depth of about 24 km, and the thrust appears as a complex zone of faults. Apparent dip of the thrust is 30°–38°, and true dip may be up to 48°. Interpretation of deep crustal reflections is complicated by the presence of multiple reflections. A structure whose complexity approaches that of basement outcrops is found in the deep crust. Gravity modeling also suggests that the fault dips moderately and that dense rocks are uplifted in the deep crust. The Moho does not seem to be presently displaced by faulting. The crust in the Wind River uplift began to deform by large‐scale folding and then broke and faulted as a rigid slab. Faulting is caused by crustal shortening from compression that is related to plate movements.