COCORP seismic-reflection profiling in northeastern Kansas

Abstract

COCORP deep seismic-reflection profiles in northeastern Kansas reveal a structurally complex Proterozoic crust, in which most prominent is the central basin and related features of the Keweenawan rift. The 40-km (24-mi)-wide rift basin is asymmetric, reaching a maximum depth of 8 km (5 mi) on its western side. The basin fill is characterized by a lower, layered sequence of strong west-dipping reflectors which may correlate with middle Keweenawan interbedded volcanic and clastic rocks exposed in the Lake Superior region. Overlying this layered sequence is a zone of weak, discontinuous reflectors correlated here with the predominantly clastic rocks which characterize the upper Keweenawan sequence near Lake Superior. The reflection character of the seismic data, the seismic-velocity distribution, and gravity modeling suggest that mafic intrusions lie beneath the main rift basin. Normal faults associated with the rift dip at moderate angles to the east. Palinspastic reconstruction indicates that the rift basin formed by the rotation of fault-bounded blocks during crustal extension. Relative to the area of the Keweenawan rift, the older 1.6-b.y.-old crust flanking the rift contains few prominent reflectors at depths less than 10 km (6 mi). However, those reflectors which are present, particularly in the area of the late Paleozoic Humboldt fault zone and the 1.3-b.y.-old Big Springs aeromagnetic anomaly, dip to the east, subparallel to the faults in the 1.1-b.y.-old rift basin. Thus, the east-dipping features appear to have been active during more than one episode of crustal deformation.