NEW CONSTRAINTS ON TRANSPORT DIRECTION DURING THE GRENVILLE OROGENY: EVIDENCE FOR A BASEMENT LATERAL RAMP IN THE EASTERN MID-CONTINENT

Shallow basement reflections beneath Ohio and Kentucky indicate that a lateral ramp developed during late stages of the Grenville orogeny and that the older Coshocton suture zone lies to the east of southern Ohio. Multi-channel seismic reflection profiles from southern Ohio and northern Kentucky were reprocessed and analyzed along 60 km east-west and north-south transects that cross at the midpoint. The east-west transect is found to display east dipping reflections that do not disturb the overlying Paleozoic strata. Similar south-dipping reflections are observed on a north-south oriented transect. Both sets of reflectors appear to penetrate to approximately 8 km depth. In east-central Ohio, west-dipping reflections of the Coshocton Zone are imaged on COCORP Ohio line 2. These reflections also begin directly beneath the Paleozoic section and dip approximately 40 degrees west. These laterally extensive west-dipping reflectors appear to be offset by bands of east dipping reflections that merge into a shallow detachment. Industry data 20 km north of the western portion of Ohio line 2 display north-east dipping reflections in the shallow Precambrian that penetrate to at least 5 km in depth. The reprocessed profiles from southern Ohio show no evidence of west dipping reflections that might be associated with the Coshocton zone. East, south and northeast dipping reflections observed in the eastern mid-continent are not consistent with a simple northwest transport direction during the Grenville orogeny. Additionally, the Coshocton zone does not appear to extend into eastern Ohio and Kentucky as previously proposed. A shift of the Coshocton suture to the east coupled with a model invoking east-west compression resulting in lateral ramps in the shallow Precambrian basement can explain these regional observations.