Interpretation of reprocessed Ohio Consortium for Continental Refl ection Profi ling (COCORP) OH-1 seismic refl ection profi les indicates four structurally complex Precambrian unconformity-bounded stratigraphic sequences that clarify the relative timing of formation of the Fort Wayne Rift and EastContinent Rift System with respect to the Grenville orogeny. Petrographic examination of sparse deep well samples in the region indicates or suggests sedimentary lithologies beneath the Paleozoic sedimentary cover. Other seismic profi les in the region, some with excellent well control, support our proposed model. A generalized model for the latter part of the Grenville orogeny suggests polyphase sedimentation and deformation with multiple episodes of crustal extension and compression. We propose the following events for Ohio and the surrounding region: (1) a major regional unconformity developed on the Eastern Granite-Rhyolite Province and accreted Grenville terranes; (2) western Ohio became the site of extensive fault-bounded rift basins, beginning with the Fort Wayne Rift and extending into west-central Ohio as the East Continent Rift System; (3) westward-advancing thrust sheets followed with deposition of sediments into newly developed basins; (4) continued Grenville thrusting created foreland basins in a westward progression; and (5) a long period of Neoproterozoic to Middle Cambrian erosion removed much of the foreland basin sedimentary sequences prior to Paleozoic deposition. Erosion in the Ohio region did not remove the large volume of rock as in Canada north of Georgian Bay. Other seismic lines in the region suggest that Grenville-age sedimentary basins are preserved beneath the Phanerozoic from Georgian Bay southward. These new fi ndings demonstrate the importance of using fault-and unconformity-bounded seismic sequences to enhance and clarify the relative timing of Proterozoic events in regions where Paleozoic sedimentary cover exists and core samples are sparse or lacking.

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Brittle fracture growth proceeds through unique stages, each marked by distinct fractographic features that can only develop during Mode I loading at the propagating crack tip. Fractographic features in any substance can be interpreted in terms of the location of the failure origin, as well as changes in propagation velocities, stress directions and stress magnitudes at the crack tip during failure. Joints in granular pervious rock, however, do not contain fractographic features, commonly formed in glass, that develop at unstable propagation rates under the influence of appreciable amounts of stored strain energy. Yet, features that develop at lower stable rates of propagation are present on fractures in glass and rock implying that the absence of certain fractographic features provides useful information.Simple laboratory experiments, primarily on glass, are discussed to provide geologists an opportunity to recognize and qualitatively interpret fractographic features diagnostic of brittle failure. The exercises demonstrate the changing propagation dynamics that control the morphological evolution of artificially-induced fractures and natural joints in rock.

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