IDEAS ON THE ORIGIN AND EVOLUTION OF INTRACRATONIC PLATEAU UPLIFTS: INSIGHT FROM STUDIES OF THE OZARK PLATEAU (USA MIDCONTINENT)

Studies of orogenic plateaus have focused, to date, on those which occur within active orogens. However, plateaus of significant area also occur within continental interiors, and may also reflect tectonism. One such plateau, the Ozark Plateau, covers about 130,000 square kilometers (1/3 the area of the Colorado Plateau) in Missouri and Arkansas, in the Midcontinent region of the United States. Topographically, it rises to elevations of up to 600 m, but structural relief, defined as the difference in elevation between the Cambrian-Precambrian contact at the top of the Ozark Plateau relative to the floor of the adjacent Illinois basin is about 7.5 km. 2-D subsidence analysis along an E-W transect crossing the basin/plateau boundary, as well as regional stratigraphy, indicate that the relative displacement between the top of the plateau and the floor of the Illinois basin began in Neoproterozoic time, and continued in pulses through the Paleozoic. Apatite fission-track analysis from the plateau indicates that the rocks in the plateau cooled from temperatures above 110°C in the Mid-Cretaceous. Structural analysis emphasizes that the plateau is bounded on the NE side and the SE side by fault zones and thus is a fault-bounded block that is tilted to the southwest. The boundary between the plateau and the deeper part of the Illinois basin, to the east, occurs in stair-step fashion. As a working hypothesis, we suggest that intracratonic plateaus, such as the Ozark Plateau, begin as relatively buoyant fault-bounded crustal blocks. In the case of the Ozark area, buoyancy may be provided by Precambrian granite/rhyolite intrusions. Once formed, the plateau has remained relatively high through geologic history. Flexural effects, caused by loading of the crustal margin by thrust sheets, or unloading during rifting, may cause pulses of renewed movement, amplified by the presence of weak faults. The Cretaceous thermal event of the Ozarks, may be related to passage of a hot spot, which left its mark in the form of local diatremes. Streams are causing deep basement incision within the Plateau, suggesting that the current landscape reflects relatively recent rejuvenation.