Accommodation zone development between the Sandwich and Plum River fault zones, Ogle County, northern Illinois

Bedrock geologic mapping of the Oregon and Mount Morris 7.5-minute quadrangles was performed to investigate whether any evidence exists to suggest that the Plum River and Sandwich fault zones are connected. Mapping led to the identification of a fault array at the northwest end of the Sandwich Fault Zone that suggests the presence of an accommodation zone between the Sandwich and Plum River fault zones. Uplifted Cambrian and early Ordovician units suggest that the fault array was formed by multiple movements on the main faults. The main faults are likely rooted in basement, and the fault array may be a group of splay faults that propagated northwestward from the tip of the Sandwich Fault Zone. Users of the geologic maps may wonder whether the newly discovered fault array will affect modern seismicity; however, models by Grollimund and Zoback (2001) suggest that seismic risk remains low in Ogle County because northern Illinois lacks a lithospheric weak zone like that underlying the New Madrid Seismic Zone.