Mapping the Deeply Eroded Roots of a Major Strike-Slip Fault System: a Summary of Recent Bedrock Mapping Along the Norumbega Fault System in Maine

Deeply eroded ancient fault systems provide a natural laboratory for the study of processes occurring in the deeper inaccessible portions of currently active fault systems. Detailed mapping of the spatial distribution of fault rocks associated with these exhumed systems, combined with microstructural and geochronological studies, provide a wealth of information on the eroded seismogenic faults that once overlay these faults and shear zones. The regionally extensive Norumbega fault system in Maine (> 400 km in length and up to 40 km in width) represents the eroded roots of a major plate boundary-parallel fault system that was active in mid-Paleozoic to Mesozoic time. A variety of both brittle (including abundant pseudotachylyte) and ductile fault rocks can be found along the length of the Norumbega and understanding their spatial distribution is critical to understanding the temporal, spatial, and kinematic evolution of this and other long-lived fault systems. Recent 1:24,000 scale mapping along the south-central Maine portion of the Norumbega fault system has been sponsored by the National Cooperative Geologic Mapping Program of the U.S.G.S. in cooperation with the Maine Geological Survey. This new mapping has provided greater detail in areas containing previously recognized structures, and revealed the existence of previously unrecognized zones of significant displacement. Complicating the mapping efforts is the distribution of a wide variety of protoliths across the strike of the fault system. Thus differences in fault rock distribution and structural style across the fault system not only reflects different episodes of displacement under different boundary conditions (e.g., temperature, pressure, fluids, differential stresses), but also differences in the rheological properties of materials upon which these conditions are operating. In addition, because of differences in the ways that different workers define and recognize various types of fault rocks, there have been some “quadrangle boundary discrepancies” that illustrate the challenges of mapping highly sheared fault rocks. Joint field trips enabling workers opportunities to compare geological relationships and fault rock mapping philosophies have been invaluable in resolving these discrepancies. Detailed mapping to date reveals significant differences both along and across the strike of the Norumbega fault system. In general, a wide zone (30+ km) of early right lateral ductile structures of Middle Devonian to Early Carboniferous age is overprinted by more localized zones (< 500 m) of higher strain. These narrow high strain zones contain both mylonite and pseudotachylyte which in some zones appears to be coeval and in other zones appears to represent discrete events. Low temperature thermochronological studies across the region (40Ar/39Ar & fission track) indicate significant episodes of Mesozoic dip slip displacement in the southern portion of the system (Casco Bay area) but no evidence for this in the south-central Maine portion of the system. The new mapping confirms the complexity and longevity of the Norumbega system.