Geomechanical aspects of subsidence in eastern Maine

Abstract

Information from several sources indicates that the eastern Maine coast is subsiding. Recent releveling surveys indicate a current rate of subsidence in eastern Maine as much as 9 mm per year relative to Bangor. The anomalously rapid rise in sea level accompanying the subsidence has led to shoreline erosion, landsliding, and submergence of archaeological sites and historical manmade structures. In situ stress measurements, well-bore breakouts, and earthquake focal mechanisms show that the maximum principal stress is horizontal, strikes N 80° E, and is correctly oriented to produce thrusting on north-south faults, right-lateral motion with minor thrusting on northeast and east-northeast faults, and major thrusting with left-lateral motion on northwest-striking faults. Maximum seismicity in Maine occurs in the subsiding zone which also includes the maximum (positive) gravity anomaly in the State. Epicenters are diffusety aligned with the Oak Bay fault which strikes N 20° W. Deflection of isobase lines drawn on ice-marginal deltas forms a trough whose axis strikes approximately parallel to the Oak Bay fault. An acceptable contemporary crustal deformation model should incorporate zones of contrasting rock density and intensity of fracturing, localized seismicity, east-west tectonic compression, and weak compression or extension in the north-south direction.

Orientation and Characteristics of Fractures in Crystalline Bedrock Determined by Surface and Borehole Geophysical Surveys, Millville and Uxbridge, Massachusetts

“…The primary northeast-to-east fracture strike observed in the geophysical data may be related to the eastnortheast regional stress maximum (Lee and Diehl, 1989;Nataraja, 1977). Fractures that strike parallel to the regional stress direction tends to have larger aperture openings than fractures that are orthogonal to the regional stress direction and tend to be forced closed.…”

Actuopaleoichnology of a modern Bay of Fundy macro-tidal flat: analogy with a Mississippian tidal flat deposit (Hartselle Sandstone) from Alabama

“…The Lubec Embayment is a fetch-limited, partly enclosed bay that is protected from significant wave influence and has no appreciable sediment source other than coastal erosion (Walsh, 1988). Tectonic subsidence in the immediate area is nine mm/year (Lee & Diehl, 1989; Tyler & Ladd, 1980) and, with current rates of eustatic sea level rise, depositional accommodation exceeds one cm/year. The sediments are derived from the Pineo Ridge Moraine Complex glacial deposits (Gates, 1989; Kaplan, 2007) and these source deposits are heterogeneous in both grain size and mineralogy.…”