Abstract The Nordland area in northern Norway is the seismically most active area on mainland Fennoscandia. It exhibits patterns of coastal extension, which contrasts with the first-order regional stress pattern that reflects compression aligned with the North Atlantic ridge push. The regional stress field has been considered to emanate from the interaction of ridge push and glacial isostatic adjustment; while the local stress pattern can be additionally influenced by gravitational, topographic stresses, as well as the flexural effects of erosion and sediment deposition. We employ finite element numerical models at a crustal scale to study the 3D stress field, using existing geometric constraints from previous geophysical studies. Internal body forces, induced by variations in density, topography or Moho depth, already yield significant deviatoric stresses. In the models tested, these can strongly influence the near-surface stress regime, in particular for the continental margin setting we are investigating. In addition, redistribution of rock mass, which occurred mainly under Pleistocene glaciation, can modify the stress field significantly on a semi-regional scale. We consider this process to be the main driver for the coastal extension, in particular in areas where erosion has been high.

ABSTRACT The Upper Ordovician Juniata Formation, Central Appalachian Basin, USA, is a thick succession of cyclically bedded arenites, wackes, and mudrocks. Sedimentary facies of the formation in West Virginia, Virginia, and Maryland indicate cyclic peritidal deposition along the northern shoreline of the basin. The subsurface Juniata Formation has been drilled throughout the basin, and long, continuous well logs are available for analysis of the cyclic deposition. A 2400-ft-long (731.52-m-long) gamma-ray (GR) log from the Preston 119 well, northern West Virginia, provides a proxy of terrigenous siliciclastic fluxes originating from the Taconic highlands, from the early Ashgillian to the Ordovician–Silurian transition. Strong cycling in the GR log shows evidence for Milankovitch-forced sea-level oscillations, hypothesized to have been produced by dynamic Late Ordovician glaciation in polar (southern) Gondwana. Juniata cycle frequencies are different from those of Quaternary Milankovitch cycles, with significantly higher obliquity and precession index frequencies, consistent with a 21.5 h Ordovician day and an Earth-Moon distance that was 95% of present day. These results support John Dennison’s long-held view that Milankovitch forcing of sedimentation took place in the early Paleozoic Appalachian Basin by action of remotely generated glacio-eustatic oscillations powered by glaciation on southern Gondwana, and that this sedimentary record has tracked “Earth’s movement through space.”

ABSTRACT Glaciotectonic deformation of glacigenic deposits in southwestern Michigan is described and analyzed to determine the source of stress of these strained sediments, which manifests as overturned folds and other deformation similar to shallow crustal décollements. The succession is exposed in 11 aggregate mining operations along the Valparaiso Upland, in portions of Berrien, Van Buren, and Allegan Counties in southwest Michigan. Observed deformation includes a complex array of folds, faults, and thrust features as much as 5 m below the surface exposure of the pit face, consistent with horizontal compressional stresses that were generally aligned with ice flow. Fabric measurement of elongated clasts in the surficial till indicates ice flow from northwest to southeast across the area and parallel to drumlins in the area. Stratigraphically, the area is dominated by fine, lacustrine deposits with coarse sand and gravel capped by the Saugatuck Till during the last glaciation. Sediment grain size, pore-water pressure fluctuations, and topographic relief are interpreted to be responsible for the deformation observed as the Lake Michigan Lobe overrode a proglacial lake basin, including fans and deltas, as it advanced eastward to the Kalamazoo moraine. The fine texture and fabric of the lacustrine sediment package restricted the flow of subglacial water and caused abrupt local increases of pore-water pressure and concomitant coupling and decoupling of the bed-substrate interface. Advancing ice deformed sediments in two stages: (1) proglacially along a décollement at the ice margin, and then (2) subglacially as ice overrode the sediments.