Subglacial squeeze-up moraines adjacent to the Vaalajärvi-Ristonmännikkö glacially-induced fault system, Finnish Lapland

Abstract

The Vaalajärvi-Ristonmännikkö glacially induced fault (GIF) system in central Finnish Lapland is attributed to multiple slip events (Mw ≈ 6.7–7.0 earthquakes), likely contributing to subglacial and postglacial sediment deformations. We applied light detection and ranging digital elevation models (LiDAR DEMs), electrical-sedimentary anisotropy (orientation) measurements and trenching to study the related geomorphology, particularly subglacial moraines on the hanging wall of the Ristonmännikkö GIF segment striking SW-NE direction. Transverse-to-ice-flow-oriented moraines and the Pulju moraines were found to cover the distal tails of the Early Weichselian rock-drumlins and they are interpreted as seismically induced crevasse infill landforms. The electrical-sedimentary anisotropy suggests squeeze-up origin rather than active ice oscillation end moraine formation processes. Based on our recent OSL-data, the squeeze-up moraines are also superimposed on a till-covered esker system yielding an age of 87 ka. Erosional V-shaped features with apices pointing up-ice were found on convex surfaces of the distal rock-drumlin tails and were attributed to subglacial mass-flows rather than glaciofluvial erosion or frozen bed fracturing mechanisms. The area lacks glaciofluvial/hummocky corridors, but a substantial outburst system on the streak of Ristonmännikkö GIF suggests an earthquake-triggered outburst of the former subglacial Lake Unari. Our recent 14C ages of paleolandslides suggest that the Vaalajärvi-Ristonmännikkö GIF terrain was ice-free already 11.3 cal kyr BP, but was subjected to lithospheric water discharge and landslide events at 2.9 and 10.6 cal kyr BP. The squeeze-up moraines most likely formed during the later phase of the Younger Dryas, between ca. 12.0–11.5 cal kyr BP. We propose that subglacial seismicity within a glacial isostatic adjustment (GIA) environment was a notable trigger for the squeeze-up and mass-flow features beneath the retreating Fennoscandian Ice Sheet (FIS) during the later phase of the Younger Dryas.