Hydraulic damage in subglacial conduits: Evidence from rock hydrofracture and hydraulic jacking for high fluid pressures during rapid melt of the Fennoscandian Ice Sheet

Abstract

When large volumes of meltwater arrive suddenly at the glacier bed, the capacity of subglacial conduits is exceeded, generating transiently high fluid pressures. Yet we currently lack reliable markers for where such conditions developed on the beds of former glaciers. Here, we describe hydraulic damage that developed at close to or above overpressure (Pw > Pi) at the bed of the last Fennoscandian Ice Sheet in Uppland, east-central Sweden. Hydraulic damage is mostly confined within sharp lateral and vertical limits at the boundaries of former large subglacial conduits. The conduits include large Röthlisberger- and Nye-channels - eskers, esker corridors, and meltwater channels – that operated at full pipeflow. Wider (0.4–1.5 km) meltwater corridors opened under sheetflow with broad, shallow Hooke-channels. Hydraulic damage found in Precambrian gneiss and granite bedrock includes hydrofracturing and hydraulic jacking, with fracture propagation, dilation, and block displacement. Dilated fractures are often plugged by massive sands and diamicts or laminated silts. The fracture fills represent brief, single and multi-phase, injections of viscous, sediment-laden fluids at the ice sheet bed. The most spectacular effects of hydraulic damage are seen in the distension, disruption, and bursting of 1–20 m high rock bumps and hills. We present a detailed taxonomy for hydraulic damage and the consequent glacitectonic damage sustained during glacial ripping.Industrial fracking provides a well understood analogue for hydrofracture in nature. In subglacial settings, pathways for the flow of pressurised fluids are determined by the tensile strengths and hydraulic transmissivity of ice, till, and rock. The easiest flow path is across the glacier bed beneath sliding ice. Abrupt increases in meltwater flow force ice-bed separation. Opening of conduits under high fluid pressures triggers further propagation of hydrofractures into ice, till, and rock. On the floors of subglacial conduits, pressurised fluids entered the hard rock bed via old, mineral coated fractures. Under Poiseuille's law, fracture flow was initially slow due to the narrow apertures, but flow accelerated rapidly as fluid wedging dilated fractures. Fluid pressures in old fractures are constrained by published measurements for near-surface fracture toughness of ≤0.7 MPa at Forsmark, east-central Sweden. Formation of new rock hydrofractures overcame gneiss tensile strengths of 8–18 MPa. Hydraulic jacking required accommodation space that developed during conduit opening. Due to buoyancy and ice bed separation, lower excess pressures of <0.3 MPa were required to lift rock sheets up to 10 m thick. Hydraulic bursting developed in large conduits under very high, but rapidly fluctuating fluid pressures as dilating rock fractures extended to the glacier bed. Distension of rock surfaces was brief due to rapid leak-off and the inherent instability of overpressured conditions in dilatant, fractured rock masses.Hydraulic damage developed in Uppland when accelerating ablation generated large subglacial meltwater fluxes in response to abrupt warming at the Younger Dryas-early Holocene transition (∼10.8 ka). The large volumes of subglacial meltwater, the sudden increases in subglacial water flow and fluid pressures, and the release and transport of 2 m b-axis boulders in esker tunnels suggest that hydraulic damage was triggered at the onset of large subglacial floods. Ice thicknesses at distances of up to 50 km behind the grounding line at the retreating subaqueous ice margin indicate that steady-state hydrostatic pressures were ∼2.9–4.5 MPa. Such pressures are consistent with (i) extensive, but brief ice-bed separation in meltwater corridors under overpressure and (ii) the development of hydraulic damage in conduits close to the ice margin. More extreme fluid pressures developed instantaneously under hydraulic shock, as rapid subglacial pipeflow was suddenly stopped or diverted. The taxonomy of hydraulic damage presented here can be applied to any hard glacier bed to identify where large volumes of subglacial meltwater reached high pressures. Similar markers are widely reported from modern and former glacier beds.