Deep Sea Drilling Project heat flow and models of the rifted West Rockall Margin

Abstract

Downhole thermal measurements for the southwest Rockall Bank rifted margin made on Deep SeaDrilling Project leg 81, plus those previously made on leg 48, give six reliable heat flow values in the narrow range 62-71 mW m -2, with an average of 66.5 mW m -2. The temperatures were obtained using a self-contained downhole temperature-recording device. A long probe is inserted through the drill bit into the undisturbed sediments below the bottom of the hole at intervals during drilling. The thermal disturbance from the penetration was extrapolated to equilibrium in situ temperatures using the cylindrical decay function. The thermal conductivities have been measured on core samples using the needle probe method supplemented with porosity-conductivity relations. Heat production in the sedimentaryvolcanic sequence above basement probably makes a negligible contribution to the heat flow. There is no systematic heat flow trend with water depth, which ranges from 1650 to 1900 m, or with position, and the mean value is not significantly different from that expected from pure oceanic crust of age equal to the approximately 50-Ma time of rifting. However, the unloaded basement depth is over 1 km shallower than expected for normal oceanic crust of the 50-Ma rifting age. The heat flow combined with subsidence data fits a simple rifting model with crustal stretching and thinning by a factor of 3 or 4 over a zone at least 100 km wide, occurring about 50 Ma. The heat production in the stretched crust is taken to contribute 4 mW m-2, based on a prerift structure comparable to once adjacent North America. A major uncertainty in thermal and subsidence models for the margin is the effect of the earlier "Iceland hot spot" which produced extensive margin volcanism on the northern British Isles and Greenland and shoaling of the oceanic crust at about the time of west Rockall rifting.

The thermal impact of Paleocene magmatic underplating in the Faeroe–Shetland–Rockall region

“…This compares with a value of 4 9 m W m -2 estimated for the heat-flow in western Rockall by Hyndman & Roberts (1987) prior to rifting and volcanic margin emplacement, as well as the 49mWm~2 used by Keen & Beaumont (1987) as the background heat-flow on the Atlantic margins of Canada. The extension and magmatism noted along the Rockall margin during continental breakup would be expected to have raised heat-flow for some time during the Cenozoic.…”

Evolution of the Vøring Volcanic Margin

“…This hinges to some extent on the crustal nature of the West Rockall Margin. Although the DSDP Leg 81 heat-flow data are compatible with oceanic crust below the dipping reflectors (Hyndman and Roberts, 1987), most evidence suggests emplacement prior to breakup as shown in Figure 10. Seismic data farther north, however, indicate that the dipping reflectors overlie both oceanic and extended continental crust in a setting similar to the Wring Plateau (White et al, 1987b).…”

Poisson's ratio of a seaward-dipping reflector series, Hatton Bank

“…The crust at ESP G is probably entirely igneous in origin , and high-temperature hydrothermal alteration is likely to have occurred within the top few kilometres of the basalts at the time of their eruption, when they were close to sea-level (Roberts et af. 1984, Hyndman & Roberts 1987. In the Bay of Islands ophiolite, the greenschist to amphibolite transition occurs at -1.5 km depth, and is associated with a reduction in Poisson's ratio (Salisbury & Christensen 1978).…”