An in situ thermal measurement was attempted using the Davis-Villinger temperature probe (DVTP) (Davis et al., 1997) after retrieving Core 180-1108B-3R. Deployment into the formation was prevented by fill within the borehole. Difficult drilling conditions and indurated formation precluded further deployment of the DVTP.
Because of the lack of in situ temperature data, an open-hole temperature record was obtained using the Adara temperature tool (Adara) after coring operations had ended (Fig. F54). The Adara was lowered in the drill pipe and held for temperature measurements at 390 mbsf for ~10 min, and at 300, 200, and 100 mbsf for ~6 min each. Measured temperature at mudline was 2.5°C. Downhole equilibrium temperatures were approximated by plotting the temperature as a function of ln[t/(t-s)], where t is the total time elapsed since the drill bit penetrated that depth, and s is the total time elapsed between the initial penetration and the cessation of circulation (Fig. F55). The line was then extrapolated to infinite time (where ln[t/(t-s)] = 0). This method was introduced by Bullard (1947) and previously applied to open-hole temperature measurements from ODP Leg 123 (Castillo, 1992). Because the approximation considers only conductive thermal transport, and recovery duration of the Hole 1108B was very limited, these data cannot be considered as accurate as in situ temperature determinations.
The data from the Adara run were supplemented by measurements taken by the TLT (see "Downhole Measurements"). Temperature data points between 159.5 and 160.5 mbsf were selected for estimation of in situ temperature at that depth (Fig. F56). For this approximation, circulation was assumed to persist for the entire period from the time that the depth was first penetrated until circulation ended as the logging tools left the bottom of pipe (105 hr). Interruptions in circulation, such as for the Adara run, were considered to be of sufficiently short duration relative to total circulation time that they could be neglected.
The profile of temperature with depth indicates a discontinuity between 160 and 200 mbsf (Fig. F57). This depth is coincident with a fault zone near 165 mbsf (see "Structural Geology") and observed perturbations in inorganic geochemical profiles (see "Inorganic Geochemistry"). The estimated thermal gradient depends on whether all data points are considered or individual intervals are examined. Linear regression of all the approximated equilibrium temperatures from both the Adara and TLT runs indicates a thermal gradient of 100ºC·km-1 if the line is required to pass through the measured mudline temperature (Fig. F57). Alternatively, gradients can be interpreted as 94ºC·km-1 above 160 mbsf, 24ºC·km-1 between 160 and 200 mbsf, and 65ºC·km-1 below 200 mbsf, as indicated by the dashed lines on Figure F57.
It is possible that normal fault movement has caused the discontinuity in thermal gradient. Calculations using the one-dimensional thermal transport equation suggest that the discontinuity can be reasonably reproduced by a 200-m vertical displacement, if the initial thermal gradient was 65ºC·km-1. However, the offset would need to be rapid because thermal conduction would significantly reduce the discontinuity in less than a few 1000 yr. An alternative explanation for the observed thermal discontinuity may be advection of warm fluids along the fault zone.