Heat flow on the western flank of the East Pacific Rise at 21 /sup 0/N

We report 48 new heat flow measurements on the western flank of the East Pacific Rise at 21 /sup 0/N. The stations were taken in 1 to 30 m of sediment on 0.4 to 1.4 m.y. old crust. The low average measured value of > or =173 mW//sup 2/ (including 12 tilted minimum values) is about 1/3 the mean heat flow predicted for crust of this age range by cooling plate theory. This may be partly due to a bias of measurement locations in sediment ponds. Nevertheless, 4-km running averages of measured heat flows, projected normally to the axis, display on oscillatory trend suggestive of cellular hydrothermal circulation in the basement. We sampled no more than one cycle of the apparent heat flow modulation, which is of roughly-equal15-km wavelength and roughly-equal200-mW/m/sup 2/ amplitude. This variation appears to be uncorrelated with the topographic variation of roughly-equal10-km wavelength. Assuming that the porosity sensitive electromagnetic results of Young and Cox (1981) reflect a permeability distribution that restricts the depth of circulation to roughly-equal1.4 km, average temperatures of circulation may be roughly-equal50-60 /sup 0/C. Deeper, higher-temperature circulation is allowable if a more uniform permeability-depth distribution holds

Heat transfer through the sediments of the mounds hydrothermal area, Galapagos Spreading Center at 86 /sup 0/W

Heat transfer processes at the mounds area of the Galapagos Spreading Center at 86 /sup 0/W are revealed by temperatures measured at roughly-equal10-m intervals in the 30 +- 10 m sediment at each of 12 holes at DSDP Leg 70 Sites 506--509 and by temperatures of up to five thermistors on eleven 8--12 m long piston cores. The 325 needle-probe values show a significant linear increase of thermal conductivity with depth in each core. About half of the temperature-thermal resistance profiles are nonlinear and are fit to a steady state, vertical pore water advection model. Results indicate high and variable total heat flow and localized hydrothermal discharge at roughly-equal10/sup -8/ m/s, associated with individual mounds. Recharge is indicated at similar rates in the low heat flow belt roughly-equal5 km south of the mounds and is suggested at slower rates in the intermediate heat flow (0.17--0.42 W/m/sup 2/) belt surrounding the mounds heat flow high. Possible slow entrained recharge within roughly-equal100 m of discharging mounds is suggested. Also suggested is strong local discharge along the major fault bounding the mounds crustal block to the north. About 95 km north of the spreading axis, at DSDP Site 510, temperatures in the 114-m sediment cover on 2.7-m.y. crust are linear, consistent with the suggestion that the hydraulic resistance of this layer is sufficient to seal off free hydrothermal exchange between basement and bottom water. The combination of heat flow data and the physical properties data of Karato and Becker (this issue) suggests that roughly-equal50 m of sediment may be a threshold thickness for sealing of hydrothermal circulation within basement, where the topography is smooth. We suggest that the formation of mounds may be associated with the forced localization of hydrothermal discharge through the sediment, as its thickness approaches this threshold value

Autocyclic behaviour of alluvial and deltaic systems. Geologica Ultraiectina (306)

Strata formation in sedimentary systems is the result of the interplay of controls generated within (autogenic) and outside (allogenic) the system. Allogenic controls include climate, sea level and tectonics. For example, tectonic tilting or sea-level variations can alter the space available for sediments to fill (accommodation), affecting the amount and characteristics of the accumulating sediments. Climate change can influence supply to the system, generating variations in (the rate of) aggradation or degradation. Since the characteristics and relative importance of the autogenic processes are still poorly known and impossible to infer from natural sedimentary systems, analogue experiments have been carried out to unravel the interacting controls. Autogenic evolution was studied separately for three different types of sedimentary systems: alluvial fans, fan deltas and a flood-tidal delta. In each experimental study the allogenic boundary conditions were held constant allowing the systems to grow and evolve in absence of externally imposed variation (base level or input of water and sediment). Autocyclic behaviour (i.e., repetitive autogenic behaviour) of alluvial fans and fan deltas was similar, consisting of alternations of sheet and channelised flow. Channelised flow was initiated by slope-driven incision, and terminated by loss of flow momentum and subsequent channel filling. Flood-tidal delta behaviour deviated from that seen on alluvial fans and fan deltas and showed lateral migration and expansion of tidal channels followed by channel abandonment and filling. All experiments demonstrated that autogenic processes and allogenic variations have a comparable impact on the morphodynamic evolution and, in the case of the alluvial fans and fan deltas, depositional architecture. The autogenic processes on the flood-tidal delta were limited to the intertidal part, and hardly left stratigraphic signals as the migrating tidal channels continuously reworked the remnants of previous channels. In addition, the effect of base level was investigated by comparing the autogenic development of alluvial fans (positioned far from base level) and fan deltas (positioned at a standing body of water, so the shoreline formed its base level). With identical input of water and sediment, alluvial-fan behaviour demonstrated remarkable differences in slope and frequency of autogenic processes, until it reached base level and started acting as a fan delta, substantiating earlier flume and numerical studies. To further apply developed concepts on autocyclic behaviour for Martian fan deltas, a series of fan deltas were formed in a mock-crater mimicking conditions similar to a specific type of deltas (stepped deltas) found on Mars. By comparing the morphology of the experimental and Martian deltas these experiments demonstrated that the Martian deltas were formed in one event of rapid water release, and the steps were generated by the interaction between the rising water level in the crater and autogenic variations in the supply related to processes in the upstream feeder channel