The Evolution of Small Rift Basins

ABSTRACT The original McKenzie (1978) model for basin formation was formulated to explain the heat flow and subsidence at the center of broad regions of crystal and lithosphere thinning. The McKenzie model has been modified (Stickler, 1980) to account for the heat flow and subsidence n narrow rift basins where lateral heat loss through the walls of the basin is important. We have used an iterative technique to apply this later modification to very narrow basins (? 50 km wide) in which sync-rift heat loss is very important. We have shown that subsidence (with a water load only) of 1 km may take place in 100,000 to 300,000 years after the initiation of rifting. Under these conditions sediment starvation may occur. We have also shown that in narrow extensional basins in which sedimentation keeps pace with subsidence, thermal maturation of in situ and early rift sediments may be quite rapid. INTRODUCTION It has been observed that deep sedimentary basins form in two different types of environments: first, where over thrust has taken place and foreland basins are formed and second in tensional regimes where rifting and crystal and lithosphere stretching occur causing subsidence. Models have been developed that may be used to explain the subsidence and thermal history of basins formed in tensional regimes or zones of pull-apart. McKenzie (1978) has shown that if the lithosphere (and crust) are uniformly thinned by stretching, subsidence or uplift may take place during the stretching (depending upon the ratio of crystal to lithosphere thickness), and that the stretching phase is followed by a long period of thermally driven subsidence which decays exponentially with time. Roy den e t al. (1980) modified the McKenzie model to incorporate the possibility of dyke injection of mantle material during the stretching phase. Both McKenzie and Roy den et al. (1980) considered that the stretched slab was of infinite horizontal extent, thus there would be no horizontal heat loss. Both the McKenzie and Roy den et al. models assumed basin forma tin to be a two stage process with a short period of rifting or stretching without significant heat loss followed by a long phase of thermally driven subsidence. Jarvis and McKenzie (1980) have shown that for the infinite slab, if the stretching phase is less than 20 m.y.syn-rift heat loss may be ignored. Stickler (1981) has modified the McKenzie model to calculate the subsidence and heat loss of basins of finite width and has shown that lateral heat loss through the sides of the basin is important; the rates of cooling and subs indene are increased. Cochran (1983) has calculated the effect of sniff heat loss and has shown that for basins less than 150 km wide, the loss of heat during rifting is important.