TECTONIC CONTROL OF FAN SIZE - THE IMPORTANCE OF SPATIALLY-VARIABLE SUBSIDENCE RATES

We study the geophysical controls on the size of alluvial fans. Simple relationships between catchment characteristics, sediment yield, subs idence patterns and fan size are developed. As predicting fan size is essentially a conservation of mass problem, our analysis is general, a pplying to all types of fan landform. The importance of spatially vari able subsidence rates has gone largely unrecognized in previous studie s of modern fans. Here we stress that the distribution of subsidence r ates in the depositional basin is a primary control on relative fan si ze. Both free coefficients in the oft-cited power-law correlation of f an area and catchment area can be shown to be set primarily by the tec tonic setting, taken to include source area uplift rate and the subsid ence distribution in the depositional basin. In the case of a steady-s tate landscape, relative fan size is shown to be independent of both c limate and source lithology; only during times of significant departur e from steady state can relative fan size be expected to vary with eit her climate or source lithology. Transients associated with (1) a sudd en increase in rock uplift rate, (2) a sudden change in climate and (3 ) the unroofing of strata with greatly differing erodibilities may pro duce variation of relative fan areas with both climate and source lith ology. Variation of relative fan size with climate or lithology, howev er, requires that catchment-fan system response to perturbations away from steady state is sensitive to climate and lithology. Neither the s trength of transient system responses nor their sensitivity to climate or lithology are known at present. Furthermore, internal feedbacks ca n significantly dampen any climatic or lithological effect. Thus theor etical considerations of the importance of climatic and lithological v ariables are inconclusive, but suggest that climatic and lithological effects are probably of secondary importance to tectonic effects. Fiel d data from an unsteady landscape in Owens Valley, California, support and illustrate theoretical predictions regarding tectonic control of fan size. Field data from Owens Valley allow, but do not prove, a seco ndary dependence on source lithology. In addition, the Owens Valley fi eld data indicate no relationship between relative fan size and climat e. Headward catchment growth and enhanced sediment bypassing of fans d uring times of increased sediment yield (glacial) are put forward as p lausible explanations.