The Payson ophiolite (~1.73 Ga) is exposed as a shallow-dipping, pseudostratigraphic sequence of gabbro, sheeted dikes, and submarine volcanic rocks, with screens and roof pendants of older arc crust. The sheeted dike complex is particularly well exposed, and analysis of measured sections reveals a threefold increase in the average dike width with depth in the complex. Magmatic temperatures and slow cooling at the base of the sheeted dike complex, indicated by gabbro-dike mingling and mutual intrusion and coarse textures, may have facilitated the growth of wide dikes. In contrast, extreme alteration of the top of the sheeted dike complex suggests that hydrothermal circulation dominated and caused rapid cooling as thin dikes. The connection between wide dikes at depth with thinner dikes toward the top requires a new model, where dikes branch upward in response to the strong vertical gradient in the rate of cooling and solidification. The branching dike model has important implications for the timing and spatial distribution of seafloor phenomena in the neovolcanic zone of spreading centers.¿ 1997 American Geophysical Union