THERMAL AND MECHANICAL CONTROLS ON FRACTURE PATTERNS IN EASTERN SNAKE RIVER PLAIN (ESRP) BASALT FLOWS, IDAHO

Most of the current knowledge on cooling fractures stems from the analysis of flood basalt flows, such as the Columbia River Basalts (CRB). However, fracture models developed for CRB flows do not predict the fracture patterns characteristic of relatively smaller ESRP flows. This is because cooling fracture patterns in pahoehoe basalt flows of the ESRP depend strongly on the presence of vesicular layers, the aspect ratio (width/height) of the lava flow, and thermal perturbations during cooling.

We recognize four distinct fracture types in ESRP basalt flows. In order of growth, they are: (1) column-bounding fractures; (2) column-normal fractures; (3) inflation fractures; and (4) entablature fractures. Types 1, 2 and 4 are cooling fractures driven by thermal stress. Type 3 is induced by pressure within a lava flow. The overall pattern created by these fractures is increasingly radial as the flow aspect ratio approaches 1.

Column-bounding and column-normal fracture patterns are strongly controlled by vesicular layers: planes of increased vesicularity that occur in three different styles in ESRP flows. Vesicular layers act as mechanical heterogeneities that concentrate thermal stress, creating preferential pathways for column-normal fracture growth and causing column-bounding fractures to either change growth direction or terminate growth.

ESRP entablature formation and resultant fracture geometries are distinctly different to CRB entablatures, which form in response to water-steam convection in fractures near flow tops after inundation by surface water. In contrast, ESRP entablatures occur near flow centers and represent rapidly cooled lava cores, preserving the shape of the lava core in the latest stages of the cooling history. Entablatures formed as inflation fractures grew toward the corners of pressurized lava cores, penetrating the lava core and causing rapid cooling and intense fracturing. Lava core shape and inflation fracture location is controlled by the aspect ratio of the lava flow. Perturbation of isotherms by convective heat loss through inflation fractures is not only responsible for entablature formation but also perturbs the overall fracture style, forming triangular and rectangular columns along the inflation fracture walls as well as columns that radiate away from inflation fractures.