EXTENSION ALONG THE GREAT RIFT, IDAHO, AS REVEALED BY FIELD MEASUREMENTS OF TENSION CRACKS: IMPLICATIONS FOR DIKE MORPHOMETRY AND ARREST MECHANISMS

Remote sensing and field study of the southern Great Rift, Eastern Snake River Plain (ESRP), Idaho, was completed to document its extensional kinematics and to understand what influenced the location of eruptive fissures. It is defined by three linear segments ranging in age from 2.2 ka to > 4.5 ka. Each segment is characterized by sub-parallel sets of tension cracks and a discontinuous central eruptive fissure. Subsurface basaltic dikes are interpreted to underlie each segment.

AIRSAR responses to various fracturing styles were qualitatively assessed by mapping fractures on images and then ground-truthing the maps. AIRSAR imagery was generally effective in identifying open fractures larger than ~ 30 cm and fractures expressed as steep depressions in rubble. Fractures spaced more closely than ~10 m were not separable in AIRSAR images. While effective in identifying areas of fracturing, AIRSAR provided little advantage over other methods in detailed fracture mapping. Surface strain associated with extension was determined by summing field measurements of tension crack and fissure widths along 22 rift-perpendicular transects. Surface strain measurements along the King's Bowl segment range from 0.64 to 4.50 m and are highest near its center. Along the New Butte segment, values range from 1.33 to 4.41 m, generally increasing toward the NW. Along the Minidoka segment, values range from 0.74 to 1.57 m, also increasing toward the NW. The width of each rift segment was used to calculate the depth of dike emplacement. Depths to dike tops are fairly constant within each segment but vary as much as 25% between segments. If dike tip arrest in the ESRP is controlled by contrasting mechanical properties of subsurface strata, this suggests the depth of this contrast (e.g. basalt over rhyolite) changes abruptly between rift segments. An alternative mechanism of dike arrest that incorporates neutral buoyancy is being investigated by modeling subsurface dike structure.

The location of eruptive fissures corresponds to regions where rift segments exhibit increased extension and reduced width. These data support the interpretation that dikes which arrested above some critical depth experienced volatile exsolution, due to reduced lithostatic pressure, which then provided the driving force for fissure eruption.