Impact fracturing and structural modification of sedimentary rocks at Meteor Crater, Arizona

摘要

Meteor Crater provides a rare opportunity to study impact deformation of sedimentary target rocks and isolate those features from preexisting tectonic deformation and impact-generated reactivation of preexisting tectonic features. This study reports over 2500 new measurements of orientations of bedding, faults, and fractures in crater walls and in surrounding bedrock. Target rocks are characterized by horizontal bedding planes that are cut by at least three prominent sets of preimpact tectonic fracture systems. The crater rim is also cut by three distinct groups of fractures: radial, concentric, and conical fractures. When the crater rim is restored to preimpact condition, the radial and concentric fractures resemble preimpact fracture populations, indicating that crater wall deformation and rim uplift were partly accommodated by activation of preexisting fractures. In contrast, the conical fractures are dissimilar to the preimpact fractures and apparently formed as a direct result of impact deformation. Some of the preimpact fractures were transformed into tear faults during the impact event, and motion along those faults appears to have controlled the geometry of the impact deformational features. The crater rim is, thus, square in plan view rather than circular. Faults occurring in the crater diagonals are prominent ones, allowing greater vertical displacement. The deformation pattern of Meteor Crater is different from that at Lonar Crater, which was excavated in basalt with fewer preimpact fractures. The differences between deformation at Meteor Crater and Lonar Crater may reflect the same disparities seen in simple craters produced in different target lithologies on Mars and other planetary surfaces.