Matthew P. Golombek

Tharsis Province of Mars - Geologic sequence, geometry, and a deformation mechanism

Abstract The early history of Mars included two large-scale events of great significance: (1) the lowering and resurfacing of one-third of the crust, followed closely by (2) evolution of the Tharsis bulge. Tharsis development apparently involved two stages: (1) an initial rapid topographic rise accompanied by the development of a vast radial fault system, and (2) an extremely long-lived volcanic stage apparently continuing to the geologic present. A deformational model is proposed whereby a first-order mantle convection cell caused early subcrustal erosion and foundering of the low third of the planet. Underplating and deep intrusion by the eroded materials beneath Tharsis caused isostatic doming. Minor radial gravity motions of surficial layers off the dome produced the radial fault system. The hot underplate eventually affected the surface to cause the very long-lived volcanic second stage. Deep crustal anisotropy associated with the locally NE-trending boundary between the highland two-thirds and the lowland one-third caused the NE elongation of many features of Tharsis.

Grabens, basin tectonics, and the maximum total expansion of the Moon

The presence of grabens and wrinkle ridges indicates that stresses sufficient to deform or fault crustal rocks have existed for at least part of lunar history. These structures have been explained as due to global expansion and contraction, basin subsidence, or both. Even if the extension implied by all lunar grabens were due to global expansion, the total radius increase permissible after terminal bombardment would be much less than that predicted by the most conservative models of lunar thermal evolution. The obvious areal association of most grabens and wrinkle ridges with impact basins suggests that a genetic relationship exists also. But it is necessary to develop either kinematic or dynamic models to determine if basin subsidence alone can account for these structures. Kinematic models are the most useful because fewer assumptions regarding material properties are needed than for dynamic models. For both types, the grabens and wrinkle ridges provide the data that must be predicted by the models. Considerations of geometry, kinematics, and mechanics suggest that the simple grabens on the moon are reliable indicators of crustal extension. Two end-member kinematic models of postimpact basin modification are derived: one assuming that subsidence is accomplished entirely by an increase in radius of curvature of the basin floor, the other assuming that subsidence is accomplished without any change in radius of curvature of the basin floor. Any combination of the two also would be possible. Both models predict enough extension to account completely for basin-related grabens, and thus they indicate that virtually all of the extension implied by lunar grabens can be provided by modest amounts of basin subsidence. In addition, both models also predict total radial and hoop shortening of the basin floor of about the correct magnitude to explain wrinkle ridges if these features are structures resulting from compression. Consequently, almost all extension and shortening of the lunar crust implied by grabens and wrinkle ridges can be explained by basin subsidence alone; significant global expansion after ∼3.9 b.y. is unlikely, and significant global contraction is unnecessary.

Geology, structure, and tectonics of the Pajarito fault zone in the Española basin of the Rio Grande rift, New Mexico

The Pajarito fault zone forms the western border of the Velarde graben, the presently active, central subbasin of the Espanola basin section of the Rio Grande rift in north-central New Mexico. The fault zone is a north-northeast-trending zone of predominantly down-to-the-east faults that cut Miocene to Pliocene volcanic rocks along the eastern flank of the Jemez Mountains. Where the fault zone cuts the 1.1-m.y.-old Tshirege Member of the Bandelier Tuff, it has produced a steep, 50- to 100-m-high fault scarp. The total displacement across the fault zone during its 5-m.y. history is between 200 and 600 m. Rates of displacement for the time periods 0–5, 0–1.1, and 1.1–5 m.y. ago range from 0.02 to 0.136 mm/yr. Abrupt facies changes between older volcanics and volcaniclastic sediments of the Jemez Mountains appear to have controlled the local position, trend, and character of the Pajarito fault zone. The fault zone bows and/or steps eastward where two large volcanic complexes are present but is found farther west in between and at either end of the volcanic complexes. One complex was sufficiently massive to interfere with the development of the Velarde graben. Slickensides on mesoscopic faults in the Tshirege Member of the Bandelier Tuff indicate that the Pajarito fault zone has undergone extension in two directions during the past 1.1 m.y., approximately parallel and perpendicular to the local trend of the fault zone. These directions indicate that the Pajarito fault zone has reoriented the regional minimum and intermediate stress directions to perpendicular and parallel, respectively, to the local trend of the fault zone, and that both minimum and intermediate stress directions are tensional. A tectonic history for the Pajarito fault zone area of the Espanola basin begins with relatively stable accumulation of prerift and synrift sediments from Eocene to Oligocene time. Sedimentation concomitant with faulting, unrelated to the Pajarito fault zone, filled deep central depressions within the Espanola basin. This faulting ceased prior to the end of the filling of the basin, around 10 m.y. ago in the local area. Jemez Mountain volcanism began about this time, before movement along the western-margin border faults of the Espanola basin caused west-tilting of old volcanics and sediments, about 7.5 m.y. ago. Volcanism continued under relatively stable conditions until ∼5 m.y. ago. At this time, the Pajarito fault zone and Velarde graben formed. Faulting has continued to the present, localized along this central subbasin.

Geometry and rate of extension across the Pajarito fault zone, Española basin, Rio Grande rift, northern New Mexico

The central Velarde graben is the active subbasin of the Espanola basin section of the Rio Grande rift in north-central New Mexico. The Velarde graben is bounded on the west, in the Jemez volcanic field, by the Pajarito fault zone. This fault zone has produced a steep fault scarp about 100 m high where it cuts the l.l-m.y.-old Tshirege Member of the Bandelier Tuff. Detailed mapping along the north-trending Pajarito fault zone has revealed a fairly simple geometry. In the Tshirege Member, the faults follow numerous vertical joints. Below this member, fault dips are ∼60° and not listric at shallow depths. This simple geometry allows calculation of a mean rate of extension of ∼0.05 mm/yr across the Pajarito fault zone for the past 1.1 m.y. If extension is not perpendicular to the fault zone, the extension rate could be as great as ∼0.07 mm/yr. Lack of transverse tilt of the Velarde graben wedge implies that the extension rate across the eastern margin is about the same as for the western margin. Comparison with a published extension rate for the northern Albuquerque-Belen basin (just to the south of the Jemez Mountains) of 0.3 mm/yr (both sides) since rifting began 26 m.y. ago indicates a slower opening for the Velarde graben during the past 1.1 m.y. If extension is localized along the margins of the Velarde graben with little activity along other fault zones in the Espanola basin, then both the mean rate of extension and the width of the actively extending region have decreased with time for this section of the Rio Grande rift.