Article,
Tectonics of mascon loading: Resolution of the strike-slip faulting paradox
Journal of Geophysical Research-Planets, 106 (E9): 20603--20620 (2001)
Abstract
Subsidence of lunar mascon maria, impact basins partly filled with
mare basalt and sites of prominent positive gravity anomalies, typically
led to the formation of concentric graben (arcuate rilles) around
the flanks of the basin, while compressive features (mare ridges)
formed in interior regions. Although previous numerical models of
the response of the lunar lithosphere to mascon loading predict that
an annulus of strike-slip faulting should also have formed around
mascon maria, no such faults have been observed. This "strike-slip
faulting paradox," however, arises from an oversimplification of
the earlier models. Viscoelastic finite element models of lunar mascon
basins that include the effects of lunar curvature, heterogeneous
crustal strength, initial stress conditions, and multistage load
histories show that the width of a predicted annulus of strike-slip
faulting may be small. The use of Anderson's criterion for predicting
fault styles may also overpredict the width of strike-slip faulting.
A faulting-style criterion that takes into account transitional faulting,
in which both strike-slip and dip-slip components are present, predicts
zones of pure strike-slip faulting that are about half of the width
predicted by the Anderson criterion. Furthermore, strike-slip faulting
should be observed only in regions in which flexural stresses are
sufficient to induce rock failure. However, since stress patterns
consistent with strike-slip faulting around mascon loads represent
a transition between compressional and extensional provinces, differential
stresses tend to be low in these regions and for at least part of
this region are not sufficient to induce rock failure. A mix of concentric
and radial thrust faulting is observed in some mascon maria, at odds
with previous models that predict only radial orientations away from
the basin center. This apparent discrepancy may be partly explained
by the multistage emplacement of mare basalt units, a scenario that
leads to a stress pattern where concentric and radial orientations
of thrust faults are equally preferred. Detailed models of the Serenitatis
basin indicate a 25-km-thick lunar lithosphere at the time of rille
formation and a 75-km-thick lithosphere at the time of late-stage
mare ridge formation. The extent of observed mare ridges and the
inferred cessation of rille formation around Serenitatis prior to
the time of emplacement of the youngest mare basalt units is consistent
with the superposition of a global horizontal compressive stress
field generated by the cooling and contraction of the lunar interior
with the local stresses associated with lithospheric loading.
Tags
- basin
- coronae;
- evolution;
- large
- lithosphere
- models;
- oceanus
- origin;
- procellarum;
- stress;
- tectonics;
- venus;
- volcanos;
