Normal Faulting Origin for the Cordillera And

Normal faulting origin for the Cordillera and

Outer Rook Rings of Orientale Basin, the Moon

Nahm, Ohman, and Kring 2013

Summarized by Dan Moriarty

Major Goals:

• Evaluate a faulting origin for the Cordillera and Outer Rook Rings of Orientale
• This is done by comparing the measured topography with a forward model of topography predicted for normal faults with several adjustable parameters, including fault height, fault length, depth of faulting, dip angle, etc.
• The results are compared with four existing models of basin formation:
• Rock Tsunami: a hydrodynamic model where impact-fluidized rubble oscillates through rebound and collapse, eventually “freezing” the basin rings in pace (no prediction or requirement for faulting).
• Ring Tectonics: basin rings are normal fault scarps resulting from crustal blocks moving radially inward toward the basin center – forming when the depth of the transient cavity exceeds the thickness of the lithosphere.
• Nested Melt Cavity: rings form at the boundary between the melt zone and displaced zone (shocked material rebounds to form peak ring), and at the edge of the displaced zone from listric normal faulting.
• Megaterracing: the most common model for basin formation – outer rings form as large normal faults, while the interior of the basin is displaced downwards.

Methods:

• Considering several topographic profiles from the Outer Rook Ring, Inner Rook Ring, and Cordillera Ring, the authors construct forward topographic models of normal faulting.
• The authors seek to ascertain if normal faulting is consistent with basin ring topography, as well as characterizing the properties of the faults.

Results:

• The Cordillera Ring is consistent with normal faulting. For the topographic profiles modeled, the modeled fault lengths range from 28-140 km; depth of faulting, 19-37 km; fault dip, 61-75 deg; displacement, 0.8-5.0 km; and fault height, 20.2-40.5 km.
• The Outer Rook Ring is also consistent with normal faulting. For the profiles considered, the modeled fault lengths range from 25-77 km; depth of faulting, 20-30 km; fault dip, 54-80deg; displacement, 1.6-5.2 km; and fault height, 20.3-31.9 km.
• The Inner Rook Ring is not consistent with normal faulting and therefore requires a different formation mechanism.

Conclusions:

• The Outer Rook Ring and Cordillera Ring can be modeled as normal faults, which probably occurred during collapse of the transient cavity (akin to the modification stage of complex craters).
• The distribution and ages of mare deposits are related to the locations of deep faulting. Thin, fractured crust at the center of the basin is the first to flood with mare. This load eventually seals the central conduit, but opens the outer deep faults at the outer rings, opening conduits for subsequent magma ascent.
• The results here predict the transient cavity rim to be somewhere between the Outer and Inner Rook Rings.
• The fault locations are mostly in agreement with the Megaterracing and Ring Tectonics models, but some parameters differ (such as the diameter of the transient cavity).
• The Inner Rook Ring does not display normal fault morphology and instead may be related to peak rings in complex craters.
• Crustal thickness differences across the basin predate basin formation.

Discussion questions:

1. The Cordillera Ring and Outer Rook Ring exhibit significantly different morphologies (~4 km scarp vs. ~1 km massifs, respectively). However, the authors attribute both rings to normal faulting during transient cavity collapse. How can one process result in such different morphologies?
2. “The Inner Ring may delineate the deepest part of the original cavity [McCauley, 1977] into which basalt was extruded but from which it was later partially withdrawn [Scott et al., 1977], causing the inner part of the basin to collapse.”
   Being at the center of the basin, it seems trivial that the inner ring would be associated with the deepest part of the transient cavity. Are there examples where the deepest part of the transient cavity are not associated with the center of a basin? Also, what does it mean for basalts to be “partially withdrawn?”
3. The paper assumed fault formation occurs prior to ejecta deposition. Is this a good assumption?
4. In Fig. 7A, the fault plane is represented in a rectangular prism. But what happens at the lower boundary of the fault?
5. In this paper, Mare Orientale and Lacus Veris are said to propagate through two different mechanisms: fractured basement and impact breccia for the central Mare Orientale, and large faults at the edge of the transient cavity for Lacus Veris. How would these propagation avenues affect the composition of mare basalts?