Seismo-tectonic geomorphology of the Moon: Lobate scarps and boulder falls

Post contributed by P. Senthil Kumar, CSIR-National Geophysical Research Institute, Hyderabad, India

Geomorphology of terrestrial planets provides important insights into how various exogenous (e.g., meteorite impact, wind activity, glaciation) and interior geological processes (e.g., tectonics, volcanism) interact with the planetary surface. The tectonic features (faults, folds and fractures) shed light on the past and on-going seismo-tectonic processes operating on a planetary body. On Earth, seismometer networks are used in the direct instrumental observations of earthquake locations and their sizes for many decades. For other planets, seismic observations are rare. On the Moon, Apollo seismometers recorded moonquakes only during a brief period of 1969-1976. Annually, about 2000 deep moonquakes originating at 800-900 km depth, tens of shallow moonquakes at 0-100 km depth and around 200 meteoroid impacts were observed (Nakamura et al., 1979; Nakamura, 1980). While the deep moonquakes are produced by tidal forces, the sources of shallow moonquakes are thought to be of tectonic in origin. However, crustal tectonic structures responsible for shallow moonquakes are poorly understood.

post_pgwg_senthil_figure_27feb2017

Figure 1: (upper panel) LROC NAC images showing the en echelon pattern of a lobate scarp, located near the southern basin wall of Schrodinger basin. Hundreds of boulder falls and their trails are found on the basin wall about 5-7 km south of the lobate scarp. The boulders are shown as points (open circles filled with yellow) and are not to the scale. Note the largest number of boulder falls is seen between 129° and 130° longitudes. (lower panel) LROC NAC (M139078014RE) image showing the boulder trails of variable lengths and widths containing some prominent boulders at the terminal ends of the trails. A 23 m diameter boulder (labelled), the largest in this scene, produces a trail (also labelled) slightly wider than the boulder indicating a possible size reduction during its transport from the source region to its current location. Most boulder trails criss-cross each other.

Geomorphology of the Moon reveals presence of ancient and recent faulting on its surface. The ancient faults are generally wrinkle ridges, mostly formed in the mare-filled impact basins.  On the other hand, lobate scarps are found both on mare and highlands with predominance in the latter. The scarps are found to occur throughout the Moon, exhibits a crisp lobate morphology and rupturing of lunar surface, and show cross-cutting relations with small fresh impact craters suggesting their young age (Lucchitta, 1976; Watters et al., 2010, 2012). In a recent study, Kumar et al. (2016) found four lobate scarps of different lengths from the interior of Schrödinger basin using Lunar Reconnaissance Orbiter and Chandrayaan-1 images. The lobate scarps display crosscutting relations with the basin materials and structures. The impact craters are one of the most common structures cross cut by the lobate scarps. Although the diameters of crosscut craters range from <10m to >500 m, a majority of them are in the range of <10 m to 30 m. Most of these small craters are characterized by crisp crater exteriors and interiors, indicating that these are young impact craters. One of the lobate scarps is about 28 km in length and ~10-100 km in width and occurs near the southern basin wall (see upper panel, Figure 1). The scarp yields a minimum age of 11 Ma, based on buffered cratering counting (a method by which the statistics of superposed impact craters allows us to estimate the age). About 5-7 km south of the scarp, the basin wall exhibits hundreds of boulder falls, in which several boulders rolled and bounced on the slopes (see lower panel, Figure 1). The average slopes of the boulder fall sites vary from 20 to 24 degrees, and are slightly gentler than the boulder source regions. A cluster of a large number of boulder falls near the scarp indicates that the scarp was recently seismically active . The relatively low runout distance of the boulders (~2.5x the fall height) suggests low to moderate levels of ground shaking, that could be related to low-magnitude moonquakes in the scarp. Therefore, the study (Kumar et al., 2016) provides an example of how tectonic geomorphology can be used to infer recent seismo-tectonic activities of the Moon. A similar approach can be extended to other terrestrial planets, but only where suitable images exist in order to resolve metre-scale boulder trails.

Further reading:

Lucchitta, B. K. (1976), Mare ridges and related highland scarps: Results of vertical tectonism, Geochim. Cosmochim. Acta, 3, 2761–2782.

Kumar, P.S., et al. (2016), Recent shallow moonquake and impact-triggered boulder falls on the Moon: New insights from the Schrödinger basin, J. Geophys. Res. Planets, 121, 147–179, doi:10.1002/2015JE004850.

Nakamura, Y. (1980), Shallow moonquakes: How they compare with earthquakes, Proc. Lunar Planet. Sci. Conf., 11th, 1847-1853.

Nakamura, Y., et al. (1979), Shallow moonquakes: Depth, distribution and implications as to the present state of the lunar interior, Proc. Lunar Planet. Sci. Conf., 10th, 2299-2309.

Watters, T. R., et al. (2010), Evidence of recent thrust faulting on the Moon revealed by the Lunar Reconnaissance Orbiter Camera, Science, 329, 936–940, doi:10.1126/Science.1189590.

Watters, T. R., et al. (2012), Recent extensional tectonics on the Moon revealed by the Lunar Reconnaissance Orbiter Camera, Nat. Geosci., 5, 181–185, doi:10.1038/NGEO1387.

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