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46th Lunar and Planetary Science Conference (2015)
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SMALL THRUST FAULT SCARPS ON MERCURY REVEALED IN LOW-ALTITUDE MESSENGER
IMAGES. Thomas R. Watters1, Sean C. Solomon2,3, Katie Daud1, Maria E. Banks1, Michelle M. Selvans1, Mark S.
Robinson4, Scott L. Murchie5, Nancy L. Chabot5, Brett W. Denevi5, Carolyn M. Ernst5, Clark R. Chapman6, Caleb I.
Fassett7, Christian Klimczak3,8, Paul K. Byrne3,9, David T. Blewett5, 1Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560, USA ([email protected]); 2LamontDoherty Earth Observatory, Columbia University, Palisades, NY 10964, USA, 3Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA; 4School of Earth and Space Exploration,
Arizona State University, Tempe, AZ 85251, USA; 5The Johns Hopkins University Applied Physics Laboratory,
Laurel, MD 20723, USA; 6Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302,
USA;7Department of Astronomy, Mount Holyoke College, South Hadley, MA, USA; 8Department of Geology,
University of Georgia, Athens, GA 30602, USA; 9Lunar and Planetary Institute, Houston, TX 77058, USA.
Introduction: The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER)
spacecraft has confirmed that the tectonic history of
Mercury since the late heavy bombardment is dominated by contraction [1-4]. The most broadly distributed
contractional tectonic landforms on Mercury are lobate
scarps, the surface manifestation of thrust faults [3,4].
These scarps are often hundreds of kilometers long and
display hundreds to thousands of meters of relief. They
are readily detected in images obtained by
MESSENGER’s Mercury Dual Imaging System
(MDIS) wide-angle and narrow-angle cameras, particularly in high-incidence-angle images and mosaics with
pixel scales of 200 m [3]. Indeed, the scale of these
prominent landforms facilitates their discrimination
from other positive-relief features and their characterization with topography acquired from the Mercury Laser Altimeter (MLA) [5] and stereo imaging [6, 7].
Lowering MESSENGER’s periapsis altitude has provided the opportunity to image the surface of Mercury
at higher resolution than was possible during the earlier
orbital phase of the mission. In low-altitude images
obtained at pixel scales as low as ~2 m/pixel, small
lobate scarps have been detected.
Dimensions of Small Scarps: The small scarps are
less than 10 km in length (Fig. 1, 2), an order of magnitude or more smaller than their larger counterparts. The
small scarps on Mercury are nonetheless comparable in
scale to similar small scarps documented on the Moon [8,
9]. Although lunar scarps were first identified in the highest-resolution Lunar Orbiter photographs and in Apollo
Panoramic Camera images, their density and distribution
could not be determined because of limited coverage until
images acquired by the Lunar Reconnaissance Orbiter
Camera (LROC) Narrow Angle Cameras (NACs) at spatial resolutions of 0.5 to 2 m/pixel revealed a globally
distributed population of previously undetected fault
scarps [8]. Their morphology, linkage between scarp
segments, along-strike reversals in vergence, and crosscutting relations all suggest that lunar scarps are generally the surface expression of thrust faults [8-12]. Forward modeling constrained by topographic profiles also
supports the interpretation that the lunar scarps formed
by thrust faulting [13]. An origin by downslope move-
ment is possible for some instances, but the fact that the
vergence direction of lunar scarps is often upslope favors a tectonic origin. Lunar scarps are found in mare
basalt units, but the great majority occur in the lunar highlands.
Small scarps on Mercury are similar in morphology
to their lunar counterparts and share characteristics such
as segment linkage, reversals in vergence, and upslope
vergence, consistent with a tectonic origin for these
landforms. Like lunar scarps, Mercury’s small scarps
often occur in clusters (Fig. 1). An example of a cluster of
at least three small scarps, the longest ~4.3 km in length,
is found in intercrater plains (Fig. 1). Estimates of relief
from shadow measurements of five small scarps indicate
a range of ~10 to 90 m (with a mean of ~40 m), similar to
the range in relief of the small scarps on the Moon [9].
Age of Small Scarps: Low-altitude MESSENGER
images also reveal that the small scarps crosscut small
impact craters. Small craters with diameters <100 m are
transected or disturbed by many of the small scarps (Fig.
1B). Transected small craters <100 m in diameter on the
Moon are Copernican (<~800 My) in age [8, 14], broadly
equivalent to the Kuiperian period on Mercury [15]. The
low relief, lack of superposed small craters, and pristine
appearance support the inference that small scarps on
Mercury are geologically recent, because small landforms
are unlikely to survive for billions of years under steady
meteoroid bombardment. Small lunar scarps are likely
less than 50 My old on the basis of estimated rates of infilling of small, shallow graben in the back-scarp terrain
[14]. A small scarp on Mercury with associated smallscale, back-scarp graben has been found (Fig. 2). Some of
these graben are estimated to have maximum widths of
~40 m, comparable to the widths of small lunar graben
[14]. The back-scarp graben on both bodies are likely the
result of uplift and flexural bending in response to slip
on the underlying scarp-related thrust fault [8, 14]. The
cratering rate has been estimated to be a factor of ~3
greater on Mercury than on the Moon [16], so the rate of
degradation of small landforms and rate of infilling of
shallow graben on Mercury is expected to be comparably
greater. Thus, thrust faults forming these scarps on Mercury could be much younger than 50 My, raising the possibility that some of these features are currently active.
46th Lunar and Planetary Science Conference (2015)
Spatial Distribution: The global distribution of
small scarps on Mercury cannot be determined because
the highest-resolution, low-altitude images are confined
to the northern hemisphere around MESSENGER’s
periapsis latitude. However, small scarps have been
found across a wide range of longitudes, suggesting that
they are broadly distributed in the northern hemisphere.
The scarps occur on both smooth plains and intercrater
plains. The small scarps are likely the smallest members of a continuum in scale of thrust fault scarps on
Mercury.
Locations of Small and Large Lobate Scarps:
High-resolution images of the large lobate scarps indicate that some of these features have been recently reactivated. For example, disturbed and crosscut small
impact craters (<100 m in diameter) on the northwest
segment of Carnegie Rupes point to recent activity on
thrust fault segments on this prominent scarp system.
Moreover, small scarps are found near Carnegie Rupes
and may have accommodated localized contractional
strain related to continued reactivation of the faults un-
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derlying the larger structure. Most of the small scarps
identified to date, however, are not found near larger,
older lobate scarps. Those small faults that are distant
from larger structures and resolvably tectonic in orgin
provide evidence that Mercury is forming geologically
young faults to accommodate the most recent phases of
interior cooling and global contraction.
References: [1] Solomon S.C. et al. (2008) Science, 321, 59–62. [2]
Watters T.R. et al. (2009) Earth Planet. Sci. Lett., 285, 283–296. [3]
Watters T.R. et al. (2013) LPS, 44, abstract 2213. [4] Byrne P.K. et
al. (2014) Nature Geosci., 7, 301–307. [5] Zuber M.T. et al. (2012)
Science, 336, 217–220. [6] Oberst J. et al. (2010) Icarus 209, 230–
238. [7] Preusker F. et al. (2011) Planet. Space Sci., 59, 1910-1917.
[8] Watters et al. (2010) Science, 329, 936–940. [9] Banks et al.
(2012) JGR, 117, E00H11, doi:10.1029/2011JE003907. [10] Schultz
P.H. (1976) Moon Morphology, Univ. Texas Press, Austin. [11]
Binder A.B. (1982) Earth Moon Planets, 26, 117-133. [12] Watters
T.R. and Johnson C.L. (2009) in Planetary Tectonics, Cambridge
Univ. Press, pp. 121-182. [13] Williams, N.R. et al. (2013) J. Geophys. Res. Planets, 118, 224-233. [14] Watters, T.R. et al. (2012)
Nature Geosci., 5, 181–185. [15] Spudis, P.D. and Guest, J.E. (1988)
in Mercury, Univ. Arizona Press, Tucson, pp. 118–164. [16] Marchi
S. et al. (2013) Nature, 499, 59-61.
Figure 1. Cluster of small lobate scarps in intercrater plains. (A) The largest scarp in the cluster (upper two arrows) is ~4.3 km in length.
(B) Crosscut small impact crater ~90 m in diameter (arrow). Location is shown in Figure 1A, ~38.9ºN, 27.9ºE, EN1029769395M.
Figure 2. Small lobate scarp with small graben. (A) Small scarp (arrows) in the northern smooth plains. (B) Small graben complex (arrows)
located in the back-scarp terrain ~3.8 km from the scarp face. Location is shown in Figure 2A, ~77.3ºN, 63.8ºE, EN1046425834M.