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46th Lunar and Planetary Science Conference (2015)
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HOW YOUNG ARE LUNAR LOBATE SCARPS? J. D. Clark1, C. H. van der Bogert1, and H. Hiesinger1,
1
Institut für Planetologie, Westfälische Wihelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
([email protected]).
Introduction: Interior cooling and contraction of
the Moon, resulted in compressional features called
lobate scarps, the surface expression of low angle
thrust faults [1-3]. Based on their generally crisp appearance and the absence of superimposed, largediameter impact craters (>400m), lobate scarps are
among the youngest landforms on the Moon (< 1 Ga)
[4-6]. Binder and Gunga [7] used crater degradation
measurements on craters transected by or superposed
on the scarps to derive ages for 20 lobate scarps. The
majority of derived model ages indicated relatively
concise scarp ages, implying a short formation period,
while a few others exhibited multiple episodes of deformation [7]. Copernican in age, the scarp age estimates ranged from 60±30 Ma to 680±250Ma [7].
These early investigations of lobate scarps were
conducted using images from the Apollo Panoramic
Camera limited to the equatorial region [7]. Today, we
have the opportunity to look at additional lobate scarps
at high resolution using images obtained by the Lunar
Reconnaissance Orbiter Camera (LROC) [5]. LROC
has revealed a true global distribution with over 2,700
lobate scarps identified thus far [8]. Moreover, the
method of crater size-frequency distriution (CSFD)
measurements was developed by [9,10] before the
work of [7]. Conventionally, CSFD measurements are
used to derive ages for well-defined geologic units,
such as impact deposits and basalt flows. However,
[11] adapted the technique for use on lobate scarps,
based on the discovery that shaking during scarp formation seems to have caused local resurfacing of small
craters. Here, we further apply the technique on 20
additional lobate scarps, to investigate the range of
scarp ages.
Overall, investigating the age of the lobate thrust
fault scarps will provide new insights into the late-stage
tectonic evolution of the Moon.
Data and Methods: We used NAC (Narrow Angle Camera) image data, which were processed using
Integrated Software for Imagers and Spectrometers
(ISIS) [12] and imported into ArcGIS. Count areas and
crater measurements were generated using CraterTools
[13]. The CSFDs were plotted and fit with CraterStats
[14], using techniques as described in [9]. The derived
absolute model ages (AMAs) are based on the chronology function (CF) and production function (PF) of
[10], valid for lunar craters >10 m and <100 km in
diameter.
For this study, we used the same method as described in [11] and illustrated in Fig. 1, where we derived AMAs for proximal and distal areas of the thrust
fault hanging walls and footwalls. Previous AMAs
derived by [11] showed that the hanging wall of the
lobate scarp generally displays a younger AMA than
the footwall. This difference in age was interpreted to
be caused by differences in shaking during scarp formation [11].
Results: Our absolute model ages from CSFD
measurements indicate that lobate scarp ages range
from 60 to <600 Ma, implying geologically recent
faulting.
This is consistent with the work of
[5,7,11,15]. Although some locations exhibited multiple resurfacing events, proximal hanging walls tend to
be younger than the footwalls as observed by [11] and
Figure 1. Locations of count areas at Barrow scarp (69.9N, 4.80E) showing CSFD measurement technique of [11] on NAC
image pair M1116516111.
46th Lunar and Planetary Science Conference (2015)
Figure 2. CSFDs measured at Barrow scarp for the
hanging wall (red) and footwall (blue).
seen in Fig. 2.
In Fig. 3, the CSFDs obtained during the present
study are plotted versus their respective fault surfaces.
All scarp proximal hanging walls are younger than
their proximal footwalls. At distal locations, the foot-
Figure 3. AMAs versus locations perpendicular to the
footwall (FW) and hanging wall (HW) of the fault, where
the fault lies between FW2 and HW2.
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wall tends to be younger than its proximal counterpart,
while the hanging walls increase in age. We interpret
those distal hanging walls in Fig. 3 with ages > 600Ma
as approaching the background surface age. Differences in fault geometry likely influences the heterogeneity in surface ages both proximal and distal from the
fault.
Comparing our AMAs to those of [7], on average
our ages are slightly younger by 10s of Ma. Both our
crater counting approach and the high-resolution NAC
images may allow craters to be measured with greater
accuracy and for smaller scarps, thus allowing us to
derive younger AMAs.
Summary and Outlook: AMAs derived to date,
along with the crisp appearance of lobate scarps, imply
geologically recent faulting, consistent with the work
of [5,11,15]. Such results have implications for the
thermal history models for either an initially totally
molten Moon or an early Moon with a magma ocean.
Additional scarps are being investigated, and the
results will be incorporated into Fig. 3 allowing a
greater ability to interpret the timing of faulting events.
Also, CSFD measurements will be done on Wide Angle Camera (WAC) images of the areas surrounding
lobate scarp to gather a background surface age for
comparison with local faulting and other geological
activity.
This ongoing investigation will also incorporate
additional model age determination techniques to compare the counting approaches and the resulting AMAs.
One of these methods includes the buffered crater
counting technique where craters are mapped that superpose the linear feature and an effective counting
buffer area is used to acquire more accurate CSFDs
[13,16].
References: [1] Hartmann W.K. and Davis D. (1975) Icarus, 24, 504-515. [2] Cameron A.G.W. and Ward W.A.
(1976) Lunar Science VII, 120-122. [3] Binder A.B. (1982)
Moon and Planets, 26, 117-133. [4] Schultz (1976) Moon
Morphology, Austin, TX. [5] Watters T.R. et al. (2010) Science, 329, 936-940. [6] Binder A.B. (1986) Papers Presented to the Conference on the Origin of the Moon, 425-433.
[7] Binder A.B. and Gunga H. (1986) Icarus, 63, 421. [8]
Watters T.R. et al. (2014) LPSC XLV, Abstract #2163. [9]
Neukum (1983) Meteoritenbombardement und Datierung
planetarer Oberflächen, Habil. Thesis, Univ. Munich. [10]
Neukum et al. (2001) Space Sci. Rev., 96, 55. [11] van der
Bogert C.H. et al. (2012) LPSC XLIII, Abstract #1847. [12]
Anderson, J.A., et al., (2004) LPSC 35, #2039. [13] Kneissel
et al. (2011) PSS, 59, 1243-1254. [14] Michael and Neukum
(2010) EPSL, 294, 223. [15] Clark J.D. et al. (2014) LPSC
XLV, Abstract #2048. [16] Kneissl T et al. (2014) Icarus,
doi:10.1016/j.icarus.2014.12.008.