HORIZONTAL FORCES WITHIN LUNAR CRUST: INTRIGUING A QUESTIONING MIND

Nilanjan Dasgupta (neelakdg@gmail.com), Trishit Ruj (trishitruj@gmail.com), Anup Das (anup@sac.isro.gov.in), Sriram Saran 1 1 2 2

1 2 Presidency University, Kolkata-700073, India, Space Application Centre, Ahmedabad-380015, India.

Fig. 7: Figure showing the systematic development of horse.(redrawn after Ramsay and Huber,

1997, p. 533).

HBridge

Step 1 Step 2

Remnant

1. INTRODUCTION

Surface morphological features of planetary bodies have been extensively used for

structural and kinematic studies by comparing such features with similar looking [1], [2], [3]

terrestrial ones; the later act as analogues and aid to a better understanding of the

processes of their formation.

The basic assumption for such a correlation is that we believe the processes that led to the

formation of any earthly regional scale feature remains same for the development of a

similar feature on planetary bodies.

We here describe a topographic feature on the lunar crust that is found in the active strike

slip zones of the earth, where the faults accommodate the horizontal displacement [4], [5],

[6]. The term 'en-echelon faults' refers to closely spaced, parallel or sub-parallel,

overlapping in a step-like minor faults in rock, which lie oblique to the overall structural trend

of the strike slip fault system. We describe similar features from the Earth's moon and

discuss on its tectonic significance. The paradox lies in the fact that presence of dynamic

stresses are absent within Earth's moon.

10km N

E

S

W

2 km

N

H

50km

o5 E o8 E

o9 N

o7 NRima Hyginus

Hyginus crater

Rima Ariadaeus

g g’

1

2

3

Fig. 1, 2, 3: LRO-WAC mosaic image of Hyginus crater (HC) showing Rima Hyginus (RH) and Rima

Ariadius (RA). Note that the RH and RA are arranged in en-echelon pattern with a cross graben similar

to a transverse fault. Within the segment 'a' sharp change in the strike of the graben margin from NW-

SE to almost E-W is seen,implying a change in the orientation of the principal stress within the overall

stress regime. Segment 'b' is a blown up image showing thickening of the graben wall within the RH.

2. STUDY AREA

We concentrate on the geometry and fracture pattern of one such rille, Rima Hyginus (RH),

located south of the Mare Vaporum in the near side of the moon (Fig.1). RH is situated

within Imbrium basin, structurally a broad structural trough, believed to have a fault related

origin, extending for km and nearly 3 km in width. The depth of the graben is around 220

500±80 m. The general topography is gently undulating (Fig. 4).

3. DATA SET USED

We have utilized Digital Elevation Models (DEM) from the LOLA and LRO-WAC

(100m/pixel), LRO-NAC datasets from the Lunar Reconnaissance Orbiter (LRO) and

processed them with J-Mars , ENVI. [7]

SUMMARY

We suggest from the study of en-echelon faults and related

features, a local scale horizontal transport is visible in lunar

crust, devoid of plate tectonics.

f’f0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Distance in km

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Fig. 4: This section is drawn in E-W section from

LRO-WAC equatorial DTM. 100 times vertically

exaggerated.

Fig. 5: 3D section showing the presence of the

horse in the study area.

4. STRUCTURAL GEOMETRY OF THE STUDY AREA

Close inspection reveals RH is segmented into smaller linear grabens, arranged in en-

echelon pattern often with a sharp change in the trend of these grabens (Fig. 1, 2 and 3).

The topography is gently undulating with a thin veneer of pyroclastic material resting upon

the basaltic floor . Our analyses of RH reveal that the trend of the graben rotates from E-[8]

W to NW-SE indicating rotation of the stress regime. This is accompanied by thickening of

the graben wall on its either flank. Profile sections drawn indicate presence of horses

(marked by H) (Fig. 5) on the southern flanks of RH. We infer that the extensional stress has

produced small scale faults en-echelon to the major fracture responsible for graben

formation (Fig. 3). The tip lines of two adjacent en-echelon faults rotate and join to form a

continuous graben and these horses remain as central remnant of en-echelon fractures

(Fig. 5). Horses, within the median portions of two en-echelon faults, are reported from the

moon for the first time.

It is seen that in some regions affected by normal faulting too, the fault propagate along

strike in an en-echelon pattern with a cross fault joining the tips of the laterally offset faults

(Fig. 6). In such cases, differential slip along the lengths of the fault may ultimately lead to

the formation of tectonic horses within the fault zone .[9]

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Fig. 6: LRO-WAC mosaic image of Rima Hyginus draped on LOLA DEM (100m), the elevation profiles

across four transects, a-a'; b-b'; c-c'; and d-d' are shown below. Note the presence of horses (marked by H on d-d'

profile plot) on Rima Hyginus (3D generated by ACT).

5. DISCUSSION

The en-echelon fault systems are mainly associated with the active strike slip tectonic zones

of the earth where simple shear plays a dominant part in the overall stress regime of the

area. Some recent reports [10] of incipient stresses leading to new fracture formation in the

lunar crust has spurted new thoughts in believing the existence of tectonic activity in the

lunar crust. Our study shows that some amount of shear stress is operative on the lunar crust

leading to limited horizontal displacement of crustal blocks.

The tectonic horses within the lunar crust can therefore form in either of the two stress

regimes a) under shear stress regime or b) tensile stress regime. The two stress regimes

greatly differ in the principal stress vector orientations [11]. Large scale horizontal

displacement as seen on earth is unlikely in moon for the absence of plate tectonics. The

major stress is impact induced [2] and this stress gets transmitted through long distances

within the lunar crust producing fractures [1] in the lunar crust. These fractures are typically

segmented and often in an en-echelon pattern to the major fault zone (Fig. 7). The obliquity

of such fracture system to the major graben wall and the slip along these oblique fracture

planes probably lead to a concentration of shear stress on a local scale. We suggest that

small scale horizontal transport occurred within the lunar crustal blocks, at least to a local

scale, even in the absence of plate tectonics.

6. REFERENCES

[1] Melosh H. J. (2010) Planetary Surface Processes, Cambridge University Press, 500p., [2] Greeley

R. (2013) Introduction to Planetary Geomorphology, Cambridge University Press, pp 1-90, [3] Watters

T. R. and Schultz R. A. (2010) Planetary Tectonics, Cambridge University Press p.518, [4] Twiss R. J. ndand Moores E. M. (2007) Structural Geology 2 Ed. Susan Finnemore Brennan p [5] Davis G. H. and .

rdReynolds S. J. (2011) Structural Geology of Rocks and Regions, 3 Edition p. 864, [6] Van Der Pluijm B. ndA. and Marshak S. (2004) Earth Structure An Introduction to Structural Geology and Tectonics, 2 Ed.

pp 656, [7] Christensen P. R. JMARS – A Planetary GIS, http://adsabs.harvard. edu/abs/2009 AGUFMI

M2 2A.06C [8] http://www.lpi.usra.edu/resources/mapcatalog/usgs/I945/ [9] Ramsay. J. G. and

Huber. M. I. (1997) The techniques of Modern Structural Geology, v2, Folds and fractures, Academic

Press, 2nd edition. [10] Watters T. R. et al. (2012) Nature Geosciences, 5, 181-185.