HORIZONTAL FORCES WITHIN LUNAR CRUST: INTRIGUING A QUESTIONING MIND
Nilanjan Dasgupta 1 ([email protected]), Trishit Ruj 1 ([email protected]), Anup Das 2 ([email protected]), Sriram Saran2 1 Presidency University, Kolkata-700073, India, 2 Space Application Centre, Ahmedabad-380015, India.
SUMMARY 5. DISCUSSION 1. INTRODUCTION We suggest from the study of en-echelon faults and related The en-echelon fault systems are mainly associated with the active strike slip tectonic zones Surface morphological features of planetary bodies have been extensively used for of the earth where simple shear plays a dominant part in the overall stress regime of the structural and kinematic studies [1], [2], [3] by comparing such features with similar looking features, a local scale horizontal transport is visible in lunar area. Some recent reports [10] of incipient stresses leading to new fracture formation in the terrestrial ones; the later act as analogues and aid to a better understanding of the crust, devoid of plate tectonics. lunar crust has spurted new thoughts in believing the existence of tectonic activity in the processes of their formation.
-100 lunar crust. Our study shows that some amount of shear stress is operative on the lunar crust
The basic assumption for such a correlation is that we believe the processes that led to the -150
-200 -250 H leading to limited horizontal displacement of crustal blocks. formation of any earthly regional scale feature remains same for the development of a -300
-350
-400 The tectonic horses within the lunar crust can therefore form in either of the two stress similar feature on planetary bodies. -450 -500
-550 regimes a) under shear stress regime or b) tensile stress regime. The two stress regimes We here describe a topographic feature on the lunar crust that is found in the active strike -600 -650 -700 RH greatly differ in the principal stress vector orientations [11]. Large scale horizontal slip zones of the earth, where the faults accommodate the horizontal displacement [4], [5], -750 f f’ -800 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 -850 displacement as seen on earth is unlikely in moon for the absence of plate tectonics. The 40 50 80 100 110 120 130 140 150 160 [6]. The term 'en-echelon faults' refers to closely spaced, parallel or sub-parallel, 0 g10 20 30 60 70 90 g’ 170 Distance in km major stress is impact induced [2] and this stress gets transmitted through long distances overlapping in a step-like minor faults in rock, which lie oblique to the overall structural trend Fig. 4: This section is drawn in E-W section from Fig. 5: 3D section showing the presence of the within the lunar crust producing fractures [1] in the lunar crust. These fractures are typically of the strike slip fault system. We describe similar features from the Earth's moon and LRO-WAC equatorial DTM. 100 times vertically horse in the study area. segmented and often in an en-echelon pattern to the major fault zone (Fig. 7). The obliquity discuss on its tectonic significance. The paradox lies in the fact that presence of dynamic exaggerated. of such fracture system to the major graben wall and the slip along these oblique fracture stresses are absent within Earth's moon. 4. STRUCTURAL GEOMETRY OF THE STUDY AREA planes probably lead to a concentration of shear stress on a local scale. We suggest that 5oE 8oE 10km N small scale horizontal transport occurred within the lunar crustal blocks, at least to a local W E Close inspection reveals RH is segmented into smaller linear grabens, arranged in en- S scale, even in the absence of plate tectonics. echelon pattern often with a sharp change in the trend of these grabens (Fig. 1, 2 and 3). 9oN Hyginus crater The topography is gently undulating with a thin veneer of pyroclastic material resting upon 2 the basaltic floor [8]. Our analyses of RH reveal that the trend of the graben rotates from E- Rima Ariadaeus W to NW-SE indicating rotation of the stress regime. This is accompanied by thickening of g N g’ the graben wall on its either flank. Profile sections drawn indicate presence of horses 7oN Rima Hyginus H (marked by H) (Fig. 5) on the southern flanks of RH. We infer that the extensional stress has Bridge 50km produced small scale faults en-echelon to the major fracture responsible for graben H 1 3 2 km Remnant formation (Fig. 3). The tip lines of two adjacent en-echelon faults rotate and join to form a Fig. 1, 2, 3: LRO-WAC mosaic image of Hyginus crater (HC) showing Rima Hyginus (RH) and Rima continuous graben and these horses remain as central remnant of en-echelon fractures 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- (Fig. 5). Horses, within the median portions of two en-echelon faults, are reported from the Step 1 Step 2 SE to almost E-W is seen,implying a change in the orientation of the principal stress within the overall moon for the first time. stress regime. Segment 'b' is a blown up image showing thickening of the graben wall within the RH. It is seen that in some regions affected by normal faulting too, the fault propagate along Fig. 7: Figure showing the systematic development of horse.(redrawn after Ramsay and Huber, 1997, p. 533). 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 2. STUDY AREA the formation of tectonic horses within the fault zone [9]. 6. REFERENCES We concentrate on the geometry and fracture pattern of one such rille, Rima Hyginus (RH), [1] Melosh H. J. (2010) Planetary Surface Processes, Cambridge University Press, 500p., [2] Greeley
-450 -400 -475
-500 -425
-575-525 -450-500 located south of the Mare Vaporum in the near side of the moon (Fig.1). RH is situated -550-600 -475 -525 -625 -550 -650 -575 -675 Elevation R. (2013) Introduction to Planetary Geomorphology, Cambridge University Press, pp 1-90, [3] Watters -600 -700 -625 -725-
Elevation -650 -750 -675 -775
-800 -700 within Imbrium basin, structurally a broad structural trough, believed to have a fault related -825 -725 -850 -750 d’ -875 -775 T. R. and Schultz R. A. (2010) Planetary Tectonics, Cambridge University Press p.518, [4] Twiss R. J. c -900 -800 -925 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.50 6.75 7.00 7.25 7.50 7.75 8.00 8.25 8.50 8.75 9.00 9.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.50 6.75 7.00 7.25 7.50 7.75 Distance in km a Distance in km a’ c c’ nd origin, extending for 220 km and nearly 3 km in width. The depth of the graben is around b c’ and Moores E. M. (2007) Structural Geology 2 Ed. Susan Finnemore Brennan p. [5] Davis G. H. and
a rd -450 b’ -700 -475 -710 -720 500±80 m. The general topography is gently undulating (Fig. 4). -500 Reynolds S. J. (2011) Structural Geology of Rocks and Regions, 3 Edition p. 864, [6] Van Der Pluijm B. -730 -740 -525 -750 -550 -760 -770 -575 -780 -600 -790 -800 -625 -810 -820 nd a’ -650 -830 H -675 -840 -850 d Elevation -700 -860 -870 A. and Marshak S. (2004) Earth Structure An Introduction to Structural Geology and Tectonics, 2 Ed. -725 -880 -750 -890
- Elevation -900 -775 -910 -920 -800 -930 -825 -940 -950 -850 -960 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.50 6.75 7.00 7.25 7.50 7.75 8.00 8.25 8.50 8.759.00 -970 -980 3. DATA SET USED b Distance in km b’ 0.00 0.50 1.00 1.50 2.00 2.50 3.0 3.50 4.0 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 d Distance in km d’ pp 656, [7] Christensen P. R. JMARS – A Planetary GIS, http://adsabs.harvard. edu/abs/2009 AGUFMI
We have utilized Digital Elevation Models (DEM) from the LOLA and LRO-WAC Fig. 6: LRO-WAC mosaic image of Rima Hyginus draped on LOLA DEM (100m), the elevation profiles M2 2A.06C [8] http://www.lpi.usra.edu/resources/mapcatalog/usgs/I945/ [9] Ramsay. J. G. and (100m/pixel), LRO-NAC datasets from the Lunar Reconnaissance Orbiter (LRO) and 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' Huber. M. I. (1997) The techniques of Modern Structural Geology, v2, Folds and fractures, Academic processed them with J-Mars [7], ENVI. profile plot) on Rima Hyginus (3D generated by ACT). Press, 2nd edition. [10] Watters T. R. et al. (2012) Nature Geosciences, 5, 181-185.