Gigantic conjugate ductile shear zones in the Moon

 Dr. Miguel de las Doblas Lavigne
 Científico Titular del CSIC, Instituto de Geociencias (CSIC-UCM), Facultad de Medicina (Edificio Entrepabellones 7 y 8),
c/ del Doctor Severo Ochoa 7, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, España, Email: doblas@mncn.csic.es

Abstract

In this paper we describe several huge ductile crustal-scale shear zones in the Moon (up to 5000 km long and 1000 km wide), that have never been studied and that appear to be the most gigantic ones in the solar system. These zones define conjugate sets of NE-dextral and NW-sinistral shear zones that are indicative of EW compressional stresses once active in this planet. This tectonic pattern confirms the hypothesis of Melosh (1977, 1980 a,b) who envisioned  this type of global conjugate strike-slip faulting in the Moon. The age of the deformations described here can be ascribed to a lower Imbrian stage as these structures are partially transected by a series of later impact structures of known lower Imbrian age (Mare Orientale and Schrödinger craters), and thus these shear zones would be the oldest ones in our galaxy. We suggest that these EW compressional stages might have arisen from the progressive slowing down of the Moon rotation speed, thus allowing the reshaping of the strongly ellipsoidal Moon towards a more spherical situation with a less accentuated equatorial bulge. These ideas are again fully in accordance with the scenario proposed by Melosh (1977, 1980 a,b) who suggested a generalized deflation of the Moon’s equatorial bulge as a result of its decreasing rotational speed. The frictional forces associated with this slowing down process might be responsible for the generation of these conjugate shear zones, possibly accompanied by a series of EW-trending massive swarms of magma-feeding dykes (concealed presently) that might have infilled the Maria lowlands with basaltic flows by upper Imbrian times in the region of the deflated equatorial bulge. Last, but not least, these gigantic ductile shear zones might hold some of the most important economic hydrothermal ores in the whole solar system.

Keywords: ductile shear zones, kinematic criteria, Moon, rotational stresses, deformation.

Introduction

            Five different ductile shear zones have been discovered in the Moon, mainly concentrated in its visible face, transecting the western half of this planet and bounding the central Maria lowlands. A series of authors have described similar sets of conjugate strike-slip systems in the Moon (and other planets), within a so-called “Lunar Grid System” scenario (Vening-Meinesz, 1947; Strom, 1964; Offield, 1971; Elston, 1972; McCauley, 1973; Karlstrom, 1974; Whitford-Stark and Fryer, 1975; Cordell and Strom, 1977; Melosh, 1977, 1979, 1980 a,b; Burt-Pechmann and Melosh, 1979; Chabot et al., 2000; Whitford-Stark, 1974, 1981). In this sense, it seems puzzling that several of these authors predicted the existence of planet-scale sets of conjugate strike-slip faults (NW sinistral and NE dextral), thus fully in accordance with what we have found in the Moon (Strom, 1964; Offield, 1971; Elston, 1972; McCauley, 1973; Karlstrom, 1974; Whitford-Stark and Fryer, 1975; Cordell and Strom, 1977; Melosh, 1977, 1979, 1980 a,b; Burt-Pechmann and Melosh, 1979; Chabot et al., 2000; Whitford-Stark, 1974, 1981). To our knowledge, no ductile shear deformation zones with SC-type mylonites (similar to the ones studied here) have ever been described in the Moon apart from a series of tectonic “zones” or brittle  regional lineaments between craters Lansberg and Palmieri, tangent to Mare Humorum (Raitala, 1978). “Structured lineaments” have also been described in Venus (Romeo et al., 2005), Mars (Márquez et al., 2004) and in the icy crust of Jupiter´s Moon, Europa (Aydin, 2006). Other authors suggest similar conjugate sets of planet-scale shear zones which might be related to the rotational characteristics of the celestial bodies via “global-scale double helix spiral tectonics” (O´Driscoll, 1980, 1981), “helicyclic tectonics” (Wezel, 1988), “global tectonic megacycles migrating eastwards” (Trurnit, 1991) or  “Earth expansion phenomena” (Pickford, 1996). It is interesting to note that most moonquakes occur in the visible side of the Moon, in sublinear regions bounding the Maria lowlands, thus possibly indicating exceptionally long-lived tectonic reactivation of the shear zones described in our paper.

            The initial "out-of-paper" figure is a collage highlighting the many sectors of the Moon displaying abundant conjugate fracture patterns and folds that have been recognized by the many authors mentioned above and that are represented in the geological map available through the Google Moon excellent application. This clearly demonstrates that the Moon is not a tectonically-dead planet and that the shear zones we describe in the following sections are perfectly conceivable.  

This work appears to follow some of the guidelines suggested by Hansen (2000), particularly in the sense that it contributes to the discovery of the types of operative processes that have shaped a planet surface, highlighting the fact that tectonic processes are vital in the geological evolution of these celestial bodies. Furthermore, and in view of the renewed interest of the NASA on the further exploration of our planet the present paper fullfills one of the key objectives outlined by this Agency (NASA, 2006): “Geology objective ID nº mGEO1” aimed at determining the internal structure and dynamics of the Moon to constrain the origin, composition, and structure of the Moon and other planetary bodies.

 Ductile shear zones in the Moon

Two major families of ductile shear zones can be observed on the Moon, corresponding to two contrasted orientations and senses of movement, thus constituting planet-wide conjugate shear zones: NE/SW dextral and NW/SE sinistral (Fig. 1).

Figure. 1. Global view of the Moon under visible light highlighting the different conjugate ductile shear zones discovered in this planet. The visible face of the Moon is shown with the dashed contour. Craters: Ab: Abdul; Ar: Ariadaeus; Av: Avogadro; B: Belkovitch; Be: Beer; C: Compton; Gr: Grimaldi; H: Hertzsprung; MO: Mare Orientale; S: Schickard; Sc: Schrödinger. Shear zones: 1 & 2: SMOH shear zone; 3: ARBE shear zone; 4: ABAV & BACO shear zones; 5 & 6: SCMOGR shear zone. Asterisks show the most favourable zones for future hydrothermal ores targets in the Moon (see text).

            All pictures shown in this paper have been obtained from the free version of the commercial program Google Moon that download global Moon images from the NASA, using the visible spectrum and the map of elevations.

            The first family of ductile shear zones corresponds to a mostly NW/SE trend and the different kinematic indicators clearly visible are consistent with a sinistral strike-slip scenario. Three different segments of this type of shear zones have been observed.

            The most outstanding case corresponds to a 1000 km wide and 2000 km long sinistral shear zone that follows a NW/SE trend along a series of later impact craters. In this sense, the name of the shear zone has been chosen according to the later impact craters that are oriented along the deformational band. This outstanding case has been named Schickard crater/Mare Orientale/Hertzsprüng crater (SMOH) shear zone (Figs. 2 and 3). In its southeastern sector (SE of the Mare Orientale), the shear zones displays the typical kinematic indicators of ductile shear zones (Fig. 2), i.e., SC mylonitic fabrics, σ and δ deformed “ phenocrysts” (previous craters in our case), fishes, drag effects, pressure shadows, etc. 

Figure 2.  Partial view of the SMOH (Schickard crater (Sc)/ Mare Orientale crater/ Hertzsprung crater) shear zone to the SE of the Mare Orientale (MO) using the elevations maps. The sinistral strike-slip character of the shear zone is obvious from the observation of SC-type deformations, σ- and δ-type deformed craters, elliptical craters (Schiller; Sch), etc. Note that in this sector this NW/SE shear zone reaches widths up to 1000 km.

It is interesting that the crater surficial crustal impacts are the ones marking the deformational behaviour of these shear zones, and they are truly acting as “phenocrysts” belonging to the inner fabric of mylonitic rocks. In this same area, the memoir of the corresponding geological map already recognized “the abundance of irregularly-shaped craters” (Karlstrom, 1974), without further interpretations. In fact, and as can be seen in figure 2, this shear zone looks strikingly similar to typical SC-deformed mylonites including the kinematic criteria cited previously as seen on hand-samples or under the microscope (e.g., Passchier and Trouw, 1996). This sector of the SMOH shear zone displays the widest deformational band in the solar system (nearly 1000 km), and in this senses it constitutes the most gigantic shear zone ever described in this galaxy (typical ductile shear zones on Earth rarely reach widths greater than 100 km). This southeastern sector of the SMOH might easily be misleaded with the classical radial patterns associated to impacts craters as it comes directly from the Mare Orientale impact crater. In this sense, the existing geological maps of this SE sector of the shear zone (Offield, 1971; Karlstrom, 1974) wrongly assume that the NW predominant lineations and valleys are related to the Mare Orientale ejecta blanket in terms of an Orientale-basin radial sculpture produced by vertical fault movements. In any case, the uniqueness of this lineated terrain nearby the Schickart crater has already been recognized in the geological map of the area (Karlstrom, 1974) in terms of a “complex, sinuous, intertwining ridge pattern with enclosed elliptical to rectilinear shallow deposits controlled by curvilinear to straight fractures” (this description closely ressembles what might be encountered in an SC-type mylonitic terrain such as the one described in our paper).

Figure 3. Partial view of the NW continuation of the SMOH shear zone reaching the Hertzsprung crater (Hc) to the NW of the Mare Orientale (MO) using the elevations maps. The sinistral strike-slip component of the SMOH shear zone is also highlighted by the abundant SC-type elements, the δ- and σ-type deformed craters, etc. Note the abundant pit crater chains that seem indicative of dilational slip faulting in this sector of the SMOH shear zone. The presence of  isolated SC-type deformations and pit chains within the MO and Hc craters indicates that the shearing events are syn/post-kinematic with respect to these pre-Imbrian impact structures.

 In its northwestern sector, the SMOH shear zone displays abundant pit-chains (sensu Wyrick et al., 2004; Ferrill et al., 2004) oriented parallel to this deformation band (NWSE; Fig. 3), probably indicating that this area constitutes an extensional/dilational termination of the deformational band. Similar NW-oriented sinistral ductile shear zones apparently belonging to this same family can be observed in the central part of the visible face of the Moon to the S of the Maria sector (Figure 4; Ariadaeus crater/ Beer crater; ARBE shear zone) and to the east of the central Maria outcrops (Figure 5; the Belkovith crater/Compton crater; BECO shear zone). In general, we might say that this type of NW-oriented shear zones clearly influences the geometry of the Maria lowlands which are bounded by NW/SE accidents.

Figure 4. Partial view of the  ARBE (Aradinaeus crater/ Beer crater (Be)) shear zone using the elevations maps. The sinistral strike-slip character of this NW-trending shear zone might be deduced from different kinematic indicators: SC-type elements, the δ- and σ-type deformed craters, etc .

Figure 5. Partial view of the ABAV (Abdul crater/ Avogadro crater (Av)) NE-trending dextral shear zone and the BECO (Belkovich crater/Compton crater) NW-trending  sinistral shear zone using the elevations maps.. Kinematic indicators: SC-type elements, the δ- and σ-type deformed craters, etc.

        The second family of lunar shear zones is constituted by two major deformation bands, both oriented NESW and with a dextral sense of movement. The most outstanding one is constituted by the Schrödinger crater/ Mare Orientale/ Grimaldi crater (SCMOGR shear zone), spanning nearly 5000 km (the longest shear zone in the solar system) from the southwestern part of the visible Moon face towards the NE in its contact with the Maria lowlands. 

Figure 6.  Partial view of the SW sector of the SCMOGR (Schrödinger crater (Sc)/ Mare Orientale crater (MO)/ Grimaldi crater) shear zone using the elevations maps. Note that this shear zone trends mainly EW to the S and it then adopts a more NE/SW orientation towards the MO crater. The dextral strike-slip character of this shear zones might be deduced by the abundant SC-type criteria, σ- and δ-type deformed craters, drag-effects, etc.

The SCMOGR shear zones in its SW sector (Fig. 6) shows a nearly EW orientation, progressively changing to the NE towards a NE/SW generalized trend. The Grimaldi geological map (McCauley, 1973) clearly shows a predominant NE lineation even if this outstanding structural element is not further interpreted by this author (Fig. 7). Another segment of this same shear zone (?) can be observed to the E of the Maria lowlands constituting the Abdul crater/Avogadro crater (ABAV) shear zone (see Fig. 5).

Figure 7. Partial view of the NE continuation of the SCMOGR shear zone reaching the Grimaldi crater (Gr) to the NE of the Mare Orientale (MO) using the elevations maps. The dextral strike-slip character of this shear zone might be deduced from a series of kinematic indicators; SC-type elements, the δ- and σ-type deformed craters, etc.

            Discussion

            We might make some guesses regarding the stresses and displacements involved in the generation of these shear zones. In the case of the SMOH shear zone (Fig. 2), we might observe a nearly perfect elliptical crater (Schiller) located within this shear zone, and if one assumes an initially circular geometry, then we might safely deduce a strain ellipse of ellipticity Rs up to 3.0 (Ramsay and Huber, 1983) acting along this sinistral deformational band, with possible horizontal displacements (which might be deduced from the inclination of the strain ellipse) reaching up to 50 kms (Ramsay and Huber, 1983). The elliptical Schiller crater has been differently interpreted in the literature as a result of either an oblique impact, two simultaneous impacts or an elongated feature associated to a NW fault (Offield, 1971).

            The spectacular width of some of these shear zones is indicative of the temperature of formation of these deformational structures. If one assumes that no erosion takes place in the Moon, then the width of these shear zones is the original one on the surface of this planet. From the experience gained from ductile shear zones on Earth, such widths as 1000 km corresponds to ductile regimes existent in crustal depths well below 10-15 km, corresponding to ductile regimes with temperatures of formation up to 200-800 ºC. We should mention that even if shear zones are thought to be “the most ubiquitous features observed in planetary surfaces” (Regenauer-Lieb and Yuen, 2003), the usual ideas suggest that mylonitic shear zones such as the ones described here in the Moon are only visible to the observer when uplifted and exposed to the surface by erosion (this is not the case in the Moon as we already argued) and they are thought to  govern the mechanical behaviour of the strongest part of the lithosphere  below 10-15 km. Furthermore, mylonitic shear zones dissect plates, thus allowing plate tectonics to develop on the Earth. In this sense, even if the Moon is presently devoided of this type of global plate tectonics scenario (O’Neill et al., 2007), one might guess that plate tectonics might have been active during the early stages of our planet evolution. As occurs on Earth, we suggest that a special role in this type of deformational bands should be attributed to the presence of water in nominally anhydrous minerals: the recent discovery that the early Moon was rich in water (Saal et al., 2008) seems to be a good confirmation of our hypothesis.

            We might speculate on the possible origin of these conjugate families of ductile shear zones. The age of formation is constrained by the fact that these deformational bands are transected and partially synchroneous with later impact craters of known lower Imbrian age (3800 to 3850 my): in the case of the SMOH shear zone we might see that this deformational band partially transects the Mare Orientale crater in its southeastern tip (of known lower Imbrian age). In the case of the SCMOGR, this shear zone is also supposed to be lower Imbrian in age as this deformational band is transected in its southwestern sector by the Schrödinger crater (of lower Imbrian age).  If these ages are true, we might be looking at the oldest known ductile shear zones in the whole solar system. The presence of NESW dextral and NWSE sinistral conjugate shear zones transecting the whole visible face of the Moon on a crustal-scale seem indicative of active EW compressional stresses. We might further speculate that this planet-wide EW compressional regime might have triggered a series of massive EW-oriented magma-feeding dyke swarms that might explain the massive basaltic outflows filling the Maria lowlands (a question presently unresolved) by upper Imbrian times (3200-3800 my) with what me considered one of  the largest LIP in the solar system. We suggest that it is conceivable that such stresses might have arisen when the Moon progressively decreased its rotational speed by pre Imbrian times, thus allowing the reshaping of the Moon elongated ellipsoid (along the equatorial plane) towards a more sphere-similar geoid. This change in the rotational behaviour of our planet has been described by many authors and it adequately explains the existence of planet-wide conjugate systems of ductile shear zones. In this sense, it seems puzzling that several authors already predicted this type of “deceleration stresses” of despun planets triggering EW compression and a series of conjugate sets of strike-slip fractures in different planets (Melosh, 1977, 1980 a,b; Burt-Pechmann and Melosh, 1979; Chabot et al., 2000; Matsuyama and Nimmo, 2008). This type of planet-wide stresses are highly probable along equatorial latitudes according to the hypothesis of “membrane stresses” (Turcotte and Oxburgh, 1974; Oxburgh and Turcotte, 1974) as a result of rotational-related stresses that are concentrated along the geoid bulge observed in these sectors of the Earth. It is puzzling that an exactly similar scenario of a progressively deflating equatorial bulge triggering membrane stresses as a result of a despinning planet has been suggested by several authors in order to account for the tectonic evolution of the Earth, the Moon, Mars and Mercury (Melosh, 1977, 1980 a,b; Burt-Pechmann and Melosh, 1979; Lambeck and Pullan, 1980; Chabot et al., 2000). Furthermore, Spohn et al. (2001) argued that the early geological evolution of the Moon might have been characterized by widespread mantle convection and upwelling and plume tectonics. These equatorial despun-related deflating stresses might explain the hypothetical EW-trending massive magma-feeding dyke swarms that we suggest in the present paper, parallel to the lunar equator that might have filled the Maria lowlands with the younger basaltic material characteristic of the visible face of the Moon. This tectonic scenario seems also to be the case for the most famous rift system in the solar system, also EW-oriented along the martian bulged equator (Vallis Marineris), even if in that case no basaltic lavas were extruded as the area of major magmatic activity concentrated further W in the Tharsis region (e.g., Olympus Mons).

            One of the most interesting possibilities of future lunar exploration programs might be related to the scientific evaluation of potential economic ore deposits in this planet. In fact, one of the lunar exploration objectives of the renewed NASA’s (2006) lunar program is aimed at “characterizing potential resources to understand their potential for lunar resource utilization” (Objective mGEO14). In this sense, we suggest that the Moon might constitute a highly interesting zone for potential gold mineralizations in that it fullfills two basic requisites of the most famous gold mineralizations on Earth: they are genetically associated to ductile shear zones that are invariably hosted by the oldest Archaean rocks (e.g.,; Eisenlohr et al., 1989; Robert and Poulsen, 1997; Weinberg et al., 2004; Micklethwaite, 2007). The gigantic lunar ductile shear zones described in this paper (apparently unique in the solar system) seem perfect in this respect as they are pre-Imbrian in age and they are affecting volcanic rocks which are often the site of interesting ore deposits and hydrothermal mineralizations. In this sense, the nearest/easier lunar possible economic ores might be even more interesting that the already plannified martian ones (Schulke-Makuch et al., 2007) in view of the uniqueness of these gigantic ductile shear zones and the recent discovery of water in the Moon’s interior (Saal et al., 2008). In particular, the intersections between the two families of shear zones or the shear zones and the lower Imbrian impacts-craters/volcanic-edifices or the trantensional sectors of these shear zones, might be highly probable economic-ores targets (see asterisks in Fig. 1). These ideas might be realistic to some degree as shown by the fact that some “theoretical” attempts have been already made at evaluating the potential economic ore deposits of the Moon (McKay and Williams, 1979; Feldman and Franklin, 2004; Gillett and Kuck, 2004). 

        Last but not least, we are well aware that the present speculative manuscript will not be accepted by most “hard-core” specialists on the Moon, as our hypothesis challenges the paradigm of a tectonically-dead satellite. Geometrically-similar features than the ones we are describing here as ductile shear zones (dual, braided, asymmetric, sigmoidal, congruent, linear, etc.), have been assigned by others to completely different scenarios: 1) Sudden flows of radial ejecta sheets wrapping around the walls of previous craters, displaying flow, slump and avalanche surface characteristics (Orientale basin; Guest and Greeley, 1977); 2) Depositional/deceleration dunes formed in places where laterally flowing violent base-surge currents broke up into turbulent eddies due to the influence of surface topography (Orientale basin; Mutch, 1972); 3) Dual sets of linear and transverse aeolian dunes diverging around topographic obstacles (Earth and Titan; Radebaugh et al., 2009).

Acknowledgements: We thank Google for the pictures downloaded from their free version of Google Moon. We thank José Arroyo, Julia de las Doblas and José María Cebriá for the figures. 

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