Geoscience Frontiers 5 (2014) 227e235

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China University of Geosciences (Beijing) Geoscience Frontiers

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Research paper Geotectonic evolution of lunar LQ-4 region based on multisource data

Jianping Chen a,b,*, Yanbo Xu c, Xiang Wang d, Shujun He a,b, Danping Yan a, Shaofeng Liu a, Yongliao Zou c, Yongchun Zheng e a School of Earth Sciences and Resources, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, China b The Institute of High and New Techniques Applied to Land Resources, China University of Geosciences (Beijing), Beijing 100083, China c Shandong Gold Geology and Mineral Exploration Co., Ltd., Laizhou 261400, China d Development Research Center, China Geological Survey, Beijing 100037, China e CAS National Astronomical Observatories, Beijing 100012, China article info abstract

Article history: The Sinus Iridum region, the first choice for China’s “Lunar Exploration Project” is located at the center of Received 17 October 2012 the lunar LQ-4 area and is the site of Chang’e-3 (CE-3)’s soft landing. To make the scientific exploration of Received in revised form Chang’e-3 more targeted and scientific, and to obtain a better macro-level understanding of the 17 May 2013 geotectonic environment of the Sinus Iridum region, the tectonic elements in LQ-4 region have been Accepted 27 May 2013 studied and the typical structures were analyzed statistically using data from CE-1, Clementine, LRO and Available online 25 June 2013 Lunar Prospector missions. Also, the mineral components and periods of mare basalt activities in the study area have been ascertained. The present study divides the tectonic units and establishes the major Keywords: Lunar tectonic events and sequence of evolution in the study area based on morphology, mineral constituents, Tectonic elements and tectonic element distribution. Tectonic units Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Evolution Elsevier B.V. All rights reserved. LQ-4 region

1. Introduction gravity and morphology, its geological conditions and evolution are still poorly understood. The macro-tectonic theory of the Moon has While China is a late entrant in lunar exploration, the successful gone through several cycles. When the geosynclines-platform launch and data acquisition of CE-1 signaled a further step into theory was prevailing, the lunar tectonic outline units had been lunar science and offered proprietary first-hand data for ongoing delineated in accordance with it, whereas the lunar tectonic divi- lunar science research in the country. The Sinus Iridum region, the sion of units was made according to the terrane blocks when the first choice for China’s ‘Lunar Exploration Project’, is located at the plate tectonics theory was prevalent. In 1969, a tectonic map of the center of the lunar LQ-4 area and is the site of Chang’e-3 (CE-3)’s Moon was compiled by Kozlov and Sulidi-Kondratiev (1969). soft landing. LQ-4 area is thus selected as the study area to obtain Raitala (1978) studied the structural domain near Mare Humorum an overall understanding of the geologic and tectonic environment at the southwest of the Oceanus Procellarum, and inferred the re- of the Sinus Iridum region, which might make the scientific gion’s geological activity based on seismic data. Though there are exploration of CE-3 more targeted and scientific. great methodological differences in the research into Earth and Though the Moon has been studied for decades, with consid- planetary geology, knowledge about planetary surface structure erable progress made in the research on material composition, helps in understanding planetary evolution and their overall tec- tonic framework. Surface structural research of the Moon will definitely aid in the study of lunar geological conditions. * Corresponding author. Tel.: þ86 13910802638. E-mail address: [email protected] (J. Chen). 2. Regional geology Peer-review under responsibility of China University of Geosciences (Beijing) The lunar LQ-4 region is located in the lunar nearside mare area between 30 and 65N, 0 and 60W, and mainly consists of the northeast part of Oceanus Procellarum, the most of Mare Frigoris Production and hosting by Elsevier and northern highlands, and the most of Mare Imbrium (Fig. 1).

1674-9871/$ e see front matter Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2013.05.006 228 J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235

Figure 1. Selenographic location of LQ-4 region (the base map is of CE-1 CCD stereo camera image 2c data).

Rocks in this region typically include the mare basalt, highland 1991; Neal and Taylor, 1992). Fig. 4a shows the TiO2 content and anorthosite, KREEP rock and impact melt breccias, with mare basalt its distribution extracted from CE-1 IIM data. It indicates that the occupying about 70% of the area. This area was highly exposed to highland area has lower TiO2 content (<2%), whereas the TiO2 impacts in the early history of the Moon and to intensive basalt content in the mare area varies considerably and is notably higher activity in the later periods (Wilhelms and McCauley, 1971; Melosh, than that in the highland area; higher TiO2 areas are mostly located 1977; Wilhelms, 1987). As a region located inside the KREEP terrane to the west of Mare Imbrium and part of Oceanus Procellarum near of Oceanus Procellarum, it plays a significant role in the research on the Mare Imbrium, while lower TiO2 content is found to the the formation and evolution of KREEP terrane (Jolliff et al., 2000). northeast of Mare Imbrium, inside Sinus Iridum and in Mare Frigoris. 2.1. Image features interpretation 2.3.2. FeO content and distribution As indicated in the image synthesized from the data obtained Fe is the basic element for understanding and classifying from the CE-1 CCD stereo camera image 2c (Fig. 2), the region is igneous rocks and one of the chemical elements contained in most composed of two subregions with noticeable albedo difference: a silicate minerals on the Moon. Obtaining the content and distri- highland anorthosite region with high albedo and rough surface, bution of Fe will improve our understanding of the origin and and a mare basalt region with very low albedo and smooth surface. evolution of the Moon (Taylor, 1975; Lucey et al., 1995; Lawrence The highland anorthosite is mainly found in the central and et al., 1998, 2000). Fig. 4b shows the FeO content and distribution northern part of the map, i.e., the northern crater area of Mare extracted from CE-1 IIM data, it indicates that the FeO content in Imbrium and the northern highland area of Oceanus Procellarum; the mare area is significantly higher than that in the highlands, and while the mare basalt is mainly found in Mare Imbrium, Oceanus the regions with higher TiO2 (wt.%) within the mare also contain Procellarum, Mare Frigoris areas and in Sinus Iridum and Plato high values of FeO, suggesting a positive correlation between them. Crater. 2.3.3. Pyroxene content and distribution 2.2. Topography and geomorphology Pyroxene is the main mineral of mare basalts and helps distin- guish mare basalt from highland anorthosite as a supplementary The DEM shows the topography and geomorphology of the re- parameter. Fig. 4c shows the pyroxene content and distribution gion: higher-altitude areas are mostly found in the area north of extracted from CE-1 IIM data. It indicates that the higher pyroxene Mare Imbrium, typically the surrounding mountains of Sinus Iri- areas are mostly found in Mare Imbrium, Oceanus Procellarum and dum and Plato Crater, and the highland area north to Oceanus Mare Frigoris, whereas the pyroxene content in the highlands area Procellarum, with altitude up to 1000 m and the mountains sur- is very low, suggesting positive correlation between the pyroxene rounding three large impact craters (Babbage, J. Herschel and South content and distribution and TiO2/FeO. Craters), which are too old and thus are largely degraded to be identified directly in the image (Fig. 2). Lower-altitude areas are 2.3.4. Al2O3 and plagioclase content and distribution mostly found in Mare Imbrium, Oceanus Procellarum area and Plagioclase is the main mineral of highland anorthosite. Al2O3 is Mare Frigoris area, and the topography in Mare Imbrium gradually the main chemical component of plagioclase and it helps in dis- smoothens from south to north (Fig. 3). The topography agrees tinguishing highland anorthosite from mare basalt. Fig. 4d,e spatially with the features identified from the CCD remote-sensed shows the Al2O3 and plagioclase content and distribution, and in- images. dicates that the higher Al2O3 and plagioclase areas are mostly found in the highlands area, while the content is very low in Mare 2.3. Mineral and rock composition Imbrium, Oceanus Procellarum and Mare Frigoris, suggesting pos- itive correlation between Al2O3 and plagioclase content, and 2.3.1. TiO2 content and distribution negative correlation with TiO2/FeO and the pyroxene content and Except for the uniformly distributed volatiles in lunar maria, distribution. mare basalt varies significantly in chemical composition, and these differences help distinguish different types of basalt. Ti, for 2.3.5. Th content and distribution example, is an important parameter for classifying basalt types Th is not only one of the most essential elements for investi- (Charette et al., 1974; BVSP, 1981; Papike et al., 1991; Taylor et al., gating the Moon and the evolution of the lunar surface (Lawrence J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235 229

Figure 2. CE-1 CCD image of LQ-4 region (synthesized from CE-1 CCD image 2c data, 120 m resolution). et al., 1998, 2000), but also is important in further understanding 2.3.6. False color composite image the formation and evolution of KREEP rock through studies of its “Standard Galileo color composite” which typically involves global distribution, as Th is enriched in the rock. KREEP terrane of false color composite by wave band ratios was used by the scientific Oceanus Procellarum is famous for containing high Th (Jolliff et al., team in the Galileo lunar exploration to identify the petrologic 2000). Th is well correlated to REE and, more importantly, Sm and differences on the lunar surface (Belton et al., 1994). This technique Ga are also well correlated to Th (Ouyang, 1988; Taylor et al., 1991). has also been widely applied to Clementine UV-VIS multi-band data The Th content is an important measure for subdividing tectonic to identify the main rock types (Bugiolacchi and Guest, 2008; structures. In this region, Th content ranges from 2.2 to 13 ppm and Kramer et al., 2008; Hackwill, 2010). The three band ratios effec- the average value is 5 ppm. The highest Th (w13 ppm) area is tively reveal the material features of the lunar surface. Fig. 4g Aristillus, near the Aristarchus Impact Craters (Fig. 4f). clearly shows material units of different compositions: the blue

Figure 3. Geomorphic map of LQ-4 region (produced with LRO laser altimeter (LOLA), with spatial resolution of up to 60 m). 230 J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235

Figure 4. Mineral and rock components in LQ-4 region. a: TiO2 content and distribution map (based on CE-1 IIM data); b: FeO content and distribution map (based on CE-1 IIM data); c: Pyroxene distribution map (based on CE-1 IIM data); d: Al2O3 distribution map (based on CE-1 IIM data); e: Plagioclase distribution map (based on CE-1 IIM data); f: Th content distribution map (based on LP GRS data); g: False color composite image (based on Clementine UV-VIS band ratio data). J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235 231 areas are typically high-Ti areas; the orange and yellow areas are widest distribution in LQ-4. They are mostly found in mare areas low-Ti basalt areas, while the red areas are typically mature anor- including Mare Imbrium, Oceanus Procellarum and Mare Frigoris, in thosite highlands with presence of comparatively fresh bluish concentric arcs inside Mare Imbrium, and in (sub)radial forms in impact craters (Pieters et al., 1994). The petrologic areas agree with Oceanus Procellarum and Mare Frigoris (Yingst and Gregg, 2009). the Ti and Fe content indicated in Fig. 4a,b. Large mountains are considered to be another typical linear structure. Physiographically, the mountains are related to large, 3. Tectonic element interpretation and basalt activity direct aerolite impact, of different scales, or mostly result from structural or tectonic activities. They can be reformed by isostasy in Tectonic elements in the region include linear structures, ring the later periods. structures, terrane structures and basin structures. The whole re- The collapse structures occur in cascades on the inner wall of gion is within the KREEP terrane of Oceanus Procellarum and is not large impact craters or basin craters, e.g., Sinus Iridum, Plato Crater subdivided further. The region was subject to intense magmatic and Archimedes Crater and contain noticeable steep slide surfaces activity when a number of basalt infill events occurred (Hiesinger and slump masses. These are mostly the result of gravitational et al., 2000). slope processes. Other types of linear structures are only found in limited cases. 3.1. Interpretation of linear structures 3.2. Interpretation of ring structures The linear structures in LQ-4 region were interpreted according to the features and identification signs of the linear structures with The most typical ring structures on the lunar surface are the the data from CE-1, Clementine, LRO and Lunar Prospector missions impact craters of different sizes ranging from below meter scale to and 10 types of linear structures were figured out: mare ridges, large impact basins. The interpretation of the ring structures is lunar rima, grabens, valleys, ruptures, pit chains, mountains, scarps, presented in Fig. 5. Large diameter impact craters mostly occur in collapses and other linear structures as presented in Fig. 5. the highlands areas, while smaller impact craters occur in the mare Lunar ridges are one of the typical linear structures on the lunar basalt unit. From the top left of Fig. 5, a few larger old impact craters surface. From the interpretation results, it is obvious that many lunar can be interpreted and inferred by their existing shapes as Nec- ridges appear in the mare basalt areas. Statistically, lunar ridges are tarian or Prenectarian craters, which are typical of large old craters. of the most numerous among the linear structures and have their J. Herschel Crater (62N, 42W, dia. 165 km), Babbage Crater

Figure 5. Tectonic map of LQ-4 region. 232 J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235

(59.7N, 57.1W, dia. 143 km) and South Crater (58N, 50.8W, dia. Regarding basin structures, LQ-4 was obviously subjected to basin 104 km) are examples, and have extremely altered the topography tectonics. Part of the boundary of Mare Imbrium basin is roughly and geomorphology of the tectonic units and whose own geo- delineated according to the geomorphology and chemical composi- morphology in turn has been severely altered by weathering and tion of the region. Since the studyarea is located inside the large basins erosion in the later periods. For example, they were overlaid by of Oceanus Procellarum, its boundary is not included in the study. material excavated from Mare Imbrium events (Fra Mauro forma- tion), making it impossible to identify them directly from remote- 3.3. Basalt activities sensing images rather than from DEM data. Many residual impact craters that are overlaid by later basalt The basalt infill activities are an important geologic event in LQ-4. and whose peripheral craters outcrop above the basalt can be The basalt filled up the basins of Mare Imbrium, in Oceanus Procella- identified in LQ-4 region on the periphery of Mare Imbrium basin rum and Mare Frigoris. The basalt infill took place in multiple periods, crater and at the interface between Mare Frigoris and Oceanus and the magma components also show certain rules of variation Procellarum, suggesting that the basalt thickness is quite limited in (Hiesinger et al., 2000, 2003, 2010;Neukum, 2001; Xu et al.,2012). The this region. basalt outcrop in the region typically occurs in late Imbrian to Era- In Fig. 5, the distribution of the impact craters in the highlands area toshenian, and can be divided by TiO2 content in the basalt as: high-Ti are obviously denser than those in the mare area and there are more basalt (TiO2 7.5%), moderate-Ti basalt (4.5e7.5%) and low-Ti basalt larger-diameter craters in the highlands area. These craters become units (TiO2 < 4.5%) (Giguere et al., 2000). The moderate- and high-Ti more complex with size. Craters of more than 20 km in diameter mare basalts in the region mostly occur in the western half of Mare generally have more complete crater structure like the bottom, wall, Imbrium basin while the low-Ti mare basalt mostly occurs in the lip and rim, with sputtering matters (rays) if formed in later eras, and northeasternparts of the Mare Imbrium basin, in Sinus Iridum, in Plato some have a central peak (Michael and Neukum, 2001; Kneissl, et al., Impact Crater, in Mare Frigoris and the other areas of Oceanus Pro- 2010). Some craters including Sinus Iridum Impact Crater, Plato Crater cellarum (Fig. 5). Calculation of the surface age of the mare basalt re- and Archimedes Crater were filled with mare basalt. veals that younger the basalt richer the Fe and Ti content. Similar Of the ring structures of LQ-4 region, the mare domes and vol- volcanic groups or mare domes occur in Rümker area of Oceanus canic craters are typically located in the Rümker area (40.8N, Procellarum, with accumulated volcano features, lava caps and vol- 58.1W) on the mare platform (plateau), with domes occurring in canic clasts (Glass, 1986). groups and with a gentle topography. Small volcanic craters are seen in the center of some of the domes, suggesting that this region 4. Discussion was subjected to volcanic activities and is an important area for investigation of such activities. The active period is equivalent to 4.1. Interaction of tectonic elements the infill period of mare basalt. There might be a certain relation between such volcanic activities and the infill manner of the broad Mare ridges in lunar mares are relatively young linear structures mare basalt. that are mostly formed after the emplacement of mare basalt. Some

Figure 6. Linear structures vs. free air gravity anomaly in LQ-4 region (Source: USGS PDS Clementine data 1996) (Free air gravity data derived from Clementine data after 70-order spheric harmonic gravity model GLGM-2, with spatial resolution of 0.25 0.25)(Zuber et al., 1994). J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235 233

Table 1 Mare Imbrium, over the lunar ridges extending in concentric rings. Division of the tectonic units in the LQ-4 region. Their extension has typically resulted from the control of the I. Primary tectonic II. Secondary tectonic unit III. Tertiary tectonic unit basement shape of the basin which has been driven by basin sub- unit sidence. Similarly, the lunar ridges extending in concentric rings Mare tectonic unit Mare Imbrium basin Mare Imbrium center within the Mare Imbrium basin are also located in the positivee tectonic unit tectonic unit negative anomaly transition zone. This suggests changes in the Mare Imbrium edge basement shape of the basin, including transition of the basin edge tectonic unit Mare Imbrium basin toward the basin center and the increasing thickness of mare basalt, crater tectonic unit which is further indication of the thin crust at the center and the Oceanus Proellarum-Mare e over-compensation of materials at depth in the Mare Imbrium Frigoris mare-like basin basin. tectonic unit Terra tectonic unit Northern terra tectonic unit e 4.2. Interaction between tectonic units and different tectonic elements very small impact craters were incised by lunar ridges, suggesting fl the ridges’ were active in recent eras (Sharpton and Head, 1988; Tectonic units not only re ect the main tectonic framework in a Watters et al., 2010). region, but also reveal the relation between them and the different Theories for the explanation of the genesis of the lunar ridges tectonic elements in a more direct way. Table 1 and Fig. 5 show the are mostly concentrated on volcanism (Strom, 1964, 1972; Hodges, tectonic units in LQ-4 divided according to geomorphology, rock/ 1973; Golombek et al., 1991), tectonism (Bryan, 1973; Muehlberger, mineral composition and tectonic element distribution. 1974; Maxwell et al., 1975; Lucchitta, 1976, 1977; Schultz, 2000) and As indicated in Fig. 5, the impact craters of the ring structures mixed volcanic-tectonic mechanisms (Scott et al., 1975; Yue et al., mostly occur in tectonic units, with the impact craters in the 2007), though recently attention has focused on the relation be- highlands unit being far denser than those in the mare tectonic tween reverse faults and lunar surface anticlines in connection with unit; larger impact craters exist in the highlands tectonic unit with the tectonic genesis theory. Some researchers believe the lunar relatively complex tectonics; there are also some older residual ridges to be anticlinal folds of reverse faults below the lunar sur- impact craters in the northwestern part of the highland tectonic ‒ face, indicating the intensity of extrusion stress (Schultz, 2000; unit in the north of Oceanus Procellarum Mare Frigoris area. Thomas, 2010; Wei et al., 2012). As indicated in Fig. 5, all the ridge structures of the linear Statistical analysis of the orientation of lunar ridges in LQ-4 structures are located in the mare tectonic unit; and those region reveals that they are mostly oriented in the NS, NE or NW extending in concentric rings in Mare Imbrium basin have mostly direction with none in the EW direction, which agrees with the formed in the Mare Imbrium basin edge tectonic unit; mountains of preferential direction of the global grid predicted earlier. Lunar the linear structures mostly occur in the mare tectonic unit and ridges in this region typically have two parts. One part is distributed were subject to large impact events; collapse structures also occur around the edge of basins including Mare Imbrium and is primarily at the inner wall of large impact craters and mostly consist of subject to basin subsidence, with the distribution possibly subject extended collapses under gravity; streams are found both in the to the basement shape of the basin (Waskom, 1975); while lunar highlands tectonic unit and the mare tectonic unit, with a larger ridges extending in the near-NW direction in the center of Mare proportion of them lying in the transition zone between the two Imbrium are distributed nearly identically to those in Oceanus major tectonic units. fi e 0 Procellarum and Mare Frigoris. This indicates that the maximum Fig. 7 shows the geologic pro le of line A A in Fig. 5 based on tectonic stress should be in the EW direction, nearly perpendicular geology of LQ-4 region. The tectonic framework in this region was to the orientation of the lunar ridges and its source of power should typically controlled by large impact events, especially in the Mare relate to regional stress, like tidal forces and thermal contraction Imbrium basin, and basalt effusion events in the later periods, (Bryan, 1973; Fagin et al., 1978; Schultz, 2000; Wei et al., 2012). accompanied by minor impact events and tectonic deformation. Mare Imbrium basin is a typical mascon basin, and no consensus has been reached on how mascons have formed (Potts and von 5. Regional tectonic evolution sequence of LQ-4 Frese, 2003; Ouyang, 2005). It reflects the thickness of the deep structures and basalt in the basin. The most noticeable feature of The tectonic evolution of lunar LQ-4 region is summarized based Fig. 6 is the control exerted by the basin structures, especially in on the discussion above and divided into the following stages:

Figure 7. Tectonic style of AeA0 geologic profile. 234 J. Chen et al. / Geoscience Frontiers 5 (2014) 227e235

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