Scholarly article on topic 'Geotectonic evolution of lunar LQ-4 region based on multisource data'

Geotectonic evolution of lunar LQ-4 region based on multisource data Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Jianping Chen, Yanbo Xu, Xiang Wang, Shujun He, Danping Yan, et al.

Abstract The Sinus Iridum region, the first choice for China's “Lunar Exploration Project” is located at the center of the lunar LQ-4 area and is the site of Chang'e-3 (CE-3)'s soft landing. To make the scientific exploration of Chang'e-3 more targeted and scientific, and to obtain a better macro-level understanding of the geotectonic environment of the Sinus Iridum region, the tectonic elements in LQ-4 region have been studied and the typical structures were analyzed statistically using data from CE-1, Clementine, LRO and 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 tectonic events and sequence of evolution in the study area based on morphology, mineral constituents, and tectonic element distribution.

Academic research paper on topic "Geotectonic evolution of lunar LQ-4 region based on multisource data"

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Research paper

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

Jianping Chena,b'*, Yanbo Xuc, Xiang Wangd, Shujun Hea,b, Danping Yana, Shaofeng Liua, 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

The Sinus Iridum region, the first choice for China's "Lunar Exploration Project" is located at the center of the lunar LQ-4 area and is the site of Chang'e-3 (CE-3)'s soft landing. To make the scientific exploration of Chang'e-3 more targeted and scientific, and to obtain a better macro-level understanding of the geotectonic environment of the Sinus Iridum region, the tectonic elements in LQ-4 region have been studied and the typical structures were analyzed statistically using data from CE-1, Clementine, LRO and 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 tectonic events and sequence of evolution in the study area based on morphology, mineral constituents, and tectonic element distribution.

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Article history: Received 17 October 2012 Received in revised form 17 May 2013 Accepted 27 May 2013 Available online 25 June 2013

Keywords: Lunar

Tectonic elements Tectonic units Evolution LQ-4 region

1. Introduction

While China is a late entrant in lunar exploration, the successful launch and data acquisition of CE-1 signaled a further step into lunar science and offered proprietary first-hand data for ongoing lunar science research in the country. The Sinus Iridum region, the first choice for China's 'Lunar Exploration Project', is located at the center of the lunar LQ-4 area and is the site of Chang'e-3 (CE-3)'s soft landing. LQ-4 area is thus selected as the study area to obtain an overall understanding of the geologic and tectonic environment of the Sinus Iridum region, which might make the scientific exploration of CE-3 more targeted and scientific.

Though the Moon has been studied for decades, with considerable progress made in the research on material composition,

* Corresponding author. Tel.: +86 13910802638. E-mail address: 3s@cugb.edu.cn (J. Chen).

Peer-review under responsibility of China University of Geosciences (Beijing)

gravity and morphology, its geological conditions and evolution are still poorly understood. The macro-tectonic theory of the Moon has gone through several cycles. When the geosynclines-platform theory was prevailing, the lunar tectonic outline units had been delineated in accordance with it, whereas the lunar tectonic division of units was made according to the terrane blocks when the plate tectonics theory was prevalent. In 1969, a tectonic map of the Moon was compiled by Kozlov and Sulidi-Kondratiev (1969). Raitala (1978) studied the structural domain near Mare Humorum at the southwest of the Oceanus Procellarum, and inferred the region's geological activity based on seismic data. Though there are great methodological differences in the research into Earth and planetary geology, knowledge about planetary surface structure helps in understanding planetary evolution and their overall tectonic framework. Surface structural research of the Moon will definitely aid in the study of lunar geological conditions.

2. Regional geology

The lunar LQ-4 region is located in the lunar nearside mare area between 30° and 65° N, 0° and 60° W, and mainly consists of the northeast part of Oceanus Procellarum, the most of Mare Frigoris and northern highlands, and the most of Mare Imbrium (Fig. 1).

1674-9871/$ — see front matter © 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

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 anorthosite, KREEP rock and impact melt breccias, with mare basalt occupying about 70% of the area. This area was highly exposed to impacts in the early history of the Moon and to intensive basalt activity in the later periods (Wilhelms and McCauley, 1971; Melosh, 1977; Wilhelms, 1987). As a region located inside the KREEP terrane of Oceanus Procellarum, it plays a significant role in the research on the formation and evolution of KREEP terrane (Jolliff et al., 2000).

2.1. Image features interpretation

As indicated in the image synthesized from the data obtained from the CE-1 CCD stereo camera image 2c (Fig. 2), the region is composed of two subregions with noticeable albedo difference: a highland anorthosite region with high albedo and rough surface, and a mare basalt region with very low albedo and smooth surface. The highland anorthosite is mainly found in the central and northern part of the map, i.e., the northern crater area of Mare Imbrium and the northern highland area of Oceanus Procellarum; while the mare basalt is mainly found in Mare Imbrium, Oceanus Procellarum, Mare Frigoris areas and in Sinus Iridum and Plato Crater.

2.2. Topography and geomorphology

The DEM shows the topography and geomorphology of the region: higher-altitude areas are mostly found in the area north of Mare Imbrium, typically the surrounding mountains of Sinus Iri-dum and Plato Crater, and the highland area north to Oceanus Procellarum, with altitude up to 1000 m and the mountains surrounding three large impact craters (Babbage, J. Herschel and South Craters), which are too old and thus are largely degraded to be identified directly in the image (Fig. 2). Lower-altitude areas are mostly found in Mare Imbrium, Oceanus Procellarum area and Mare Frigoris area, and the topography in Mare Imbrium gradually smoothens from south to north (Fig. 3). The topography agrees spatially with the features identified from the CCD remote-sensed images.

2.3. Mineral and rock composition

2.3.1. TiO2 content and distribution

Except for the uniformly distributed volatiles in lunar maria, mare basalt varies significantly in chemical composition, and these differences help distinguish different types of basalt. Ti, for example, is an important parameter for classifying basalt types (Charette et al., 1974; BVSP, 1981; Papike et al., 1991; Taylor et al.,

1991; Neal and Taylor, 1992). Fig. 4a shows the TiO2 content and its distribution extracted from CE-1 IIM data. It indicates that the highland area has lower TiO2 content (<2%), whereas the TiO2 content in the mare area varies considerably and is notably higher than that in the highland area; higher TiO2 areas are mostly located to the west of Mare Imbrium and part of Oceanus Procellarum near the Mare Imbrium, while lower TiO2 content is found to the northeast of Mare Imbrium, inside Sinus Iridum and in Mare Frigoris.

2.3.2. FeO content and distribution

Fe is the basic element for understanding and classifying igneous rocks and one of the chemical elements contained in most silicate minerals on the Moon. Obtaining the content and distribution of Fe will improve our understanding of the origin and evolution of the Moon (Taylor, 1975; Lucey et al., 1995; Lawrence et al., 1998, 2000). Fig. 4b shows the FeO content and distribution extracted from CE-1 IIM data, it indicates that the FeO content in the mare area is significantly higher than that in the highlands, and the regions with higher TiO2 (wt.%) within the mare also contain high values of FeO, suggesting a positive correlation between them.

2.3.3. Pyroxene content and distribution

Pyroxene is the main mineral of mare basalts and helps distinguish mare basalt from highland anorthosite as a supplementary parameter. Fig. 4c shows the pyroxene content and distribution extracted from CE-1 IIM data. It indicates that the higher pyroxene areas are mostly found in Mare Imbrium, Oceanus Procellarum and Mare Frigoris, whereas the pyroxene content in the highlands area is very low, suggesting positive correlation between the pyroxene content and distribution and TiO2/FeO.

2.3.4. Al2O3 and plagioclase content and distribution

Plagioclase is the main mineral of highland anorthosite. Al2O3 is

the main chemical component of plagioclase and it helps in distinguishing highland anorthosite from mare basalt. Fig. 4d,e shows the Al2O3 and plagioclase content and distribution, and indicates that the higher Al2O3 and plagioclase areas are mostly found in the highlands area, while the content is very low in Mare Imbrium, Oceanus Procellarum and Mare Frigoris, suggesting positive correlation between Al2O3 and plagioclase content, and negative correlation with TiO2/FeO and the pyroxene content and distribution.

2.3.5. Th content and distribution

Th is not only one of the most essential elements for investigating the Moon and the evolution of the lunar surface (Lawrence

et al., 1998, 2000), but also is important in further understanding the formation and evolution of KREEP rock through studies of its global distribution, as Th is enriched in the rock. KREEP terrane of Oceanus Procellarum is famous for containing high Th (Jolliff et al., 2000). Th is well correlated to REE and, more importantly, Sm and Ga are also well correlated to Th (Ouyang, 1988; Taylor et al., 1991 ). The Th content is an important measure for subdividing tectonic structures. In this region, Th content ranges from 2.2 to 13 ppm and the average value is 5 ppm. The highest Th (~13 ppm) area is Aristillus, near the Aristarchus Impact Craters (Fig. 4f).

2.3.6. False color composite image

"Standard Galileo color composite" which typically involves false color composite by wave band ratios was used by the scientific team in the Galileo lunar exploration to identify the petrologic differences on the lunar surface (Belton et al., 1994). This technique has also been widely applied to Clementine UV-VIS multi-band data to identify the main rock types (Bugiolacchi and Guest, 2008; Kramer et al., 2008; Hackwill, 2010). The three band ratios effectively reveal the material features of the lunar surface. Fig. 4g 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).

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).

areas are typically high-Ti areas; the orange and yellow areas are low-Ti basalt areas, while the red areas are typically mature anor-thosite highlands with presence of comparatively fresh bluish impact craters (Pieters et al., 1994). The petrologic areas agree with the Ti and Fe content indicated in Fig. 4a,b.

3. Tectonic element interpretation and basalt activity

Tectonic elements in the region include linear structures, ring structures, terrane structures and basin structures. The whole region is within the KREEP terrane of Oceanus Procellarum and is not subdivided further. The region was subject to intense magmatic activity when a number of basalt infill events occurred (Hiesinger et al., 2000).

3.1. Interpretation of linear structures

The linear structures in LQ-4 region were interpreted according to the features and identification signs of the linear structures with the data from CE-1, Clementine, LRO and Lunar Prospector missions and 10 types of linear structures were figured out: mare ridges, lunar rima, grabens, valleys, ruptures, pit chains, mountains, scarps, collapses and other linear structures as presented in Fig. 5.

Lunar ridges are one of the typical linear structures on the lunar surface. From the interpretation results, it is obvious that many lunar ridges appear in the mare basalt areas. Statistically, lunar ridges are of the most numerous among the linear structures and have their

widest distribution in LQ-4. They are mostly found in mare areas including Mare Imbrium, Oceanus Procellarum and Mare Frigoris, in concentric arcs inside Mare Imbrium, and in (sub)radial forms in Oceanus Procellarum and Mare Frigoris (Yingst and Gregg, 2009).

Large mountains are considered to be another typical linear structure. Physiographically, the mountains are related to large, direct aerolite impact, of different scales, or mostly result from structural or tectonic activities. They can be reformed by isostasy in the later periods.

The collapse structures occur in cascades on the inner wall of large impact craters or basin craters, e.g., Sinus Iridum, Plato Crater and Archimedes Crater and contain noticeable steep slide surfaces and slump masses. These are mostly the result of gravitational slope processes. Other types of linear structures are only found in limited cases.

3.2. Interpretation of ring structures

The most typical ring structures on the lunar surface are the impact craters of different sizes ranging from below meter scale to large impact basins. The interpretation of the ring structures is presented in Fig. 5. Large diameter impact craters mostly occur in the highlands areas, while smaller impact craters occur in the mare basalt unit. From the top left of Fig. 5, a few larger old impact craters can be interpreted and inferred by their existing shapes as Nec-tarian or Prenectarian craters, which are typical of large old craters. J. Herschel Crater (62° N, 42°W, dia. 165 km), Babbage Crater

Figure 5. Tectonic map of LQ-4 region.

(59.7°N, 57.1°W, dia. 143 km) and South Crater (58°N, 50.8°W, dia. 104 km) are examples, and have extremely altered the topography and geomorphology of the tectonic units and whose own geo-morphology in turn has been severely altered by weathering and erosion in the later periods. For example, they were overlaid by material excavated from Mare Imbrium events (Fra Mauro formation), making it impossible to identify them directly from remote-sensing images rather than from DEM data.

Many residual impact craters that are overlaid by later basalt and whose peripheral craters outcrop above the basalt can be identified in LQ-4 region on the periphery of Mare Imbrium basin crater and at the interface between Mare Frigoris and Oceanus Procellarum, suggesting that the basalt thickness is quite limited in this region.

In Fig. 5, the distribution of the impact craters in the highlands area are obviously denser than those in the mare area and there are more larger-diameter craters in the highlands area. These craters become more complex with size. Craters of more than 20 km in diameter generally have more complete crater structure like the bottom, wall, lip and rim, with sputtering matters (rays) if formed in later eras, and some have a central peak (Michael and Neukum, 2001; Kneissl, et al., 2010). Some craters including Sinus Iridum Impact Crater, Plato Crater and Archimedes Crater were filled with mare basalt.

Of the ring structures of LQ-4 region, the mare domes and volcanic craters are typically located in the Rumker area (40.8° N, 58.1°W) on the mare platform (plateau), with domes occurring in groups and with a gentle topography. Small volcanic craters are seen in the center of some of the domes, suggesting that this region was subjected to volcanic activities and is an important area for investigation of such activities. The active period is equivalent to the infill period of mare basalt. There might be a certain relation between such volcanic activities and the infill manner of the broad mare basalt.

Regarding basin structures, LQ-4 was obviously subjected to basin tectonics. Part of the boundary of Mare Imbrium basin is roughly delineated according to the geomorphology and chemical composition of the region. Since the study area is located inside the large basins of Oceanus Procellarum, its boundary is not included in the study.

3.3. Basalt activities

The basalt infill activities are an important geologic event in LQ-4. The basalt filled up the basins of Mare Imbrium, in Oceanus Procella-rum and Mare Frigoris. The basalt infill took place in multiple periods, and the magma components also show certain rules of variation (Hiesinger et al., 2000,2003,2010; Neukum, 2001; Xu et al., 2012). The basalt outcrop in the region typically occurs in late Imbrian to Era-toshenian, and can be divided by TiO2 content in the basalt as: high-Ti basalt (TiO2 > 7.5%), moderate-Ti basalt (4.5—7.5%) and low-Ti basalt units (TiO2 < 4.5%) (Giguere et al., 2000). The moderate- and high-Ti mare basalts in the region mostly occur in the western half of Mare Imbrium basin while the low-Ti mare basalt mostly occurs in the northeastern parts of the Mare Imbrium basin, in Sinus Iridum, in Plato Impact Crater, in Mare Frigoris and the other areas of Oceanus Pro-cellarum (Fig. 5). Calculation of the surface age of the mare basalt reveals that younger the basalt richer the Fe and Ti content. Similar volcanic groups or mare domes occur in Rumker area of Oceanus Procellarum, with accumulated volcano features, lava caps and volcanic clasts (Glass, 1986).

4. Discussion

4.1. Interaction of tectonic elements

Mare ridges in lunar mares are relatively young linear structures 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° x 0.25°) (Zuber et al., 1994).

Table 1

Division of the tectonic units in the LQ-4 region.

I. Primary tectonic II. Secondary tectonic unit III. Tertiary tectonic unit

Mare tectonic unit Mare Imbrium basin Mare Imbrium center

tectonic unit tectonic unit

Mare Imbrium edge

tectonic unit

Mare Imbrium basin

crater tectonic unit

Oceanus Proellarum-Mare —

Frigoris mare-like basin

tectonic unit

Terra tectonic unit Northern terra tectonic unit —

very small impact craters were incised by lunar ridges, suggesting the ridges' were active in recent eras (Sharpton and Head, 1988; Watters et al., 2010).

Theories for the explanation of the genesis of the lunar ridges are mostly concentrated on volcanism (Strom, 1964,1972; Hodges, 1973; Golombek et al., 1991), tectonism (Bryan, 1973; Muehlberger, 1974; Maxwell et al., 1975; Lucchitta, 1976,1977; Schultz, 2000) and mixed volcanic-tectonic mechanisms (Scott et al., 1975; Yue et al., 2007), though recently attention has focused on the relation between reverse faults and lunar surface anticlines in connection with the tectonic genesis theory. Some researchers believe the lunar ridges to be anticlinal folds of reverse faults below the lunar surface, indicating the intensity of extrusion stress (Schultz, 2000; Thomas, 2010; Wei et al., 2012).

Statistical analysis of the orientation of lunar ridges in LQ-4 region reveals that they are mostly oriented in the NS, NE or NW direction with none in the EW direction, which agrees with the preferential direction of the global grid predicted earlier. Lunar ridges in this region typically have two parts. One part is distributed around the edge of basins including Mare Imbrium and is primarily subject to basin subsidence, with the distribution possibly subject to the basement shape of the basin (Waskom, 1975); while lunar ridges extending in the near-NW direction in the center of Mare Imbrium are distributed nearly identically to those in Oceanus Procellarum and Mare Frigoris. This indicates that the maximum tectonic stress should be in the EW direction, nearly perpendicular to the orientation of the lunar ridges and its source of power should relate to regional stress, like tidal forces and thermal contraction (Bryan, 1973; Fagin et al., 1978; Schultz, 2000; Wei et al., 2012).

Mare Imbrium basin is a typical mascon basin, and no consensus has been reached on how mascons have formed (Potts and von Frese, 2003; Ouyang, 2005). It reflects the thickness of the deep structures and basalt in the basin. The most noticeable feature of Fig. 6 is the control exerted by the basin structures, especially in

Mare Imbrium, over the lunar ridges extending in concentric rings. Their extension has typically resulted from the control of the basement shape of the basin which has been driven by basin subsidence. Similarly, the lunar ridges extending in concentric rings within the Mare Imbrium basin are also located in the positive— negative anomaly transition zone. This suggests changes in the basement shape of the basin, including transition of the basin edge toward the basin center and the increasing thickness of mare basalt, which is further indication of the thin crust at the center and the over-compensation of materials at depth in the Mare Imbrium basin.

4.2. Interaction between tectonic units and different tectonic elements

Tectonic units not only reflect the main tectonic framework in a region, but also reveal the relation between them and the different tectonic elements in a more direct way. Table 1 and Fig. 5 show the tectonic units in LQ-4 divided according to geomorphology, rock/ mineral composition and tectonic element distribution.

As indicated in Fig. 5, the impact craters of the ring structures mostly occur in tectonic units, with the impact craters in the highlands unit being far denser than those in the mare tectonic unit; larger impact craters exist in the highlands tectonic unit with relatively complex tectonics; there are also some older residual impact craters in the northwestern part of the highland tectonic unit in the north of Oceanus Procellarum-Mare Frigoris area.

As indicated in Fig. 5, all the ridge structures of the linear structures are located in the mare tectonic unit; and those extending in concentric rings in Mare Imbrium basin have mostly formed in the Mare Imbrium basin edge tectonic unit; mountains of the linear structures mostly occur in the mare tectonic unit and were subject to large impact events; collapse structures also occur at the inner wall of large impact craters and mostly consist of extended collapses under gravity; streams are found both in the highlands tectonic unit and the mare tectonic unit, with a larger proportion of them lying in the transition zone between the two major tectonic units.

Fig. 7 shows the geologic profile of line A—A' in Fig. 5 based on geology of LQ-4 region. The tectonic framework in this region was typically controlled by large impact events, especially in the Mare Imbrium basin, and basalt effusion events in the later periods, accompanied by minor impact events and tectonic deformation.

5. Regional tectonic evolution sequence of LQ-4

The tectonic evolution of lunar LQ-4 region is summarized based on the discussion above and divided into the following stages:

Figure 7. Tectonic style of A—A' geologic profile.

(1) Early magma ocean stage on the Moon.

In its early years, the Moon was a partially melted or totally melted magma ocean before it started to crystallize and differentiate in the form of cumulus crystals, as widely accepted, and developed the proto-crust, mantle and residual magmas (KREEP components) (Ouyang Ziyuan, 2005).

(2) Large impact stage.

Under the impact of large aerolites in the Prenectarian, the large Oceanus Procellarum impact basin formed. Subsequently at around 3.85 Ga, Mare Imbrium basin was formed under large impact events. KREEP rock features appeared in the highlands area around craters, suggesting that the impact events of Oceanus Procellarum and Mare Imbrium had dug out the residual KREEP magma at depth. After that, the Sinus Iridum Impact and Plato Impact Craters, and Archimedes Impact Crater in the southeast of Mare Imbrium took shape.

(3) Mare basalt infill stage.

After the large impact stage, this region was primarily subjected to the infill activities of Mare Imbrium basalt. The mare basalt infill periods in the region mainly include late Imbrian and Eratoshenian. The late Imbrian mare basalt is mostly located to the east of Mare Imbrium, at the interface between the western edge of Mare Imbrium and Oceanus Procellarum, in Mare Frigoris and the northern part of Oceanus Procellarum. In Sinus Iridum area, the Plato Impact Crater was also overlaid by late Imbrian mare basalt, whereas the late Eratoshenian basalt mostly occurs in the west of Mare Imbrium and in Oceanus Procellarum.

The composition of the basalt shows regular changes with time. The TiO2 and FeO contents in the Eratoshenian mare basalt are noticeably higher than that in the late Imbrian mare basalt, demonstrating different origins of the magma (Hiesinger et al., 2000, 2003, 2010; Xu et al., 2012).

(4) Tectonic deformation stage.

Ring structures are typically the result of impact actions, which have never stopped in any of the eras of the Moon. The linear structures, on the other side, remain relatively complete since Imbrian, though the older ones might have been obliterated or damaged by impact events and basalt infills. The ridge structures, in particular, mostly occurred in or after late Imbrian and some even in Copernican, suggesting that this region was exposed to a relatively strong extrusion stress, probably coinciding with the thermal state evolution across the Moon and isostasy in local areas. The spatial distribution of the ridge structures, which reflects the geometry of the basement to a certain degree, is the combined product of basement and regional stresses (Bryan, 1973; Waskom, 1975; Schultz, 2000; Wei et al., 2012).

Acknowledgment

We express our gratitude to the key project (No. 2009AA122201) under the 863 program sponsored by Ministry of Science & Technology that has funded our research, to CAS National Astronomical Observatories that has offered us the Chang'e-1 data, and to NASA that has offered us the Clementine, Lunar Prospector and LOLA data.

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