The Manicouagan impact structure as a terrestrial analogue site for lunar and martian planetary science

ARTICLE IN PRESS Planetary and Space Science 58 (2010) 538–551

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The Manicouagan impact structure as a terrestrial analogue site for lunar and martian planetary science John G. Spray , Lucy M. Thompson, Marc B. Biren, Catherine O’Connell-Cooper Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada

a r t i c l e in f o

a b s t r a c t

Article history: Received 26 February 2009 Received in revised form 28 August 2009 Accepted 10 September 2009 Available online 20 September 2009

 The 90 km diameter, late Triassic Manicouagan impact structure of Quebec, Canada, is a well-preserved, undeformed complex crater possessing an anorthositic central uplift and a 55 km diameter melt sheet. As such, it provides a valuable terrestrial analogue for impact structures developed on other planetary bodies, especially the Moon and Mars, which are currently the focus of exploration initiatives. The scientific value of Manicouagan has recently been enhanced due to the production, between 1994 and 2006, of  18 km of drill core from 38 holes by the mineral exploration industry. Three of these holes are in excess of 1.5 km deep, with the deepest reaching 1.8 km. Here we combine recent field work, sampling and the drill core data with previous knowledge to provide insight into processes occurring at Manicouagan and, by inference, within extraterrestrial impact structures. Four areas of comparative planetology are discussed: impact melt sheets, central uplifts, impact-generated hydrothermal regimes and footwall breccias. Human training and instrument testing opportunities are also considered. The drill core reveals that the impact melt and clast-bearing impact melts in the centre of the structure reach thicknesses of 1.4 km. The 1.1 km thick impact melt has undergone differentiation to yield a lower monzodiorite, a transitional quartz monzodiorite and an upper quartz monzonite sequence. This calls into question the previous citing of Manicouagan as an exemplar of a relatively large crater possessing an undifferentiated melt sheet, which was used as a rationale for assigning different composition lunar impact melts and clast-bearing impact melts to separate cratering events. The predominantly anorthositic central uplift at Manicouagan is comparable to certain lunar highlands material, with morphometric analogies to the King, Tycho, Pythagoras, Jackson, and Copernicus impact structures, which have similar diameters and uplift structure. Excellent exposure of the Manicouagan uplift facilitates mapping and an appraisal of its formation and collapse mechanisms, enhanced by drill core data from the centre of the structure. Recent field studies at the edge of the central island at Manicouagan, and multiple drill core sections through footwall lithologies, provide insight into allochthonous (clastic and suevitic) and autochthonous breccia formation, as well as shock effects. The hydrothermal regimes developed at Manicouagan are akin to systems proposed for Noachian ( 43.5 Ga) Mars that involve alteration of impact melts via meteoritic and surface waters, with the generation of phyllosilicates, zeolites, hematite, sulfates and sulfides that can contribute to martian soil formation and sedimentation processes. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Manicouagan Hydrothermal systems Central uplifts Mars Moon Terrestrial analogues

1. Introduction Current international initiatives for planetary exploration are captured by the statement ‘‘The Moon, Mars and beyond’’ (NASA et al., 2007). This strategy proposes a return to the Moon, including increased orbiter, surface-based robotic and, eventually, human missions, with the establishment of a permanent lunar base within the next decade or so. These missions would also

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serve as a springboard for continued Mars exploration and a potential future human visit to the Red Planet (e.g., Rapp, 2008). Specific interest in the characterization of asteroids and comets continues (e.g., NASA’s Dawn and ESA’s Rosetta missions). Following numerous workshops, the importance of terrestrial analogue sites in furthering planetary exploration initiatives has been recognized by the global community (e.g., the Canadian Analogue Research Network; Hipkin et al., 2007). Analogue sites facilitate the development, testing and design optimization of equipment. This includes scientific experiments, proving rover and in situ resource utilization technologies, and training astronauts, mission scientists and program managers. The Apollo

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astronauts visited various sites in North America and Europe in order to familiarize themselves with lithologies similar to those that they anticipated encountering on the Moon. This involved field excursions to volcanic terrains, but also included visits to the Barringer (USA), Ries (Germany), and Sudbury (Canada) impact structures, where they could examine lunar-like shocked, brecciated and melted rocks first hand (Schaber, 2005). As such, these terrestrial structures provided the only analogues for the impactgenerated rocks that dominate the lunar surface. Analogue studies also allow us to further our understanding of both past and present planetary processes, including geological, biological and atmosphere–hydrosphere-surface interactions. The hypervelocity collision of meteorites and comets has played an important role throughout the history of our solar system, particularly during the first 600 Ma, as revealed by the heavily cratered surfaces of many solid planetary bodies. Impact was dominant during the inception and assembly of the solar system, through to the purported terminal cataclysm at 3.9 Ga. Prior to this time, repeated global sterilization due to megaimpacts by projectiles 100 s km in diameter prevented the survival of life (e.g., Taylor, 1998), although the notion of continuous bombardment during the Hadean has been challenged (Ryder, 2002). With the decrease in projectile flux since that time, life forms were more easily able to develop, at least on Earth, with their evolution being periodically disrupted by environmentally catastrophic impacts. Today, asteroid and comet impact continues to be a threat to existing life forms on Earth, albeit at a reduced level (Lewis, 1999). Impact may also be responsible for transporting life between planets and solar systems (e.g., Kamminga, 1982). Life, or its building blocks, may have come to Earth via asteroidal or cometary collision, or via the entry of cosmic dust (e.g., Hoyle and Wickramasinghe, 1993). Impact-induced hydrothermal systems may have generated conditions conducive to the formation of life, whether on Earth or on Mars. Such ‘‘oases’’ could have also provided safe niches for existing life forms that allowed them to survive otherwise hostile planetary conditions (e.g., Osinski et al., 2001). Understanding impact cratering as a planetary process therefore aids us in understanding how our solar system and planets have evolved, including life itself. Given that the surfaces of the Moon and Mars preserve much of their early cratering record, the majority of the rocks encountered on these relatively less active planetary bodies, as well as asteroids, can be expected to be generated and modified, or at least displaced, by impact processes. Consequently, familiarization with impact-generated lithologies on Earth is a valuable prelude to future extraterrestrial planetary exploration. Here we discuss the Manicouagan impact structure of Canada as a lunar and martian analogue site. The preservation of an almost complete suite of impact lithologies facilitates the construction of a near intact slice through a large, complex crater. The central uplift is dominated by anorthositic lithologies, akin to components of the lunar highlands. Only fallback breccia and very top of the impact-melt sheet have been removed by erosion. Manicouagan is the only impact crater of the larger terrestrial structures ( Z90 km) that is well exposed (in contrast to Chicxulub, which is buried), that has not been subject to significant erosion (unlike Vredefort, which is deeply incised), that exhibits no tectonic-metamorphic overprinting (in contrast to Sudbury), and which has been extensively drilled to a considerable depth (three holes are in excess of 1.5 km deep). Acquisition in 2006 by the University of New Bunswick’s Planetary and Space Science Centre (PASSC) of  10 km of drill core from the private sector, as well as access to logs for a further 8 km of core, provides for an unprecedented view of the third dimension of an impact crater. Manicouagan’s twin, Popigai, located in northern Siberia, is less accessible and less well exposed

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(Whitehead et al., 2002). There is a significant post-impact cover. However, Popigai has been drilled via more than 700 boreholes, totaling over 130 km of core, as a result of exploration for impact diamonds. Much of this core has been effectively lost, and most holes were significantly less than 500 m deep (the deepest is 780 m), so did not penetrate the entire structure. Because of Manicouagan’s state of preservation, good exposure and its surviving morphometry, comparisons can be made with lunar and martian craters of similar morphology. Here we examine aspects of comparative planetology between this well-preserved terrestrial crater and craters developed on other planetary bodies. We also consider how we can use this site for testing exploration technologies and training future mission personnel.

2. Crater setting and geology The  90 km diameter Manicouagan impact structure (Fig. 1) is currently the fourth largest terrestrial impact crater known (Earth Impact Database, 2009), a position jointly held with Popigai in Siberia (Whitehead et al., 2002). The earliest recorded geological studies of the Manicouagan area were provided by Low (1897), followed by Hammond (1945), Rose (1955), Be rard (1962) and Kish (1962, 1963, 1968), all of whom acted for federal or provincial geological surveys. It was not until Carlyle Beals and his group, based at the Dominion Observatory in Ottawa, initiated a systematic search for impact craters within the Canadian Shield that Manicouagan was proposed as being the result of asteroid or comet impact (Beals et al., 1963). Subsequently, Michael Dence and co-workers identified shock-generated features at Manicouagan and the impact origin was confirmed (Dence, 1964; Bunch et al., 1967). Significant mapping was done by Currie (1972) and Murtaugh (1976) prior to completion of the Manic 5 dam by Hydro-Que bec in 1968. The filling of the reservoir created a  65 km diameter annular lake and near circular  55 km diameter central island (ˆIle Rene -Levasseur), that are clearly visible from space (Dence, 1977), and which account for the moniker ‘‘the eye of Que bec’’ (Fig. 2). Ironically, while the flooding made Manicouagan something of a remote sensing icon as a terrestrial impact structure, it also made access to the island and the centre of the crater more difficult. The  5 km wide moat of water surrounding the island renders it reachable only by boat, float plane or helicopter. Because of its relative isolation, postflooding research in the area has been limited. Exploration after 1970 has been mainly carried out by the private sector, notably Mine raux Manic (1994–98), Great Legends Mining (1998–2000) and Manicouagan Minerals (2003–06). All these companies targeted magnetic anomalies on the main island and alluded to potential similarities with the world-class nickel–copper– platinum group mineralization at Sudbury. To date, these exploration initiatives have yielded limited economic indicators. However, they have produced a significant amount of drill core. Fig. 3 shows the location of drilling sites on the island. The island comprises forested, rugged terrain within Canada’s subarctic Boreal forest. Away from the lake shore, access can be difficult, but excellent exposures occur in the higher elevation areas that comprise Mont de Babel and Maskelynite Peak, which together define part of the anorthosite-dominated central uplift (Fig. 4a). Coastal outcrops provide good exposures of the impact melt sheet (Fig. 4b), complemented by drill core (Fig. 4c), and underlying footwall, which also sporadically outcrop inland. Manicouagan was generated at 215 Ma (Hodych and Dunning, 1992; Ramezani et al., 2005) predominantly in Precambrian crystalline metamorphic and igneous rocks of the  1000 Ma Grenville Province. It is located within the Mesoproterozoic Manicouagan Imbricate Zone and underlying Gagnon Terrane,

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Fig. 1. Regional geological province location (a) and simplified geological setting (b) of the Manicouagan impact structure. The Gagnon and Hart Jaune Terranes and Canyon and Gabriel Domains are tectonometamorphic units of the  1 Ga Grenville Province. MIZ =Manicouagan Imbricate Zone, which comprises the Lelukuau (L) and Tschenukutish (T) Terranes. GF =Grenville Front. White dotted circle indicates probable impact melt source envelope within the Tschenukutish Terrane. Modified after Hynes et al. (2000).

Fig. 2. Colour-coded relief map of the Manicouagan impact structure and surrounding terrain computed from NASA Shuttle Radar Topographic Mission (SRTM) data. Height given bottom left in metres.

although reworked Archean components survive (Fig. 1). A thin ( o200 m) discontinuous cover of Middle Ordovician carbonates and shales was present at the time of impact (Nowlan and Barnes, 1987). The impact lithologies present include a well-developed

impact-melt sheet (Fig. 4b, c), clast-bearing impact melts (Fig. 4d), allochthonous breccias, which may be clastic (Fig. 4e) or suevitic (Fig. 4f), and autochthonous breccias (Fig. 4g) overlying fractured, faulted and brecciated basement. Shock effects beneath the

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Fig. 3. Drilling site locations on the main island: ˆIle Rene -Levasseur. Stars indicate PASSC acquisitions; circles represent other drill hole locations. Lines show positions of drill holes featured in Figs. 5 and 6.

impact-melt sheet and within the central uplift include planar deformation features in quartz and feldspar, shatter cones (Fig. 4h and the development of maskelynite (e.g., Dworak, 1969; Dressler, 1990). Pseudotachylyte is present as anastomosing vein and dyke systems pervading the basement lithologies (Fig. 4i), including the central uplift. Figs. 5 and 6 show diagrammatic representations of selected drill hole logs from the island, which reveal the relationships of these impact lithologies.

3. Comparative planetology 3.1. Impact melt sheets The pockmarked and heavily cratered surfaces of many planets and moons, such as Mercury, Mars and the Moon, attest to the role that impact cratering has played throughout the history of these bodies and our solar system in general: it has been a dominant geological process. One of the more conspicuous products of the hypervelocity collision of larger projectiles with planetary surfaces is the generation of impact-melt sheets. A large proportion of the kinetic energy of the projectile is transformed to thermal energy that is focussed into a relatively small volume (Melosh, 1989). This highly shocked material is melted and vaporized. It then moves outward at velocities of several km/s (O’Keefe and Ahrens, 1975), overriding and incorporating slower moving, colder and less shocked debris; the whole mass becoming turbulently mixed. As a relatively large terrestrial impact structure, Manicouagan exhibits a well-developed impact melt sheet. For an estimated average original thickness of 400 m and a diameter of 55 km, the total volume of melt would have been  950 km3, assuming a simple cylinder form. This would have been initially superheated (well above liquidus) to yield a melt pool at 41700 1C (Ivanov and Deutsch, 1999; Zeig and Marsh, 2005; Spray, 2006). The melt equilibrates between the liquidus and solidus (Simonds et al., 1976a). Extensive digestion of clastic material takes place, the degree of which is controlled by the liquidus and solidus temperatures of the melt. This process cools the melt. Melts with lower liquidus and solidus temperatures will take longer to crystallize and therefore assimilate entrained clastic debris more completely. This explains how the intermediate-composition melt at Manicouagan, averaging 58 weight % SiO2 (Spray and Thompson, 2008), is relatively clast poor compared to many lunar impact melts, which are more basic and therefore have higher freezing points (Simonds et al., 1976a). To date, the only way to directly sample in situ impact-melt sheets in the context of their original

crater setting is by examining terrestrial examples. The presence of a well-preserved, drilled and extensive impact-melt sheet at Manicouagan provides a valuable terrestrial analogue. Our current lunar inventory comprises Apollo (382 kg) and Luna (0.3 kg) samples, as well as lunar meteorites ( 33 kg; Korotev, 2009). The community has attempted to put these samples into context with respect to their geological evolution and, more specifically, their impact cratering history. In addition, the resumption of lunar surface missions in the near future will produce a new inventory of lunar samples that will require interpretation. Where impact processing is the dominant geological feature of a planetary surface, there is typically evidence of continuous modification, such that lithologies become reworked by subsequent events. Continuous micrometeorite bombardment of the atmospherebereft Moon also contributes to the gardening of near-surface material and the production of agglutinates (e.g., McKay et al., 1991). Many of the lunar samples are breccias that comprise fragments of one or more impact-melt sheets, which have been generated via multi-impact reworking (e.g., highland polymict breccias; Taylor et al., 1991). In this respect, Earth provides limited analogue scenarios, with few surviving areas witnessing more than one impact. However, Wanapitei (Eocene) superimposed on the much larger Sudbury Structure (Paleoproterozoic) is an example (Earth Impact Database, 2009). Conversely, Earth provides the only impact-generated samples whose context can be directly linked to their location within the crater framework. This is invaluable for attempting to interpret impact-generated or impact-modified planetary materials whose source location is typically unknown on other planetary bodies (e.g., much of the lunar surface comprises displaced regolith). When Apollo samples were first returned to Earth, comparisons were made with several terrestrial impact melt sheets to aid in the interpretation of the samples. Popigai and Manicouagan have been the most commonly cited examples (e.g., Grieve et al., 1974; Simonds et al., 1976a, b). Specifically, Manicouagan was considered representative of a 60–100 km size complex crater with an undifferentiated, chemically homogeneous, although somewhat texturally heterogeneous, impact-melt sheet (Murtaugh, 1976; Floran et al., 1976; Phinney and Simonds, 1977; Floran et al., 1978; Ryder and Stockstill, 1995). Based on the belief that craters in the size range of Popigai and Manicouagan produced chemically homogeneous melt sheets, Simonds et al. (1976b) identified four distinct compositions of lunar melt in Apollo 16 breccia samples, attributing each to four different impact-melt sheets formed during discrete cratering events. However, the recent drilling activities at Manicouagan, combined with surface sampling followed by

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Fig. 4. (a) View looking west from Mont de Babel, central uplift at Manicouagan; (b) columnar-jointed impact-melt sheet along the shores of ˆIle Rene -Levasseur; (c) split drill core samples showing the medium grain size of the impact melt from drill hole MAN-06-08; (d) impact melt breccia (IMB) field outcrop; (e) allochthonous clastic breccia; (f) basal suevite with melt-rimmed lithic clast; (g) clast of autochthonous breccia in basal suevite; (h) large (metre-size) shatter cones in anorthosite from the central uplift (note hammer for scale on right-hand group of shatter cones); and (i) pseudotachylyte in gneissose drill core.

geochemical analysis, reveal that its impact-melt sheet is not of uniform composition as suggested by past field work. This calls into question some of the previously held assumptions regarding the identification and interpretation of lunar impact melts (Spray and Thompson, 2008). Drilling has revealed an unexpectedly varied topography to the melt sheet-basement contact in the centre of the structure at Manicouagan. An elongate, impact-melt filled, N–S trough extending at least 8 km from the southern flanks of the uplifted Mont de Babel anorthosite has been identified. The trough varies in depth from 600 m at the northern and southern extremes, to 1430 m in the middle, resulting in substantially thicker melt sections than

previously identified, estimated to be  200–300 m, based on cliff sections, exposures and elevation (Currie, 1972; Murtaugh, 1976; Floran et al., 1976, 1978; Phinney and Simonds, 1977). Geochemical analysis of 88 core and field impact melt samples (Spray and Thompson, 2008) reveal that the more typical, r300 m thick sections (e.g., Man-03-02, 03-03, 05-02, 05-13, 06-03) and the newly discovered 600 m thick sections intersected within the central trough in Man-05-01 and 05-11 (Figs. 3 and 5), exhibit a homogeneous, quartz monzodiorite composition comparable with the average impact melt compositions of Currie (1972) and Floran et al. (1978). In contrast, the 1100 m clast-free melt sequence encountered in the centre of the graben in hole

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Fig. 5. Drill core sections from the three deepest holes: MAN-05-11 (  1.8 km), MAN-06-08  1.5 km) and MAN-05-01 (  1.6 km). All three holes are located within 7 km of the centre of the structure (Fig. 3) and were drilled vertically.

Man-06-08 is segregated into two compositionally distinct layers, separated by a transition zone: a  450 m thick lower monzodiorite; a 180 m thick transition zone of quartz monzodiorite (the same as the average composition of the impact-melt sheet intersected in the other drill holes), and a  450 m thick upper quartz monzonite (Fig. 5). Notably, the weighted average composition of the impact-melt sheet in Man-06-08 is the same as the average for the other drill holes, indicating that all were derived from a common parent melt, but that the thicker section underwent differentiation. The identification of a thicker, differentiated impact-melt sheet section at a crater the size of Manicouagan (90 km) has implications for the interpretation of lunar samples. It is apparent that samples previously assigned to separate impact events on the Moon may be differentiates of a common impact-melt sheet. Critically, this may occur at smaller diameters than previously considered. Future work should aim to test for consanguinity

between lunar impact melts and clast-bearing impact melts. This has implications for cratering rates and the critical volumes of melt required to initiate differentiation. Manicouagan has also been used by various workers in an attempt to integrate theoretical models, cratering experiments and field observations for the formation of impact-melt sheets (Phinney and Simonds, 1977; Simonds et al. 1976a, b; Onorato et al. 1978; Grieve and Floran, 1978; Simonds and Kieffer, 1993). Given the recently identified complexities revealed by drilling the impact-melt sheet at Manicouagan, these models need to be revisited. 3.2. Crater morphometry and tectonics The majority of crater classification schemes rely primarily on topographic observations from the Moon and, to a lesser extent, other planetary surfaces (e.g., Pike, 1985). Currently, terrestrial impact craters provide the only means with which to ground truth

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Fig. 6. Vertical sections based on four drill holes representing a partial cross-section through the island. MAN-06-08 was drilled vertically. The others have been corrected from their original inclinations to yield vertical profiles. See Fig. 5 for lithology key.

such observations, as well as providing an additional (third) dimension due to erosion, drilling and/or geophysics. Manicouagan is an example of a complex crater that is exposed, drilled and moderately eroded. As such, it is an excellent analogue candidate to examine the link between morphometry and tectonic processes. The polygonal, nearly circular form of the Manicouagan structure, as defined by the shores of the reservoir (Murtaugh, 1976), may be analogous to polygonal impact craters observed on ¨ hman et al., 2008) and other planetary bodies, such as Mars (O Venus (e.g., Aittola et al., 2007). These workers have suggested that the orientation of polygon segments reflect the control of preexisting structure within the target on the tectonics of the crater formation process. At Manicouagan, straight segments are observed defining the eastern- and western-most outer edges of the reservoir (Figs. 1 and 2), which may be related to shear zones and faults manifest within the Grenville target (Murtaugh, 1976). The well-exposed central uplift at Manicouagan provides insight into the mechanics of central uplift formation, both here on Earth, and on other planets where we are typically limited to topographic data obtained by orbiters. At Manicouagan, the central uplift is dominated by anorthositic rocks. The central uplift forms a rugged, horseshoe-shaped terrain some 23 km in diameter, reaching 950 m above sea level, which is 4300 m above the current highest exposure of impact melt. Given the final rim diameter (D) of Manicouagan (90 km), the amount of structural uplift (SU) can be estimated at 9 km, using the formula SU=0.086D1.03 of Therriault

et al. (1997). For a geothermal gradient of 30 1C/km, this yields a temperature of 270 1C for the uppermost central uplift rocks (neglecting shock heat). This exhumed heat source can contribute to generating a hydrothermal regime (see next section). Recent studies of Mont de Babel and Maskelynite Peak reveal that the anorthositic central uplift exhibits discrete deformation features related to impact (Biren et al., 2008). These are: (1) anastomosing white veinlets (up to a few mm wide) showing no, or limited, offset, comprising feldspar glass with fluidal textures, and maskelynite, which is preferentially developed at vein margins. These are akin to shock veins developed in certain meteorites; (2) pseudotachylyte systems, typically a few cm wide, that individually exhibit cm to 10 s of cm offset and which commonly occur in subparallel, interleaved groups. Whilst individual pseudotachylyte veins are typically only a few cm wide, melt pockets can reach up to a metre across. Zones of increased pseudotachylyte development can be several metres to 10 s of metres wide and strike for considerable distances (100 s of metres); (3) ball breccia (Murtaugh, 1976), which is manifest as polygonal clasts surrounded by shock-generated glassy material, with polygons being 1–10 cm in diameter. Ball breccia zones form sheets that are several cm to metres in thickness, which cut across the otherwise undisturbed pre-impact metamorphic fabric of the anorthosites. The glassy matrices of the ball breccias are particularly susceptible to zeolitization; and (4) shatter cones, some of which reach considerable size (2 m length); Fig. 4h.

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Fig. 7. Four hydrothermal regimes for an idealized complex impact structure, with emphasis on Manicouagan. Vertical scale exaggerated (  5). See text and Table 1.

Larger displacement systems within the central uplift are defined by combinations of pseudotachylyte and ball breccia, which may together attain several metres in width, and strike for several km. Many of these zones correlate with steep cliff faces, and we interpret them to be fault systems that were generated during the emplacement and/or collapse of the uplift. This points to a localized mechanism of deformation within the central uplift at Manicouagan, rather than bulk motion (i.e., there is no evidence of gross fluidization, although the scale of observation must be borne in mind). This is substantiated by the preservation of Grenvillian metamorphic fabrics in the anorthosites, which can commonly be traced undisturbed for kilometres on the well-exposed uplift. Our field-based results to date indicate that the central uplift at Manicouagan comprises a limited number of very large coherent blocks (several km in dimensions), which are most probably fault bound. Some of these faults are low-angle normal and reverse, while others are high-angle. Drilling at Manicouagan has revealed an unexpectedly complicated structure to the central region of this complex crater. The anorthosite massifs of Mont de Babel and Maskelynite Peak were uplifted and breached the impactmelt sheet, following the strict definition of a central uplift (e.g., French, 1998). However, drilling reveals basement displacements of 100 s of metres concealed beneath the melt sheet. This has important implications for correctly determining melt sheet volumes, recognizing compositionally layered and differentiated systems, and understanding the mechanics of sub-melt sheet target response. In terms of assessing models for central uplift formation (e.g., Bray et al., 2008), our evidence tends to support floor elevation rather than the inward collapse of the rim as the controlling process. Further details of the tectonics of central uplift formation at Manicouagan are discussed by Biren et al. (2008) and Spray and Thompson (2008). Manicouagan appears to be a central peak basin with rings. However, this does not follow current complex crater classification schemes (e.g., Melosh, 1989) because, although it resembles a peak ring crater, the peak ring-crater diameter ratio is noncompliant (Spray and Thompson, 2008). Extensive drilling and good exposure within the central region at Manicouagan provide a unique opportunity to link the morphological planetary data with ground-based structural data and further refine our classification of complex craters and the formation mechanisms of central uplifts.

3.3. Impact-generated hydrothermal systems: Earth and Mars The notion that impact events can be destructive and catastrophic to living organisms is well appreciated, but the aftermath of an impact can generate new niches amenable to the development and survival of life. For planets that have, or had,

hydrospheres, the formation of an impact-generated topographic low (basin) can facilitate conditions suitable for water accumulation and the creation of a lake (e.g., New Que bec crater; Earth Impact Database, 2009). Such basins provide additional ‘‘wet’’ habitats. Moreover, the impact process can generate heat from three principal sources: (1) shock decompression, with waste heat being released to the shocked medium on decompression; (2) uplift and exhumation of previously buried target rocks, thus elevating hotter material and juxtaposing it with cooler surface materials, and (3) impact-generated melt sheets and clast-bearing impact melts, including associated hot ejecta blankets. The degree to which heat is added to the system depends on the energy of the collision. Larger impacts produce more intense shock, more melt and greater core uplift. Simple impact structures on Earth (diameter o3 km) yield little impact melt, negligible structural uplift and suffer weaker shock, so their thermal budgets are limited and transient (perhaps a localized elevation of a few 10 s of degrees for a few years). Conversely, complex craters have considerably more potential to generate relatively long-lived geothermal systems that can last 104–105 years, with temperatures being elevated several hundred degrees in footwall and cooling impact-melt rocks. This is largely due to the creation of superheated impact-melt bodies (e.g., Carstens,1975; Zeig and Marsh, 2005). In the presence of water, this instantaneously generated heat engine will create convective systems and a warm wet oasis that may be conducive to supporting life (e.g., Corliss, 1990; Osinski et al., 2001; Cockell, 2006). As such, paleohydrothermal systems developed in extraterrestrial impact craters remain important exploration targets for the search for life on other planets, especially Mars (e.g., Rathbun and Squyres, 2002). We identify four main potential hydrothermal regimes within complex impact structures, as exemplified by our preliminary work at Manicouagan: (1) intra- and supra-melt sheet cooling cells that affect overlying fallback breccias and the melt sheet; (2) sub-melt sheet cells that operate within the footwall; (3) intra-central uplift convection systems and (4) peripheral hot springs located at the collapsed rim (Fig. 7). Regimes (1) and (2) are likely to facilitate supercritical H2O activity (4375 1C), at least initially, due to proximity to the superheated melt sheet, while regimes (3) and (4) are likely to be low temperature (o270 1C) because they are driven by residual heat from uplifted basement (regime 3), or are peripheral to the melt sheet (regime 4). Each regime may develop distinct mineral parageneses in response to different host lithologies and thermal conditions. Naumov (2005) suggests that most impact-generated hydrothermal systems are dominated by alkaline to near-neutral aqueous fluids possessing low salinity and low PCO2 that are supersaturated in silica. There is limited evidence of silica precipitation at Manicouagan; the predominance of basic to intermediate target rocks may be unsuited to SiO2 leaching. However, quartz (including amethyst) has been observed at the

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Table 1 Hydrothermal regimes and associated mineral parageneses for a complex crater, with emphasis on Manicouagan. Regime a

1. Intra- and supra-melt sheet 2. Sub-melt sheet footwall (allochthonous and autochthonous breccias, basal suevites, fractured and faulted basement) 3. Central uplift (mainly anorthositic rocks) 4. Peripheral hot springs (slumped cavity margins) a

Temperature H 4375 1C; L o 270 1C

Mineral paragenesis

H-LT (initially supercritical H2O) H-LT (initially supercritical H2O)

hematite, chlorite, zeolites, clays, limited quartz precipitation smectites (montmorillonite, saponite), hematite, chlorite, scapolite, limited quartz precipitation

LT LT

analcime, thomsonite, hematite, prehnite, pectolite, anhydrite sulfides, sulfates, hematite, chlorite

Fallback and uppermost melt sheet have been removed by erosion at Manicouagan.

periphery of the structure in the lower parts of the impact-melt sheet and underlying suevite in the form of crystal aggregates filling vugs, cavities and veins There are some similarities with the hydrothermally-driven alteration assemblages developed in the Charlevoix impact structure, which possesses a comparable Grenvillian target with Ordovician carbonate cover (Trepmann et al., 2005), although Ca-metasomatism appears not to have been significant at Manicouagan. Table 1 summarizes the characteristics of hydrothermal alteration, with emphasis on Manicouagan. These results should be considered preliminary in anticipation of more detailed study. Regimes 1 and 2 are likely to be higher temperature pressurized systems associated with supercritical H2O activity. In particular, submelt sheet conditions may facilitate the creation of isolated convective cells trapped beneath the overlying relatively impermeable melt sheet. Open system exchange and depressurization is likely to be limited to fault systems bounding the central uplift and the collapsed margins (Fig. 7). Initial contact metamorphic conditions in the immediate sub-melt sheet footwall probably caused the dehydration and hornfelsing of the target lithologies, with hydrothermal systems becoming active during the later stages of cooling. Cooling of the upper parts of the impact melt sheet has, in places, generated spectacular columnar jointing (Fig. 4b), and these joints would have allowed the ingress of oxygenated water. Reddening of the uppermost parts of the melt sheet, as commonly seen in the field and in core samples, is attributed to associated hematization. Similar oxidation effects occur locally in the footwall and within mafic minerals in the central uplift. For regime 3 (central uplift), aside from contact effects where the anorthosite is directly juxtaposed with the impact-melt sheet, this will result in relatively low-temperature alteration. Plagioclase and maskelynite breakdown has resulted in the formation of hydrated Na- and Ca-bearing aluminosilicates, including zeolites (e.g., thomsonite and analcime), pectolite and prehnite. Given an An50 composition for the plagioclase the following reactions are likely: 2ðCa; NaÞðAl; SiÞ4 O8 þ 7H2 O-NaCa2 ½ðAl; SiÞ5 O10 6H2 O þ Na½AlSi2 O6 H2 O labradorite thomsonite analcime ðmaskelyniteÞ

biotite and amphibole, have undergone local hematization, typically where cracks, fractures and fault systems provide pathways for water access. The development of hematite is responsible for the reddening of fracture margins within the central uplift. Anhydrite is also developed locally. This is considered to form by the oxidation of accessory sulfide phases (pyrrhotite, pyrite, chalcopyrite). The central uplift regime at Manicouagan can be viewed as a closed system, with alteration involving the oxidation and hydration of existing phases, with minimal evidence of metasomatism (i.e., overall, it appears to have behaved isochemically). Regime 4 is not well exposed at Manicouagan, with many relevant outcrops now being submerged beneath the waters of the reservoir. However, there are indications in peripheral drill core (i.e., at the edge of the main island) of chlorite, sulfide mineralization, hematite and sulfates associated with faulting and possible large-scale slumping, which may equate with transient cavity collapse tectonics. Interest in the role of hydrothermal systems on Mars is strong, primarily because of their importance in facilitating suitable conditions for life, in particular thermophilic autotrophs (e.g., Woese, 1987). Early life on Earth, and possibly Mars and other suitable planetary bodies, may have evolved in impactgenerated hydrothermal settings when the impact flux was high (i.e., pre-3.9 Ga; Sleep and Zahnle, 1998). In fact, it has been suggested that it is only during Mars’ Noachian period (43.5 Ga), when impact rates were elevated and larger (4500 km) craters were formed, that suitably long-lived hydrothermal systems operated for durations sufficient to create conditions conducive to harboring and preserving early life forms (Pope et al., 2006). The circulation of steam in martian melt sheets (regime 1, Table 1) may have produced iron-rich clays, ferric hydroxides and near-surface salts, potentially resulting in a substantial fraction of martian soil being derived from the erosion products of hydrothermally altered impact melt sheets (e.g., Newsom, 1980). Understanding the conditions of secondary phyllosilicate formation at Manicouagan may have important implications for interpreting the presence of Fe3 + rich-phyllosilicates on Mars, which point to the existence of an early (i.e., Noachian) martian atmosphere with low levels of carbon

ðIÞ 4ðCa; NaÞðAl; SiÞ4 O8 þ10H2 O-Ca2 Al½AlSi3 O10 ðOHÞ2 þCa2 NaSi3 O8 ðOHÞ þ 3Na½AlSi2 O6 H2 O þ11H þ labradorite ðmaskelyniteÞ

prehnite

pectolite

Alteration tends to be localized in fracture and fault systems, although the mineralization can be patchy in places. Experimental work (summarized in Deer et al., 1978, 2004) indicates that these products are generated at To270 1C, which is compatible with the predicted thermal state of the uplift (see Section 3.2). Mafic minerals within the anorthosite, including garnet, pyroxene,

analcime

ðIIÞ

dioxide interacting with meteoric and surface liquid water (Chevrier et al., 2007). Hydrothermal alteration and ongoing weathering of the impact-melt sheet at Manicouagan together provide good analogue potential for processes operating on early Mars. In terms of technology testing, visible and near-IR reflectivity, Mossbauer and X-ray diffraction data were obtained from

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hydrothermally altered impact-melt sheet samples from Manicouagan for comparison with martian samples by Morris et al. (1995).

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heterogeneous glasses, (6) variable loss of Na and Ca in shocked plagioclases, and (7) quench crystals in shock-melted and thetomorphic glass phases (e.g., Short, 1970). Manicouagan possesses shocked central uplift features comparable to those of the Moon, as listed above.

3.4. Anorthosite and the lunar highlands Anorthositic rocks are common on the Moon, but relatively uncommon on Earth (Ashwal, 1993). The lunar upper crust has a bulk composition equivalent to anorthositic norite, and is probably underlain by a noritic lower crust (Ryder and Wood, 1977). The nonmare lunar highlands constitute  83% of the Moon’s surface, the remaining 17% being composed of mare basalts (Head and Wilson, 1992). Initial differentiation of the lunar magma ocean resulted in the segregation of relatively low-density plagioclase to generate the anorthositic suites that dominate the lunar highlands. The first stable crust was probably formed by 4.56–4.47 Ga (Jolliff et al., 2006). Intense bombardment since crystallization of the lunar magma ocean resulted in fracturing and brecciation to depths of at least 20 km, as well as the generation of impact-melt sheets (most of which have themselves been reworked by impact) and associated polymict ejecta blankets. This bombardment impregnated the upper crust with siderophiles derived from the impactors, such that pristine magma ocean products remain scarce (Warren, 1993). In this respect, the anorthositic central uplift at Manicouagan is valuable in that it retains a history of shock, but has not undergone siderophile addition through multiple impact, melting and mixing. It is therefore analogous to more pristine lunar highlands material, although its anorthite content is comparatively low (An50). Manicouagan’s central uplift rises to 950 m above sea level (Fig. 4a). The exhumed anorthosite is part of a 1700–1170 Ma regional anorthosite-mangerite-charnockite-granite (AMCG) suite that was subsequently metamorphosed and deformed under amphibolite to granulite and, locally, eclogite facies conditions during the Grenville orogeny (Indares et al., 1998), prior to shocking and impact-induced uplift at 215 Ma. The anorthosite is characteristic of the Proterozoic massif-type variant, typical of the Adirondacks of New York State and Nain Plutonic Suite of Labrador (Ashwal, 1993). Two other terrestrial impact structures possess anorthosite-bearing targets (Earth Impact Database, 2009): Charlevoix in Quebec (54 km diameter) and Mistastin in Labrador (28 km diameter). Compared to Manicouagan, their anorthosite volumes are significantly smaller. It has been argued that the massif-type, rather than the stratiform and megacrystic anorthosites on Earth, is the closest analogue to lunar highlands anorthosites (Hargraves and Buddington, 1970). However, the plagioclase of Proterozoic anorthosites is typically andesine to labradorite, in contrast to the anorthite-dominant plagioclases of the lunar highlands. This has implications concerning the liquidus and solidus temperatures, which are 100–2001 higher for the more calcic lunar feldspars. The anorthositic rocks at Manicouagan are dominated by labradorite plagioclase (An50). Nevertheless, characterization of the shock metamorphic, melting and deformation effects of these rocks at Manicouagan provides for comparisons with more pristine lunar highlands anorthosite and its response to the impact process in the absence of significant reworking. On the Moon, evidence for shock effects resulting from impactinduced pressures (in the 10 s of GPa range) and temperatures occurs in varying abundance in the lunar soil and microbreccias collected around the Apollo 11 site. For example: (1) multiple sets of planar features in silica phase(s) and plagioclase, (2) shockinduced twin or deformation lamellae, and kink bands in clinopyroxenes, (3) the occurrence of maskelynite in crystalline rock fragments within the breccias, (4) isotropization and partial decomposition of clinopyroxenes (leading to anomalously low refractive indices with respect to initial compositions), (5) partial fusion of mineral and rock fragments, with the generation of

3.5. Footwall breccias Of the 382 kg of lunar samples returned to Earth by the Apollo missions, 33% of that mass constitutes breccia (Heiken et al., 1991). Possible breccia dykes and parautochthonous breccias have also been identified on Mars (Head and Mustard, 2006; Tornabene et al., 2009). At Manicouagan, the fallback breccia and uppermost sections of the impact-melt sheet have been lost (supposedly due to as much as 1.7 km of erosion) during post-impact glaciation(s) (Degeai and Peulvast, 2006). However, the preservation of a variety of breccias at the melt sheet-footwall contact, and as dykes within the crater floor, render Manicouagan a good analogue site for comparison with certain lunar breccia samples, as well as future martian sample returns. Breccias at Manicouagan include (1) lithified, purely clastic breccias and (2) suevitic breccias, both of which can occur as lenses or sheet-like bodies at the contact of the impact-melt sheet and underlying basement, and as dykes and irregular bodies within the crater floor; (3) pseudotachylyte that typically occurs as anastomosing, cm-thick veins and dykes (up to at least 1 m wide) within the crater floor; and (4) clast-bearing impact melts, which we will not discuss further in this section. Clastic breccias range from in situ and clast-supported, through matrix-supported with angular to rounded fragments exhibiting no, or limited, transport (i.e., autochthonous or parautochthonous; Fig. 4g), to matrix-supported, polymict breccias indicating substantial transport (i.e., allochthonous; Fig. 4e). The majority of clastic breccias are characterized by weakly consolidated, green to brown, very fine-grained matrices (such that individual grains are indistinguishable in hand specimen) enclosing angular to rounded lithic and mineral fragments of the same material, up to metres in size. Within the crater floor, a continuum is observed from relatively intact footwall with fine, anastomosing shears, through clast-supported to matrix-supported breccias. These constitute the autochthonous or parautochthonous breccias, which typically comprise fragments of proximal basement lithologies, indicating relatively little transport. These breccias are analogous to the lunar monomict breccias ¨ ¨ of Stoffler et al. (1980) and Stoffler and Grieve (2007), although the Manicouagan breccias may comprise several lithologies owing to the relatively varied nature of the crater floor at Manicouagan, and should therefore be referred to as polymict. Allochthonous breccias are typical of the impact-melt sheetcrater floor contact, although they also occur as dykes within the basement. They are similar in appearance to the parautochthonous and autochthonous breccias but are matrix supported and polymict, with rock and mineral fragments exhibiting varying degrees of shock metamorphism, indicating derivation from disparate locations. They may also contain megaclasts (tens of metres diameter) of what were autochthonous and parautochthonous monomict breccias. Clastic breccias may grade into suevitic breccias. Suevitic rocks at Manicouagan have only been described from the contact between the impact-melt sheet and the underlying fractured and brecciated basement. Owing to their position, and the fact that they comprise predominantly clastic matrices with unshocked and shocked mineral and lithic fragments with blebs and schlieren of melt and glass, Murtaugh (1976) referred to them as basal suevites. The absence of aerodynamically-shaped melt and glass bombs and their occurrence within the final crater rim

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indicate that they can also be termed crater suevites, after von von Engelhardt (1997). The suevites at Manicouagan are easily identifiable at the outer edge of the impact-melt sheet where they form green to red/ brown friable layers, lenses and dykes. This is in contrast to the more massive, resistant and blocky weathering impact melt and clast-bearing impact melt. There is an interfingering and intermingling of the suevite and basal impact-melt sheet and clastbearing impact melt, indicating that they were moving relative to one another. Breccias intersected in drill holes from the centre of the structure (e.g., MAN-06-08) may represent hornfelsed suevitic breccias where they underlie thick ( 41 km) sections of impactmelt sheet. Further work will examine these breccias in more detail, but they may provide an analogy to lunar granulitic breccias, which are interpreted as breccias that have undergone heating to  1000 1C (e.g., Hudgins et al., 2008). The source of this heat is still debated, but may be due to juxtaposition with superheated impact melt or ejecta blankets (Hudgins and Thompson, 2008). In the field and in hand specimen, the suevites possess friable, clastic matrices enclosing rounded to angular mineral and rock fragments ranging in size from omm to metres, which themselves may be brecciated and/or contain pseudotachylyte-like veins and shears. Shatter cones are observed in some of the larger lithic fragments. The suevites are polymict, although in a given area, an abundance of one lithology may dominate. Also entrained within the clastic matrices are blebs, schlieren and larger masses of melt (Fig. 4f). Many of the larger lithic clasts are rimmed by melt. The melt rims typically exhibit a ductile behaviour with the surrounding matrices. Preliminary petrographic and analytical scanning electron microscope observations of the suevitic breccias reveal matrices now cemented by clay minerals, with larger monominerallic and lithic fragments exhibiting varying degrees of brecciation, shock metamorphism and melting. Shock metamorphism is evident primarily as planar deformation features in quartz and, to a lesser extent, feldspar. There is some evidence of partial melting of clasts. Discrete melt blebs and schlieren exhibit microcrystalline igneous textures comprising the same mineralogy and composition as the impact-melt sheet. This supports the field observation that the majority of melt entrained within the suevite (as elongate, flowing blebs and schlieren) is derived from the immediately overlying clast-bearing impact melt. The fact that these melt inclusions exhibit fluidal textures and a lack of obvious cooling against the clastic matrix indicates that they were still hot, plastic, and not completely solid, upon incorporation into the suevite. Suevites are traditionally thought to have originated as material (clastic and melted) that was blasted out during the

initial target excavation phase, most of which leaves the crater, becoming airborne and falling back as an ejecta deposit that blankets the whole structure and surrounding area. Crater suevites are considered to represent the excavated material that either fell back within the crater (e.g., the Onaping Formation at Sudbury) and/or never left the crater, and instead formed a lining to the crater floor (von Engelhardt, 1997). Evidence at Manicouagan suggests that some of the suevites may also originate from the interaction of clastic material that has slumped down from the peripheral crater walls with the base/edge of the impact-melt sheet. Pseudotachylyte is ubiquitous throughout the crater floor and central uplift in many terrestrial impact craters, e.g., Vredefort (Shand, 1916; Reimold et al., 1999) and Sudbury (Speers, 1957; Dressler, 1984; Thompson and Spray, 1994). At Manicouagan they are manifest as mm to cm thick veins and up to 1 m thick dyke-like bodies. This is evident from field observations (particularly within the well-exposed central uplift; see Section 3.2), as well as from the extensive drill core data (Fig. 4i). They exhibit characteristically aphanitic, black, purple or red matrices with o mm to cm size, typically subangular to rounded, mineral and rock clasts. Despite the erosion of the fallback breccia, Manicouagan preserves a variety of impact breccia lithologies. In particular, the excellent exposures of basal/crater suevite at the edge of the central island, as well as in drill core sections, provide new insights into this relatively little-studied variant of suevite, as well as important links to lunar and martian breccias.

4. Analogue training and testing potential Manicouagan possesses a number of attributes conducive to analogue research and development in support of future planetary mission activities. These can be broadly expressed as human training and instrument testing opportunities, and their viability in the Manicouagan context (Table 2). Trainees are here divided into four groups: astronauts, mission managers, science team and engineers. There can be overlap between groups; for example, some science team members might undertake management-type decisions during a mission, and science teams may include engineers. Engineers, as defined here, are those responsible for building infrastructure and optimizing its performance (e.g., satellite, surface instrument). The human component includes training in aspects of geology, communications and psychology. Instrument testing includes the ability to ground truth remote sensing data; for example, verifying the presence of plagioclase-dominant rock (anorthosite) using hyperspectral instruments. We have ranked Manicouagan in terms of it

Table 2 Human training and instrument testing potential for Manicouagan with high, medium or low applicabilities. Human training

Instrument testing

Geological

Communications, command and control

Psychology of human isolation

Remote sensing

Surface instruments

ISRU technologies

Lithology recognition

Sampling protocols

Manicouagan potential

High

High

Medium

Medium

Medium

High

Med-Low

Astronauts Mission managers Science team Engineers

High Medium

High Medium

High High

High Medium

Low Medium

High Medium

High Medium

High Medium

High Medium

High Low

Low Low

High High

High High

High High

See text for details.

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providing opportunities for training and testing. It is considered ‘‘High’’ for geological training, ‘‘Medium’’ value for telecommunications, psychology and remote sensing potential, ‘‘High’’ for surface instrument testing and ‘‘Medium to Low’’ for performing In Situ Resource Utilization (ISRU) experiments. ISRU limitations are primarily due to the surface not comprising planetary regolith (this remains a problem for all terrestrial sites). The human group rankings with respect to ‘‘High’’, ‘‘Medium’’ and ‘‘Low’’ indicate a given group’s need to acquire an appropriate skill set. For example, it is important for the astronauts and science team to understand the geology of a field location, but less so for the mission managers and engineers. Overall, Manicouagan is a good site for geological training (it exhibits many features characteristic of impact craters that would be encountered on other planetary surfaces), as well as surface instrument testing and validation. Vegetation cover limits realistic remote sensing testing, but does permit some ground truthing for especially lunar-like lithologies and impact melts.

5. Conclusions Manicouagan is one of the few relatively accessible, large ( Z90 km diameter), terrestrial impact craters that is well preserved, well exposed, only moderately eroded, undeformed and drilled. It has a predominantly crystalline target with an

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anorthositic central uplift; the latter being uncommon in terrestrial craters. As such it provides an excellent site to study the mechanics of terrestrial complex crater formation and apply the results to craters on other planets where we are currently limited to remote, morphometric studies. The preservation of a wide range of impact lithologies and structures and, more specifically, the presence of anorthosite and a post-impact hydrothermal overprint make Manicouagan an exemplary analogue site for lunar and martian studies. We identify six areas of interest relevant to comparative planetology research and planetary exploration initiatives:

(1) Impact-melt sheets and clast-bearing impact melts—Manicouagan possesses a well-preserved impact-melt sheet underlain by clast-bearing impact melts. The castellated melt sheet floor facilitated accumulation of melt sections up to 1400 m thick (in contrast to the average of 300–400 m), which underwent differentiation (see Fig. 2 in Spray and Thompson, 2008). Assigning lunar impact melt and clast-bearing impact melt samples to individual impact events is unwarranted if differentiation has taken place. Manicouagan shows that this can happen at smaller crater diameters than previously recognized. We need to re-evaluate lunar impact melt and impact melt breccia-crater associations in the light of this discovery.

Fig. 8. Lunar (a–d) and martian (e–f) impact craters showing similar morphometries to Manicouagan. (a) Pythagaros, 142 km diameter; (b) Jackson, 128 km diameter; (c) King crater, 75 km diameter; Apollo 16 metric image AS16-M-2379; (d) Tycho, 85 km diameter; image taken from earth; (e) Burton, 125 km diameter; Viking I and II image and (f) Martz, 97 km diameter; Mars Global Surveyor image. Images (a) and (b) are high definition TV images from the JAXA, Selene, Kaguya mission.

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(2) Central uplift formation—the exact mechanism of formation remains a subject of debate. Many excellent comparable examples of central peak and peak ring craters are developed on the Moon, Mars and other planetary bodies (Fig. 8). Manicouagan can help determine how they formed, based on careful ground-based structural mapping. Our evidence so far indicates that the Manicouagan central uplift is a fault-bound and intra-faulted horst complex. This needs to be explored in greater detail. Interestingly, the diameters of impact structures on both Mars and the Moon that are comparable in form to Manicouagan are similar, which appears to contradict the scaling predicted based on gravitational differences (Melosh 1989). (3) Hydrothermal regimes—impact craters can generate hot rockwater systems where meteoritic and ground waters interact with impact-melt sheets, exhumed basement (central peaks) and shock-heated target rocks. Together these can create warm, wet niches for the protection of existing life forms and/ or provide conditions suitable for the development of life. Manicouagan represents an analogue for such sites on Mars, as well as providing insight into how alteration and weathering processes generate phyllosilicates, zeolites and other hydrous phases that can contribute to soil and sediment formation on Mars. (4) Anorthosite-lunar highlands comparisons—most of the lunar highlands have undergone multiple impacts since crystallization of the lunar magma ocean. This has resulted in modification via brecciation, melting and contamination with projectile siderophiles, as well as by subsequent igneous activity. Manicouagan provides a shocked anorthosite uplift comparable to the more pristine lunar highlands material. It also provides a case study for the disposition of shock features within the preserved anorthositic rocks. (5) Footwall breccias—the new drill core and ongoing field studies at Manicouagan reveal a well-preserved sequence of sub-melt sheet breccias, including allochthonous clastic and basal suevitic varieties, and autochthonous breccias. This allows us to better place certain lunar and other planetary breccia samples in context within the structure and stratigraphy of impact craters. (6) Training—Manicouagan provides a training ground for astronauts, mission managers, science teams and engineers wishing to prepare for operations on cratered planetary surfaces. It is also a venue to test and optimize the design of surface-based analytical equipment (e.g., laser-induced breakdown spectroscopy, Raman spectroscopy, and alpha-particle X-ray spectrometry) and to ground truth remote sensing technologies (e.g., hyperspectral imaging, laser altimetry, RADAR).

Acknowledgements The Manicouagan Impact Research Program (MIRP) is supported by Canada Research Chairs funding, the Canadian Space Agency, the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of New Brunswick. We thank Karen Shea and undergraduates Kyle Blades, Laura Malone, Thomas Race and Catrina Russell for their assistance with field logistics. This manuscript benefited from constructive comments from an anonymous reviewer and Paul Warren. Planetary and Space Science Centre contribution 62. References ¨ hman, T., Leitner, J.J., Raitala, J., 2007. The characteristics of polygonal Aittola, M., O impact craters on Venus. Earth, Moon and Planets 101, 41–53.

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