The summit of Olympus Mons stands at 21,229 m - the highest elevation on Mars - and the mountain is surrounded by several blocky, lobate haloes that stretch for hundreds of kilometres. Unit names and labels consist of the following components: (1) age, (2) unit group, and (3) subtype (optional). When units span more than one epoch within a period, epochs are not discriminated in unit names and labels - the Correlation Map Units (CMU) specifies the interpreted epoch range.
Portions of the crater size–frequency distributions that fit the production function well over continuous diameter intervals were used to determine model ages. A new early Amazonian referent is the areally small early Amazonian basin unit (eAb)—the only map unit restricted to this epoch despite the epoch's apparently approximately 2-b.y. To show this lack of constraint, the bottom edges of the unit boxes have sawtooth-edged age limits in the CMU.
Geologic History
Noachian Period
Early Noachian Epoch
Middle Noachian Epoch
Unit lNh surveys indicate that the Thaumasia highlands rose mainly during the Middle Noachian, and the development of closely spaced, narrow grabens and rifts in the Thaumasia highlands at this time indicates their contemporaneity with uplift (Dohm and others, 2001a; Hauber and others). , 2010).
Late Noachian Epoch
Hesperian Period
Crustal extension associated with increased regional magmatism resulted in the formation of graben sets in volcanic rocks (unit eHv) within and along the margin of the Tharsis Rise (*, Anderson and others, 2001). Some Early Hesperian rocks are cut by fluvial channels, including those in southwestern Hesperia Planum, suggesting persistent, volcanically driven groundwater drainage toward Hellas Planitia in Hesperia (*, Squyres et al., 1987; Crown et al. Tanaka and Leonard, 1995; Price, 1998). The Late Hesperian Volcanic Unit (lHv) covers several areas of the Tharsis region and a small area extending southwest of Tyrrhenus Mons (Greeley and Crown, 1990).
The Late Hesperian volcanic field unit (lHvf) includes many small shields and rift vents around Syria Planum. Unit lHvf was modified by extensional tectonism during the Late Hesperian, resulting in narrow grabens oriented both radially and concentrically to shield structures, as well as the elliptical ridge of Syria Planum (*, Anderson et al. Hesperian Transition Unit late (lHt ) covers many parts of the lower-elevation, plateau-forming montane-lowland transition zone, as well as the floor and some flanking areas of Valles Marineris and a rocky plateau southeast of Hesperia Planum.
The Amazonian and Hesperian Transitional Undivided Unit (AHtu) forms the later of two transitional undivided units that may have been emplaced in the Late Hesperian. We also include in this unit an outcrop located west of Kasei Valles, surrounded by and stratigraphically within the AHv unit, which is perforated and possibly layered, as evidenced by changes in pit size with exposure levels. The end of the Late Hesperian is here defined as the widespread cessation of sedimentary plains in the northern lowlands, as represented by the Late Hesperian Lowland Unit, lHl (Tanaka, 1986; Parker et al., 1989; Tanaka et al. (Werner et al., 2011).
The lHl unit includes scattered, highly modified materials forming bas-relief and perhaps thinly buried knobs and mesas, some of which predate the Late Hesperian. The Late Hesperian Basin Unit (lHb) forms the bulk of Hellas Planitia and may result from a combination of volcanic, lacustrine, glacial and aeolian deposits (Kargel and Strom, 1992; Tanaka and Leonard, 1995; Moore and Wilhelms) .
Amazonian Period
Unit lHvf also occurs in the central and northeastern parts of the Tharsis region and on the western flank of Elysium Mons (*, Tanaka et al., 1992; Moore, 2001; Hauber et al., 2011). Other contributing processes may include mass wasting and fluvial dissection of the highland-lowland transition zone (*, Frey and others, 1988; Maxwell and McGill, 1988; Tanaka and others volcanism, and aeolian erosion and deposition. The Amazon Volcanic Building Unit (Ave) ) consists of the largest shield volcanoes in the Tharsis region—Olympus, Alba, Ascraeus, Pavonis, and Arsia Montes.
The ages of the material buried within these buildings are unknown and may be Hesperian or earlier in age, as surrounding lava plains of the Tharsis outcrop date back to at least the Late Noachian (unit lNv). Northeast of Alba Mons, the northernmost extensions of densely ordered narrow graben dissect the Late Hesperian lowland unit (lHl); the overall fault system deforming Alba Mons may be the result of regional faulting due to northwest-southeast extension and local, focal-induced buoyant doming of the Alba Mons shield (*, Tanaka, 1990; . Cailleau et al., 2005) . Northwest of Alba Mons, fold ridges oriented around the edifice may have formed in response to lithospheric loading imposed by the growth of the shield and by the overall loading of the Tharsis rise (*, Banerdt et al., 1992; Watters, 1993 ; Head and others, 2002).
This epoch represents the setting of the Middle Amazon Lowland (mAl) unit, based on overlap relationships and crater counts (Werner et al., 2011; Skinner et al., 2012). The unit may be mainly dust and ice derived from the erosion of the undivided Hesperian polar unit (Hpu); subsequent gradual erosion of the mAl unit led to the formation of the pedestal crater and possibly redeposition as the undivided Amazonian Polar Unit (Apu) (Skinner et al., 2012). The Late Amazonian Volcanic Field (lAvf) unit is composed of groups of small basaltic shields, fissure vents, and erupting lava flows; the unit occurs in the central parts of the Tharsis uplift and in the Cerberus Fossae, commonly along graben stacks (*, Bleacher et al., 2007; Vaucher et al., 2009; Hauber et al., 2011).
These deposits are overlain by residual ice forming the late Amazonian polar mantle unit (lApc). The late Amazonian foreland unit (lAa) occurs along the western to northwestern flanks of Tharsis Montes and Olympus Mons and was mapped in more detail by Morris and Tanaka (1994), Scott and Zimbelman (1995), and Scott and others (1998).
Major Results
This epoch includes significant coverage of the lowlands of Amazonis Planitia by sparsely cratered lava flows originating southwest of Olympus Mons and flow fields south and east of the Elysium uplift that erupted from Cerberus Fossae (*, Fuller and Head, 2002; Plescia, 20 Tanaka et al., 2005; Dohm et al., 2008; Vaucher et al., 2009; Hamilton et al., 2010; Hauber et al., 2011; Platz and Michael, 2011). Part of the Cerberus Fossae flows reach Amazonis and Arcadia Planitiae, where older volcanic flows, as well as transitional and plain units, are superimposed. These outcrops, as well as other flows surrounding the base of Olympus Mons and south of Alba Mons, are mapped as the Late Amazonian Volcanic Unit (lAv).
Formation of narrow grabens and wrinkle ridges under the late Amazon is restricted to volcanic areas and generally follows older patterns of deformation. The Amazonian Polar Undivided Unit (Apu) was emplaced on the polar plateaus, Planum Boreum and Planum Australe, during the Late Amazon (Tanaka and Kolb, 2001; Koutnik and others, 2002; Tanaka, 2005; Tanaka and others, 2008; . Tanaka and Fortezzo, 2012). The southern polar ice sheet is colder than the northern polar ice sheet due to its higher elevation and includes carbon dioxide ice overlying water ice (*, Byrne, 2009), as well as a buried, submerged radar-transparent material up to 300 m thick that interpreted to be carbon dioxide (Phillips and others, 2011).
Planum Boreum is surrounded by the most extensive dune sand accumulations on Mars, designated as the Late Amazonian polar dune unit (lApd) (Tanaka and Fortezzo, 2012); smaller, unmapped dune patches occur globally (*, Hayward et al., 2007).
Acknowledgments
Michael of the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt, Bonn), grant 50QM1001 (High Resolution Stereo Camera on Mars Express), on behalf of the German Federal Ministry of Economics and Technology.
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