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Lunar and Planetary Science XXXI 2023.pdf

THE ISIDIS PLAINS UNIT, : POSSIBLE CATASTROPHIC ORIGIN, TECTONIC TILTING, AND SEDIMENT LOADING. K. L. Tanaka1, T. Joyal1, and A. Wenker1, 1U. S. Geological Survey, Flagstaff AZ, 86001; [email protected].

Introduction: We propose that an extensive de- gradually lower to ~-3750 m. The southern contact posit covering most of formed by cata- also appears to descend toward the east from ~-3650 m strophic erosion of highland materials along the west- to ~-3900 m. ern margin of the topographic basin. The deposit ap- Generally, the Isidis plains unit’s surface lessens in parently initiated as a large lake of debris, which sub- elevation away from its contact, such that the overall sequently underwent mud volcanism as volatiles were surface is roughly bowl shaped and tilted to the - expelled. The deposit also may have depressed the west. The deepest parts of the basin range from ~-3950 lithosphere. Although not part of the major interior m along the west edge of the unit (near 12°N, plains unit that covers Utopia and north polar basins, 277.5°W) and trends southeastward to ~-3800 m (near the Isidis deposit does share similarities with that unit 8°N, 269°W). in morphology, age, and subsequent deformation. A Unexpectedly, the deepest part of the basin (-4000 northeastward rise of the Isidis unit contact may be m) occurs within the knobby and hummocky materials explained by peripheral uplift produced by the response along the western margin of Isidis Planitia. Along the of the lithosphere to loading by the interior plains unit northeast edge of Syria Planum, an abrupt scarp nearly in , more than 150 km northeast of the 700 m high marks the edge of the planum, whereas Isidis plains unit. farther south, the surface of the planum gently descends Stratigraphy. Geologic mapping indicates that into Isidis Planitia. Isidis Planitia is covered by a relatively smooth plains The Isidis plains deposit and interior plains unit in material [1-2]. This unit is confined within the basin Utopia Planitia occur more than 150 km apart, sepa- [1-3] and does not connect with the proposed water [4] rated by somewhat more densely cratered, knobby and or mud [3,5] ocean margin observed nearby in Utopia hummocky plains material. Although the topographic Planitia (which outlines the interior plains unit). It rise between these two units is only ~100 m, no spill- mostly overlies and embays older plains units that ways across the rise are seen in Viking images. We may be deposits produced by degradation of ancient identify two margins of the interior plains unit in this highland and high plains materials [1-2]. The unit area (260-270°W). The upper contact ranges from contact in most areas is approximate because a sharp –3480 to –3580 m, whereas the lower contact ranges contact signature or morphologic distinction with adja- from –3580 to –3680 m. cent units is generally lacking in Viking images. The Isidis Mounds and Other Landforms. Unusual, relative age of the unit, based mainly on crater counts mostly pitted mounds occur on the Isidis plains unit from Viking images of the western part of the unit [6], and may be fundamental to understanding the most straddles the / boundary [7] recent resurfacing in the basin. Detailed mapping of The unit grades into knobby plains materials in the mounds indicate that they occur either (a) isolated western Isidis Planitia, which appears to have formed but locally following arcuate patterns or (b) continu- by the degradation of the wrinkle ridged, Hesperian ously connected into arcuate ridges [6]. The mounds plateau rocks of Syrtis Major Planum and the underly- have a mean diameter of 370±90 m, and the ridges ing and adjacent cratered materials of Arabia mostly range from 0.5 to 1 km in width and 10 to 40 Terra [8]. Along the southern margin of Isidis basin, km in length [6]. Based on shadow measurements and appear to be massifs formed by the early assumed maximum slopes, mound heights were esti- impact that initially formed the basin during the Early mated to be 150 to 300 m [6]. Photoclinometric Noachian [7]. Valleys dissect the intermassif areas, measurements, which should be more accurate but are through which debris had been shed into Isidis basin less numerous (272 profiles taken), indicate a mean probably sometime during the Hesperian [9]. basal diameter of 750±130 m, a mean height of 41±17 MOLA Topography. The Isidis plains unit con- m, and central pit depths of 5±4 m [11]. The mounds tact appears to vary systematically in elevation, from have been interpreted as volcanic pseudocraters [12], higher in the northeast to lower in the southwest. fracture-controlled volcanoes “unique to Mars” [13], However, because of uncertainties in precise location of moraine-like remnants of an ice-rich debris deposit in the contact in many places, especially for the southern which removal of ice cores formed the summit craters section, measurement of the contact elevation is ap- [6,14], pingos [15], remnants of mudflows [3], and proximate; measurement accuracy probably varies by a mud volcanoes [11,16]. few to several tens of meters. Most of the northern and Along the northern and southern margins of the Isi- eastern sections are within a few tens of meters of dis plains unit, several sinuous troughs with distinct –3600 m. The northwestern contact with the margin medial ridges trend generally perpendicular to the local of is not clearly defined but appears to unit contact. The troughs appear to have shallow Lunar and Planetary Science XXXI 2023.pdf

THE ISIDIS PLAINS DEPOSIT, MARS: K. L. Tanaka et al.

slopes and less relief compared to that of the medial lated (unchained) pitted domes and sinuous troughs ridges. These trough/ridge structures are mostly a few occur within northern plains deposits in Utopia, tens of kilometers in length and singular, although at Chryse, Acidalia, and Arcadia Planitiae. However, least one branching “Y” structure occurs. These fea- such features seem to be absent in , tures, which also occur along the margins of the inte- which may indicate that either they didn’t form at high rior plains deposit in Chryse and Utopia Planitiae [16], latitudes or they were largely obliterated by later geo- have been interpreted to be channels with medial bars, logic activity. sandbanks, or elongate pingos [17]; coastal spits and Tectonic Tilting. If we assume that the Isidis barriers [18]; subglacial tunnel valleys with medial plains unit originated by erosion of Syrtis Major eskers [19]; and contractional features within either Planum, then the unit contact should have been either floating ice streams [20] or partly frozen mass-flow level or lower to the east. Instead, the contact rises deposits [16]. perhaps as much as 200 to 300 m to the northeast. Plateau Breakup Features. The Isidis plains unit This means that either the erosional breakup of the grades into knobby and hummocky material along the edge of Syrtis Major had nothing to do with emplace- west edge of Isidis Planitia. The east margin of Syrtis ment of the plains unit or that the unit has been tilted. Major Planum is complexly fractured and degraded. In Possibly, the unit was tilted in response to isostatic places, fractures widen into troughs, separating a mo- adjustment of the loading produced by the interior saic of large, irregular plateaus. A few plateaus extend plains unit in Utopia Planitia. Similar peripheral tilt- out into Isidis Planitia; some are isolated from the ing has been proposed as an explanation for reversed planum. outflow channel gradients south of [21- Along the north edge of the planum, remnant mesas 22]. and knobs outline what appears to be a shallow, broad Sedimentary Loading. The depressed interior (~20 km wide) channel extending more than 100 km part of the Isidis plains unit may be caused by sedi- into Isidis Planitia. This channel may have been a ment loading. Rapid sedimentation would gradually late-stage mass-flow pathway. produce subsidence as isostatic adjustment occurred On the southern part of the plateau breakup, a few beneath the elastic lithosphere. If the load were en- marginal plateaus are surrounded by troughs and fis- tirely compensated (assuming that flexural support by sures lined by narrow ridges (see Viking image the lithosphere did not occur), the amount of depres- 377S60). Toward the planum, the troughs become sion would be proportional to the load thickness, narrower, until they are gone and ridges with central, based on the relative densities of the sediment load and shallow troughs remain atop the unbroken plateau the lithosphere. Assuming a load-to-lithosphere den- rocks. The innermost ridges appear to be linear chains sity ratio of ~0.8 and a depression of about 50 to 150 of pitted domes, similar to those occurring within the meters, the thickness of the deposit would be about 60 basin. These domes and ridges may be products of to 190 meters (compare [21]). mud volcanism. References. [1] Tanaka K.L. (1997) LPSC XXVIII, Catastrophic Origin for Isidis Plains Unit. The 1411-1412. [2] Tanaka K.L. and MacKinnon D.J. foregoing observations perhaps are best reconciled in a (1999) LPS XXX, #1217. [3] Jöns H.-P. (1987) LPSC scenario in which catastrophic breakup of Syrtis Major XVIII, 470-471. [4] Parker T.J. et al. (1989) Icarus 82, Planum rocks produced a series of mass flows that be- 111-145. [5] Tanaka K.L. and Banerdt W.B. (2000) come deposited within Isidis basin as essentially one this vol. [6] Grizzaffi P. and Schultz P.H. (1989) Ica- unit. Possibly only one unit was deposited, based on rus 77, 358-381. [7] Tanaka K.L. (1986) JGR 91, (1) the lack of observed contacts within the Isidis PLPSC 17, E139-E158. [8] R. and Guest J.E. plains unit, and (2) the pitted mounds, which, assum- (1987) USGS Map I-1802-B. [9] Crumpler et al. (2000) ing that they are mud volcanoes, indicate rapid sedi- this volume. [10] Head J.W. (1999) Science 286, 2134- mentation [16]. Moreover, the occurrence of possible 2137. [11] Davis P.A. and Tanaka K.L. (1995) LPSC mud volcanic features on the plateau and apparently XXVI, 321-322. [12] Frey H. (1986) LPSC XVII, 239- along cracks indicates that volatiles could have been 240. [13] Hodges C.A. and Moore H.J. (1994) USGS venting as the plateau material broke up. Catastrophic Prof. Pap. 1534, 194 p. [14] Rossbacher L.A. and release of volatiles may have been effective at pulveriz- Judson S. (1981) Icarus 45, 39-59. [15] Witbeck N.E. ing and fluidizing the plateau rocks, allowing them to and Underwood J.R. (1984) USGS Map I-1614. [16] transform into mass flows. Tanaka K.L. (1997) JGR 102, 4131-4149. [17] Scott More speculative is the reason why and when such D.H. (1982) NASA Tech. Mem. TM 85127, 216-218. an event took place. Magmatic activity at Syrtis Ma- [18] Parker T.J. et al. (1993) JGR 98, 11,061-11,078. jor Planum may have instigated hydrothermal activity [19] Kargel J.S. et al. (1995) JGR 100, 5351-5368. along the Isidis margin [8]. [20] Lucchitta B.K. et al. (1986) JGR 91, PLPSC 17, If our interpretations are generally correct, then the E166-E174. [21] Tanaka K.L. and Banerdt W.B. fine-scale morphologies of the Isidis plains unit may be (2000) this volume. [22] Skinner J. and Tanaka K.L. indicative of the depositional processes involved. Iso- (2000) this volume.