https://doi.org/10.1130/G48903.1

Manuscript received 25 September 2020 Revised manuscript received 3 February 2021 Manuscript accepted 5 March 2021

© 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 17 May 2021

Pityusa Patera, : Structural analyses suggest a mega-caldera above a magma chamber at the crust-mantle interface Hannes Bernhardt and David A. Williams School of and Space Exploration, , Tempe, Arizona 85287, USA

ABSTRACT Yellowstone-like magma chamber collapse was Pityusa Patera is the southernmost of four paterae in the 1.2 × 106 km2 wrinkle-ridged tentatively favored by some previous investiga- plains-dominated Malea Planum region of Mars. Based on their texture, morphology, and tions (Peterson, 1978; Head and Pratt, 2001; Wil- uniqueness to Pityusa Patera, we interpret layered, folded massifs as pyroclastic deposits em- liams et al., 2009), no indicative observations placed during patera formation as a collapse caldera. Such deposits would not be expected in were made in support of such a scenario. Because a previously suggested scenario of patera formation by subsidence from lithospheric loading. Pityusa Patera is potentially the oldest and one of Our structural measurements and modeling indicate that the folding and high relief of the the largest extant caldera on Mars (Bernhardt and massifs resulted from ∼1.3%–6.9% of shortening, which we show to be a reasonable value Williams, 2021), further assessment of its origin for a central plug sagging down into an assumed piston-type caldera. According to a previ- is crucial to understanding Mars’ early evolution ously published axisymmetric finite-element model, the extent of shortening structures on and inform future exploration. a caldera floor relative to its total diameter is controlled by the roof depth of the collapsed We present new observations including the magma chamber beneath it, which would imply Pityusa Patera formed above a chamber at analyses of previously undescribed massifs of 57.5–69 km depth. We interpret this value to indicate a magma chamber at the crust-mantle folded, potentially pyroclastic deposits as well interface, which is in agreement with crust-penetrating ring fractures and mantle flows ex- as structural investigations based on model- pected from the formation of the Hellas basin. As such, the folded massifs in Pityusa Patera, ing by Zuber and Mouginis-Mark (1992) that which are partially superposed by ca. 3.8 Ga wrinkle-ridged plains, should consist of primor- indicate Pityusa Patera to have formed not by dial mantle material, a theory that might be assessed by future hyperspectral observations. loading-induced lithospheric subsidence, but as In conclusion, we do not favor a formation by load-induced lithospheric subsidence but sug- an actual volcanic mega-caldera from collapse gest Pityusa Patera to be one of the oldest extant volcanic landforms on Mars and one of the of a magma chamber, potentially at the crust- largest calderas in the solar system, which makes the folded, likely mantle-derived deposits mantle interface. on its floor a prime target for future exploration. PATERA SIZE INTRODUCTION AND SETTING sill intrusions to form paterae with such muted Based on data from the High The Malea Planum region is an ∼1.2 × 106 topography (so-called “Arsia-type calderas”). Resolution Stereo Camera (HRSC; e.g., Neu- km2 physiographic domain defined by Noa- Larson (2007) then suggested such subsidence kum et al., 2004; Gwinner et al., 2009) and by chian to early Hesperian wrinkle-ridged plains from loading, e.g., by mid-crustal intrusions the Mars Orbiter Laser Altimeter aboard Mars (including Malea Planum) located in Mars’ akin to Idaho’s Snake River Plain (northwest- Global Surveyor (Smith et al., 2001), Pityusa southern hemisphere, southwest of the Hel- ern United States), to have formed Pityusa Patera Patera has a best-fit radius of∼ 115 km as las basin (Fig. 1A; Peterson, 1978; Tanaka and cited its large diameter, lack of discernible defined by the largest long-wavelength (25 km) and Scott, 1987; Williams et al., 2009, 2010a, bounding scarps or an edifice-like rise, and the slope change (Fig. 1C) around it. Mars’s largest 2010b; Tanaka et al., 2014). The Malea Planum apparent absence of caldera-typical volcanic unambiguous volcanic caldera outside of the region hosts four large and morphologically dis- flows and/or vents as reasons. However, recent Malea Planum region is that on the summit of tinct “paterae”, i.e., irregularly to round-shaped, mapping by Bernhardt and Williams (2021) Arsia Mons and has a best-fit radius of ∼65 km rimless, flat-floored depressions. The second- showed that Pityusa Patera is superposed by the (e.g., Crumpler and Aubele, 1978). However, largest and southernmost, Pityusa Patera, is cen- younger, ubiquitous wrinkle-ridged plains char- the larger depression flanked by Nili and Meroe tered at 37.37°E, 67.17°S and has been ascribed acterizing the Malea Planum region (Fig. 1D, Paterae in the center of Syrtis Major Planum a maximum depth of ∼1.5 km and variable diam- unit Npr—Noachian ridged plains), thus explain- likely also represents a collapse caldera that eters ranging from ∼170 to ∼400 km due to its ing the apparent lack of tectonic and volcanic was later superposed by wrinkle-ridged plains very gently sloping inner walls (Figs. 1B and landforms expected for calderas. Nevertheless, and, with a best-fit radius of∼ 120 km, is even 1C) (Head and Pratt, 2001; Plescia, 2003; Wil- while the formation of Pityusa Patera as a cal- larger than Pityusa Patera (Kiefer, 2004). Fur- liams et al., 2009). Crumpler et al. (1991, 1996) dera reflecting vertical motion of the surface in thermore, the largest potential volcanic calderas suggested lithospheric subsidence caused by response to volume changes at depth by a giant, on Earth and are Apolaki caldera in the

CITATION: Bernhardt, H., and Williams, D.A., 2021, Pityusa Patera, Mars: Structural analyses suggest a mega-caldera above a magma chamber at the crust-mantle interface: , v. 49, p. 1020–1024, https://doi.org/10.1130/G48903.1

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C D

Figure 1. (A) Topographic view of Mars centered on the Malea Planum region. Black box outlines Pityusa Patera shown in B–D. Orthographic projection centered at 45°E, 67°S. (B) Blended Mars Orbiter Laser Altimeter (MOLA)–High Resolution Stereo Camera digital terrain model over daytime image mosaic by the Mars Odyssey Thermal Emission Imaging System (Christensen et al., 2004) of Pityusa Patera. Same projection as in A. (C) MOLA slope map of same scene as in B. Black dots outline best-fit circle (radius≈ 115 km) of Pityusa Patera as defined by main slope break mostly visible along its western rim. Black dashed line shows best-fit circle (radius ≈ 103 km) defined by the geographic extent of unit Nml (red unit in D). (D) Geologic map by Bernhardt and Williams (2021) of the same scene as in B. Labeled black polygon outlines location of Figure 2A. Unit labels (prefixes A, H, and N stand for Amazonian, Hesperian, and Noachian): Add—dark dunes; Am—mantling; Apc—pedestal crater (upper); Acm—fresh crater materials; Hpc—pedestal crater (lower); Hs—smooth plains; Hcm—degraded crater materials; HNst—stream- lined plains; Npr—ridged plains; Nml—lobately layered massifs. The cross-hatched pattern outlines areas of low thermal emissivity.

West Philippine Basin and Sacajawea Patera than Pityusa Patera. We therefore submit that FOLDED MASSIFS AND BEDDING on Lakshmi Planum, which have best-fit radii Pityusa Patera’s size is still comparable to that MEASUREMENTS of ∼80 km and ∼100 km, respectively (Rob- of other examples and should not be considered Massifs as much as ∼1.2 km high with lobate erts and Head, 1990; Barretto et al., 2020), thus as an argument against an origin as a collapse lineations (Fig. 1D, unit Nml—Noachian lobately making them only ∼30% and ∼13% smaller caldera. lineated massifs) cover 6030 km2 (∼17%) of

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/8/1020/5362076/g48903.1.pdf by guest on 01 October 2021 Pityusa Patera’s topographically defined floor. A Pityusa Patera is the only caldera-like depres- sion on Mars to host such kind of material. Unit Nml is embayed by, and therefore older than, all adjacent units including the wrinkle-ridged plains (unit Npr), for which Bernhardt and Wil- liams (2021) derived a model age of ca. 3.8 Ga. The massifs are characterized by lineations at an average spacing of ∼150 m formed by ridges that are as much as several tens of meters high and wide (Fig. 2). Some ridges expose meter-scale blocks and commonly form extensive, subpar- allel, lobate patterns, including (semi)circular arrangements as much as ∼3 km wide (Fig. 2B, two black arrows). Patterns formed by the ridges are neither predominantly perpendicular nor par- allel to slopes, as would be expected if the ridges resulted from gravity-driven surface processes. Only four impact craters with widths >2 km occur on unit Nml, two of which show linea- tion ridges traversing their rims without visible deviation (Fig. 2B, white arrow). Based on these observations, we interpret the small ridges to be BC surface expressions of truncated, folded layers and used the LayerTools add-in for ArcMap software (https://desktop.arcgis.com/en/arc- map/; Kneissl et al., 2010) to obtain 58 bedding attitudes (strikes and dips). To measure a bed- ding attitude, a layer was traced with five to 27 points depending on its exposed length, thereby mitigating errors in point placement and geore- ferencing (Fig. 2A; see Table S1 in the Supple- mental ­Material1). The estimated average vertical error of the HRSC digital terrain model (DTM) of ∼10 m (Gwinner et al., 2009) should result in dip errors of no more than 1.1° given that all measurements extend across >100 m of eleva- tion and >1 km of baseline distance. We were able to measure antithetic dips along three fold trains, i.e., adjacent anti- and synforms (“north”, “center”, and “south” in Fig. 2). Because the res- olution of the DTM prevented an exact determi- nation of each fold limb’s length, we weighted each fold in a train equally based on the rela- tively equal spacing between apparent fold axes (Fig. 2B, black lines with arrows). Assuming we Figure 2. (A) Cropped Context Camera (CTX, Mars Reconnaissance Orbiter; Malin et al., 2007) were able to include the entire lengths of the image of unit Nml in Pityusa Patera, Mars; location is shown in Figure 1D. CTX swath cut in three fold trains and neglecting potential tilts and two halves shown side by side. White lines are 100 m contour lines based on High Resolution deformations of fold axes, the estimated aver- Stereo Camera digital terrain model h2276_0000; black box is location of B. Fifty-eight (58) bed- ding attitudes (red symbols) result in 14 interpreted fold axes (black lines with converging or age horizontal shortening strain for unit Nml, diverging arrows; see Fig. S1 [see footnote 1] for a full-size version of this figure). Along three ԑ(Nml), would range between −1.3% and −6.9% lines (“north”, “center”, “south”; outlined by dashed ellipses), antithetic dips form fold trains, (accounting for measurement/fitting/data errors; i.e., adjacent anti- and synforms (see Table S1). (B) Enlargement of part of panel A, zooming see also Table S1). in on fold train “south”. Scene also shows circular ridge arrangement (two black arrows) and one of four impact craters wider than 2 km (∼2.6 km diameter). White arrow shows where small Given that unit Nml in its current form is ridges cross the crater rim without visible deviation. Image location is outlined by black box older than the wrinkle-ridged plains and unique in A. (C) Schematic of fold train “south” seen in panel B. to Pityusa Patera’s interior (Bernhardt and Wil- liams, 2021), we infer that the compressive stress retention, eroded appearance, and apparent abil- 1Supplemental Material. Figure S1 (full-size field that folded unit Nml was confined to the ity to accommodate strain via folding indicating version of Figure 2A), and Table S1 (all 58 measured patera and unrelated to wrinkle-ridge forma- a comparatively low rock strength. In this case, bedding attitudes, including various error parameters tion. Furthermore, we suggest the material of unit Nml would be the oldest unit of volcanic as well as median and average dip values). Please visit https://doi.org/10.1130/GEOL.S.14417636 to access the folded massifs to be genetically related to origin in the Malea Planum region and among the supplemental material, and contact editing@ Pityusa Patera, potentially pyroclastic deposits, the oldest volcanic materials on the surface of geosociety.org with any questions. as indicated by unit Nml’s low impact-crater Mars (e.g., Xiao et al., 2012; Tanaka et al., 2014).

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/8/1020/5362076/g48903.1.pdf by guest on 01 October 2021 However, due to the widespread superposition that θ for piston-type calderas can be as shallow the caldera width, which is very likely in our by the wrinkle-ridged plains, neither the exact as ∼50°, although values >60° are more common piston-type scenario). For Pityusa Patera, the

timing of unit Nml emplacement during caldera (Roche et al., 2000; Kusumoto and Takemura, fraction of RC(Pityusa) can be constrained by unit formation nor unit Nml’s true geographic extent 2005; Hardy, 2008). The current depth of Pityusa Nml, which our observations indicate to have (possibly indicating an uneven distribution from Patera is ∼1.5 km, although this is a minimum undergone folding by a stress field that was lim- a trapdoor-style collapse, etc.) can be reliably value due to the substantial post-collapse infill, ited to within the caldera. Unit Nml can be found determined with currently available data. including wrinkle-ridged plains material as much as much as ∼103 km from the caldera center, for as ∼2.7 km thick as indicated by ghost crater and which we estimate a physiography-based best-fit

STRUCTURAL MODEL AND DEPTH OF structural wrinkle-ridge analyses. For a range of radius RC(Pityusa) of ∼115 km, thereby resulting in

MAGMA CHAMBER θ = 50°–70° and DPityusa = 1.5–4.2 km, SPityusa a minimum radius fraction of ∼0.9 × RC(Pityusa). Shortening within calderas (e.g., Fernan- would be ∼0.9–7 km. Assuming the geographic According to the model by Zuber and Mougi-

dina caldera [Earth] or Zeus Patera [Mars]) as extent of unit Nml across ∼205 km to be the nis-Mark (1992), 0.9 × RC(Pityusa) would cor- indicated by folded unit Nml may be the result minimum diameter of the sagged plug in Pityusa respond to a magma chamber depth d of

of the interior caldera floor sagging down as a Paterae, the calculated shortening value results in 0.5–0.6 × RC(Pityusa) = 57.5–69 km. This depth

single lid or coherent “plug” within a piston- a strain of ԑ(Pityusa plug) = −0.5% to −3.4%. This range should be regarded as maximum estimate, type caldera (Fig. 3; e.g., Zuber and Mougi- strain range is compatible with the strain derived given that the magma chamber radius might be nis-Mark, 1992; Roche et al., 2000; Kusumoto from the folded layers of unit Nml within Pityusa smaller than that of the caldera (Roche et al., and Takemura, 2003, 2005; Howard, 2010; Patera (∼−1.3% to −6.9%). 2000; Hardy, 2008; Howard, 2010). Holohan et al., 2015). While this inner plug is Piston-type calderas (Fig. 3) bound by bound by a vertical or outward-dipping reverse inward-dipping normal faults tend to form above DISCUSSION AND CONCLUSIONS fault forming early during caldera collapse, it collapsing magma chambers whose roof depth The strain of −1.3% to −6.9% we derived causes little to no horizontal strain, whereas d is larger than their radius (e.g., Lipman, 1997; from the folding of the massifs in Pityusa Patera subsequently forming inward-dipping normal Roche et al., 2000; Cole et al., 2005; Kusumoto is compatible with the −0.5% to −3.4% that faults accommodate nearly all of the vertical and Takemura, 2005; Hardy, 2008; Howard, would be created in a central plug sagging into movement and cause the majority of exten- 2010). This not only results in the above-men- Pityusa Patera assuming a piston-type caldera sion and shortening of the outer and central tioned compressive stress affecting the interior model. Based on their texture and morphology, caldera zones, respectively (e.g., Kusumoto plug of the caldera, but also is in agreement with the layered, folded massifs lend themselves and Takemura, 2003; Hardy, 2008; Howard, an earlier axisymmetric finite-element model to an interpretation as volcanic deposits, pos- 2010). Therefore, assuming that all horizontal of the stress field in Zeus Patera, the oldest and sibly pyroclastics of a caldera-forming erup- compression is accommodated by it, the central largest of Olympus Mons’s summit calderas tion sequence. Because their exclusive location plug is shortened according to the dip angle (Zuber and Mouginis-Mark, 1992). The model within Pityusa Patera indicates these deposits to of the normal fault as it is sagging into a nar- showed that the change in surface stress indi- be genetically related to patera formation, and rowing funnel during and after collapse. In a cated by ridges giving way to graben at half of because such deposits would not be expected

simplified, axisymmetric case, this shortening, Zeus Patera’s best-fit radius R( C) is not strongly in the previously suggested alternative scenario S, can be expressed by: sensitive to the magma chamber’s aspect ratio, of patera formation by subsidence from litho- internal pressure, or relative stiffness but is spheric loading (Crumpler et al., 1991, 1996; 2D S = , (1) highly dependent on the radius and depth of Larson, 2007), we interpret Pityusa Patera as an tanθ the chamber. Therefore, this stress transition at ancient collapse caldera, potentially the largest

where θ is the dip angle of the normal fault and a certain fraction of RC can inform the depth and oldest of its kind on Mars. D is the rim-to-floor depth of the caldera. Experi- d to the roof of the magma chamber (assum- According to the axisymmetric finite-ele- ments and terrestrial observations have shown ing the chamber width to be ∼0.75× to 1× ment model by Zuber and Mouginis-Mark (1992), the extent of shortening structures on a caldera floor relative to its total diameter also informs the roof depth of the collapsed magma chamber beneath it, which would imply Pity- usa Patera’s chamber was at a depth of 57.5– 69 km. This would correspond to the current crustal thickness, for which models predict a moderate local thinning within Pityusa Patera to ∼55–60 km (Parro et al., 2017). Although crustal thickness might have been different during patera formation at ≥3.8 Ga, models suggest it has not changed much since then (e.g., Breuer and Spohn, 2003), therefore put- ting the Pityusa magma chamber at the crust- mantle boundary. This contrasts with Olympus Mons or comparable volcanoes on Earth such as Kīlauea (Hawaiʻi), whose magma chambers were estimated to be within the edifice; i.e., at depths of ≤16 km (e.g., Zuber and Mouginis- Figure 3. Schematic illustration of piston-type caldera structure including geometric parameters Mark, 1992). On Venus, crust-mantle–boundary explained in text. RC is the caldera radius; d is the depth to the roof of the magma chamber, θ magma chambers have been invoked to explain is the dip angle of the normal ring fault bounding the caldera, and D is the depth of the caldera. the diameters and depths of its largest calderas

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J., 2017, Present-day heat flow model of Mars: Elkins Tanton, L.T., Grove, T.L., and Donnelly-Nolan, Nature Scientific Reports, v. 7, 45629, https://doi​ dicted mantle-fed volcanism facilitated by deep J., 2001, Hot, shallow mantle melting under the .org/10.1038/srep45629. ring fractures and mantle upwelling caused by Cascades volcanic arc: Geology, v. 29, p. 631– Peterson, J.E., 1978, Volcanism in the Noachis-Hellas the Hellas impact event (e.g., Peterson, 1978; 634, https://doi​.org/10.1130/0091- region of Mars, 2: Proceedings of the 9th Lunar Williams et al., 2009). If unit Nml represents 7613(2001)029<0631:HSMMUT>2.0.CO;2. and Planetary Science Conference, Houston, Gwinner, K., Scholten, F., Spiegel, M., Schmidt, R., Texas, March 13–17, 1978: New York, Pergamon deposits derived from such a mantle source, Giese, B., Oberst, J., Heipke, C., Jaumann, R., Press, v. 3, p. 3411–3432. hyperspectral observations uncompromised by and Neukum, G., 2009, Derivation and valida- Plescia, J.B., 2003, Amphitrites-Peneus Paterae/Malea dust and revealing corresponding signatures tion of high-resolution digital terrain models Planum geology: Abstract 1478 presented at the (e.g., high-Mg olivine and low-Ca pyroxene) from Mars Express HRSC data: Photogrammetric 34th Lunar and Planetary Science Conference, that are spatially associated with units Nml’s Engineering & Remote Sensing, v. 75, p. 1127– League City, Texas, 17–21 March. 1142, https://doi.org/10.14358/PERS.75.9.1127​ . Roberts, K.M., and Head, J.W., 1990, Lakshmi Pla- ridges, i.e., layers, would be vital to further this Hardy, S., 2008, Structural evolution of calderas: num, Venus: Characteristics and models of origin: discussion, but are not available at present. In Insights from two-dimensional discrete element Earth, , and Planets, v. 50–51, p. 193–249, conclusion, we suggest Pityusa Patera to be simulations: Geology, v. 36, p. 927–930, https:// https://doi​.org/10.1007/BF00142395. one of the oldest extant volcanic landforms on doi​.org/10.1130/G25133A.1. Roche, O., Druitt, T.H., and Merle, O., 2000, Experi- Head, J.W., and Pratt, S., 2001, Malea Planum Hes- mental study of caldera formation: Journal of Mars and one of the largest calderas in the solar perian Volcanic Province: Characterization using Geophysical Research, v. 105, p. 395–416, system, which makes the folded, likely mantle- MOLA data: Abstract 1627 presented at the 32nd https://doi​.org/10.1029/1999JB900298. derived deposits on its floor a prime target for Lunar and Planetary Science Conference, Hous- Smith, D.E., et al., 2001, Mars Orbiter Laser Altim- future exploration. ton, Texas, 12–16 March. eter: Experiment summary after the first year of Head, J.W., III, and Wilson, L., 1986, Volcanic pro- global mapping of Mars: Journal of Geophysical cesses and landforms on Venus: Theory, predic- Research, v. 106, p. 23,689–23,722, https://doi​ ACKNOWLEDGMENTS tions, and observations: Journal of Geophysi- .org/10.1029/2000JE001364. This work was funded by the German Research Foun- cal Research, v. 91, p. 9407–9446, https://doi​ Tanaka, K.L., and Scott, D.H., 1987, Geologic map dation (grant number BE 6457/1-1) and conducted at .org/10.1029/JB091iB09p09407. of the polar regions of Mars: U.S. Geological the Ronald Greeley Center for Planetary Studies at Holohan, E.P., Schöpfer, M.P.J., and Walsh, J.J., 2015, Survey Geologic Investigations Map I-1802-C, Arizona State University (USA). We would like to Stress evolution during caldera collapse: Earth scale 1:920,425, https://doi.org/10.3133/i1802C​ . extend special thanks to Christian Klimczak from the and Planetary Science Letters, v. 421, p. 139– Tanaka, K.L., Skinner, J.A., Dohm, J.M., Irwin, R.P., University of Georgia (USA) for his input concerning 151, https://doi​.org/10.1016/​j.epsl.2015.03.003. III, Kolb, E.J., Fortezzo, C.M., Platz, T. Michael, our structural measurements. We are also very grate- Howard, K.A., 2010, Caldera collapse: Perspectives G.G., and Hare, T.M., 2014, Geologic map of ful to Walter S. Kiefer from the Lunar and Planetary from comparing Galápagos volcanoes, nuclear- Mars: U.S. Geological Survey Scientific Inves- Institute, as well as to four anonymous reviewers for test sinks, sandbox models, and volcanoes on tigations Map 3292, scale 1:20,000,000, with their valuable feedback. Mars: GSA Today, v. 20, no. 10, p. 4–10, https:// 43 p. text, https://doi​.org/10.3133/sim3292. doi​.org/10.1130/GSATG82A.1. Williams, D.A., et al., 2009, The Circum-Hellas Vol- REFERENCES CITED Kiefer, W.S., 2004, Gravity evidence for an extinct canic Province, Mars: Overview: Planetary and Barretto, J., Wood, R., and Milsom, J., 2020, Benham magma chamber beneath Syrtis Major, Mars: A Space Science, v. 57, p. 895–916, https://doi​ Rise unveiled: Morphology and structure of an look at the magmatic plumbing system: Earth and .org/10.1016/​j.pss.2008.08.010. 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