A thick crustal block revealed by reconstructions of early Mars highlands

Abstract

The global-scale crustal structure of Mars is shaped by impact basins, volcanic provinces, and a hemispheric dichotomy with a thin crust beneath the northern lowlands and a thick crust beneath the southern highlands. The southern highlands are commonly treated as a coherent terrain of ancient crust with a common origin and shared geologic history, plausibly originating from a giant impact(s) or a hemispheric-scale mantle upwelling. Previous studies have quantified the contribution of volcanism to this crustal structure; however, the influence of large impacts remains unclear. Here we present reconstructions of the past crustal thickness of Mars (about 4.2 Gyr ago) where the four largest impact basins (Hellas, Argyre, Isidis and Utopia) are removed, assuming mass conservation, as well as the main volcanic provinces of Tharsis and Elysium. Our reconstruction shows more subdued crustal thickness variations than at present, although the crustal dichotomy persists. However, our reconstruction reveals a region of discontinuous patches of thick crust in the southern highlands associated with magnetic and geochemical anomalies. This region, corresponding to Terra Cimmeria–Sirenum, is interpreted as a discrete crustal block. Our findings suggest that the southern highlands are composed of several crustal blocks with different geological histories. Such a complex architecture of the southern highlands is not explained by existing scenarios for crustal formation and evolution.

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Fig. 1: A global view of the crustal structure of Mars.
Fig. 2: The radial crustal structure of Hellas Planitia.
Fig. 3: Histograms of crustal thickness of three domains (northern lowlands, southern highlands and Cimmeria–Sirenum) of the Martian crust.
Fig. 4: The geophysical and geochemical signature of the Cimmeria–Sirenum block.

Data availability

All data used in this study are publicly available on the NASA Planetary Data System Geoscience Node (topography, elemental composition; https://pds-geosciences.wustl.edu/), Planetary Plasma Interactions Node (magnetic field; https://pds-ppi.igpp.ucla.edu/index.jsp) or the NASA Goddard Space Flight Center Planetary Geodynamics Data Archive (crustal thickness; https://pgda.gsfc.nasa.gov/). Source data for the map of Mars’s crustal thickness without impact basins and volcanoes (Fig. 1e,f), the primary output of this manuscript, are provided with the paper.

Code availability

All analyses in this study were conducted using original MATLAB code, which is described in the Methods. The MATLAB code is available from J.T.K. upon reasonable request (jkeane@caltech.edu).

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Acknowledgements

We thank S. Goossens for kindly sharing crustal thickness and density maps17. This research was funded by the Programme National de Planétologie of INSU-CNRS. J.T.K. acknowledges support from the Caltech Joint Center for Planetary Astronomy postdoctoral fellowship. I.M. was financially supported by NASA under grant no. 80NSSC17K0724 issued through the NASA Solar System Workings programme. B.L. was financially supported by a project (NEWTON) that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement 730041. This research has made use of NASA’s Astrophysics Data System.

Author information

S.B. first identified the anomalous crustal structure of Cimmeria–Sirenum. S.B. and D.B. developed the hypothesis and its implications, and guided the research effort. J.T.K. created and executed the crustal thickness correction method and created all figures. The manuscript was collectively written by J.T.K., S.B. and D.B. (in order of the importance of the contribution). I.M. performed preliminary calculations of Mars without Tharsis. B.L. provided the Martian magnetic field model. All authors provided input on the manuscript and the broader implications of this work.

Correspondence to Sylvain Bouley.

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The authors declare no competing interests.

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Peer review information Primary Handling Editors: Tamara Goldin; Stefan Lachowycz.

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Extended data

Extended Data Fig. 1 Topography and crustal structure around Hellas Planitia.

a, MOLA topography of Mars (ref. 9). b, crustal thickness of Mars based on crustal model B (ref. 17, Methods). In a–b, the maps are in Lambert azimuthal equal-area projection, centred on Hellas Planitia. Each map covers all of Mars except for a small region antipodal to the map centre. Maps are draped over present-day topography for reference. c, radial profiles of surface and mantle topography measured from the centre of Hellas. d, radial profiles of crustal thickness. In c–d, each coloured line represented a different radial profile, coloured by the azimuth of that profile. The white lines are the mean of the surface and mantle topography profiles.

Extended Data Fig. 2 Topography and crustal structure around Utopia Planitia.

a, MOLA topography of Mars (ref. 9). b, crustal thickness of Mars (ref.17). In a–b, the maps are in Lambert azimuthal equal-area projection, centred on Utopia Planitia. Each map covers all of Mars except for a small region antipodal to the map centre. Maps are draped over present-day topography for reference. c, radial profiles of surface and mantle topography measured from the centre of Utopia. d, radial profiles of crustal thickness. In c–d, each coloured line represented a different radial profile, coloured by the azimuth of that profile. The white lines are the mean of the surface and mantle topography profiles.

Extended Data Fig. 3 The parameter space for finding the mass-conserving crustal structure for Hellas Planitia.

a, the fitting parameter space. Colour and contours show how well each tested model conserves mass: reds are models dominated by mass excesses, blues are models dominated by mass deficits, and green are models where mass is approximately conserved. Contour levels are indicated by tick marks in the colour bar. The yellow dash-dot region indicates the confining region where we look for the best mass-conserving solutions that are plausibly the result of Hellas. b–h, example solutions from the parameter space (marked with an ‘x’ in panel a). Symbols and lines are the same as in Fig. 2, and are labelled in panel c. In this example model run, panel c is the best-fitting model.

Extended Data Fig. 4 Mass balance for each feature of interest removed in our crustal reconstruction.

a, mass excess (red) and mass deficit (blue) for each feature of interest. b, the total mass excess (mass excess + mass deficit) for each feature of interest. Mass-conserving structures have a total mass excess of zero. c, the ratio between mass excess and mass deficit. Mass-conserving structures have a ratio of one.

Extended Data Fig. 5 Crustal thickness map and simplified radial profile after the removal of each feature.

a, Mars today. b, removing Elysium Mons. c, removing Olympus Mons. d, removing Pavonis Mons. e, removing Arsia Mons. f, removing Ascraeus Mons. g, removing Alba Patera. h, removing central Tharsis. i, removing Tharsis rise. j, removing Argyre. k, removing Isidis. l, removing Hellas. m, removing Utopia. n, removing Borealis. On simplified radial profiles, the mass excess is in red and the mass deficit in blue.

Extended Data Fig. 6 Boundary of the Cimmeria-Sirenum block with >50 km thick crustal regions indicated in green (c-d).

The question mark to the east of the block indicates the unclear boundary between the Thaumasia region and the Cimmeria-Sirenum block due to the possible different origin of Thaumasia19.

Extended Data Fig. 7 Topography before Tharsis.

a–b, MOLA topography of Mars (ref. 9), with the Cimmeria–Sirenum region outlined. c–d, topographic model of Tharsis, and the rotational deformation arising from TPW due to the formation of Tharsis (ref.20). e–f, topography of Mars without the effects of Tharsis. Panel e is equal to panel a minus panel c; panel f is equal to panel b minus panel d. The Cimmeria–Sirenum region has anomalously high topography in this corrected map. This early work hinted at the unusual nature of this terrain. Maps are in Lambert azimuthal equal-area projection, centred on 0°E (left column) and 180°E (right column). Each map covers all of Mars except for a small region antipodal to the map centre. Maps are draped over present-day topography for reference. Grid lines are in increments of 30° of latitude and longitude.

Source data

Source Data Fig. 1

Mars’s crustal thickness without impact basins and volcanoes (Fig. 1e-f), as described in the text. The file is an ascii table of corrected crustal thickness (in kilometers) as a function of longitude (0°E to 360°E) and latitude (−90°N to 90°N). The file is 1350 (latitude) x 2700 (longitude), and the first entry is −90°N, 0°E. This resolution of this map is higher than the actual resolution of the crustal thickness data for data visualization purposes.

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Bouley, S., Keane, J.T., Baratoux, D. et al. A thick crustal block revealed by reconstructions of early Mars highlands. Nat. Geosci. (2020). https://doi.org/10.1038/s41561-019-0512-6

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