Patrick J. McGovern

Sapas Mons is a large shield volcano in Atla Regio on Venus. Its summit region is partially encircled by a system of pits and extensional faults (graben). The existence and configuration of this system have been attributed to stresses generated above the margin of a magma chamber spanning the region beneath the summit. The proposed stress‐generating mechanism includes either withdrawal of magma from or solidification of magma within such a chamber (Keddie & Head, 1994, https://doi.org/10.1007/bf00644896). To explore these hypotheses, we calculate Finite Element Method models of stresses and deformations resulting from magma chamber depressurization beneath a Sapas Mons‐sized edifice with axisymmetric geometry. For a range of magma chamber depths and vertical thicknesses, we determine the minimum under pressure that produces a stress state predicting failure in circumferential normal mode at the observed position of the graben system. We also determine maximum under pressure under two conditions: (1) no failure (thrust fault mode) predicted at the summit, and (2) predicted failure (thrust fault mode) limited to within 10 km of the summit. We find that successful models require sill‐like chamber geometry with vertical thicknesses 66:1), and chamber depth <8 km beneath the summit. Calculated reductions in chamber volume are comparable to volumes of late‐stage eruptive units mapped at Sapas Mons, favoring the magma withdrawal hypothesis for graben system formation. Evidence that unit 5 of Keddie and Head (1994, https://doi.org/10.1007/bf00644896) overlapped in time with, but largely postdated, the graben forming event renders it the most likely destination for magma removed from the chamber.

Sputnik Planitia on Pluto is a vast plain consisting of a nitrogen ice deposit filling a broad topographic depression, likely an impact basin. The basin displays a broad, raised rim and is surrounded by numerous extensional fault systems, each with characteristic orientations with respect to the basin center. The nitrogen ice exerts a large mechanical load on the water ice outer shell crust (here also containing the lithosphere). We calculate models of stress and deformation related to this load, varying dimensional, mechanical, and boundary condition properties of the load and Pluto's lithosphere, in order to constrain the conditions that led to the formation of the observed tectonic and topographic signals. We demonstrate that the tectonic configuration is diagnostic of a particular set of conditions that hold for the Sputnik basin and Pluto, including moderate elastic lithosphere thickness (40–75 km, with higher values favored if initial basin topography is compensated) and a basin that was pan‐shaped and shallow (∼3 km) at the time of nitrogen deposition initiation. These tectonic systems show the contributions of both flexural (bending) and membrane (stretching) responses of the lithosphere, with the latter dominating in proportion to the importance of spherical geometry effects. Rim topography may also show an influence of primordial annular trans‐basin ice shell thickening from the impact process. Analysis of stress‐driven cryomagma transport shows that loading stresses can facilitate ascent of cryomagmas in annular zones around the basin, the locations of which overlap the observed distances from Sputnik of several candidate cryovolcanic sites.

Sputnik Planitia on Pluto is a vast plain consisting of a nitrogen ice deposit filling a broad topographic depression, likely an impact basin. The basin displays a broad, raised rim and is surrounded by numerous extensional fracture systems, each with characteristic orientations with respect to the basin center. The nitrogen ice exerts a large mechanical load on the water ice outer shell crust (here also containing the lithosphere). We calculate models of stress and deformation related to this load, varying dimensional, mechanical, and boundary condition properties of the load and Pluto’s lithosphere, in order to constrain the conditions that led to the formation of the observed tectonic and topographic signals. We demonstrate that the tectonic configuration is diagnostic of a particular set of conditions that hold for the Sputnik basin and Pluto, including moderate elastic lithosphere thickness (50 ± 10 km) and a wide load set into a basin that was pan-shaped and shallow (~3 km) at the time of nitrogen deposition initiation. These tectonic systems show the contributions of both flexural (bending) and membrane (stretching) responses of the lithosphere, with the latter dominating in proportion to the importance of spherical geometry effects (i.e., wide loads). Rim topography may also show an influence of primordial annular trans-basin ice shell thickening from the impact process. Analysis of stress-driven cryomagma transport shows that loading stresses can facilitate ascent of cryomagmas in annular zones around the basin, the locations of which overlap the observed distances from Sputnik of several candidate cryovolcanic sites.

The NASA InSight mission, landing in late November 2018, promises to revolutionize our understanding of Martian interior structure via analysis of seismic data returned by the SEIS instrument. The extent to which the mission’s potential is realized will depend on the number of detectable seismic events that occur during the period of operation. Here I estimate the rate of detectable events generated by volcano-tectonic activity on Mars based on extrapolation of Hawai’i’s seismic record. I use a catalog of 5603 earthquakes spanning 48 years [IRIS, 2018], with moment magnitudes MW ranging from 3.0 to 6.9, to derive the Gutenberg-Richter (G-R) frequency-magnitude relation for the Island of Hawai’i, expressed as log(N) = a – b MW, where N is the number of earthquakes with magnitudes greater than or equal to MW, and a and b are constants. By this analysis, one earthquake with MW 5.1 or greater can be expected every year at Hawai’i. Under the assumption that the mechanisms of seismicity associated with edifice building are similar at Hawai’i and Olympus Mons (supported by observation of decollement-based volcanic spreading at both), I use the same b for both settings and scale a according to estimates of magmatic volume flux rates dV/dt at both settings. Over the 80 Myr history of the Hawaiian-Emperor volcanic chain, dV/dt ≈ 1.7 x 10-2 km3/yr. An estimate of dV/dt for the Olympus Mons volcano on Mars was derived from paleotopographic analysis of a set of lava flows south of Olympus Mons with discordant topography. Given the mean flow age from crater counts (210 Ma), an estimate of the amount of volcanic material needed to cause deflections of flow orientations of the required magnitudes yields estimates of dV/dt over this timespan ranging from 6.33 x 10-4 to 6.43 x 10-3 km3/s. Taking the mean of these values and scaling a by the ratio of dV/dt values for Mars and Hawai’i yields a rate of at least 1 quake of MW = 4.4 or greater per year (Figure 1). Thus, under several assumptions (including a steady recent magma supply rate for Olympus Mons), we can expect ≈ 2 volcano-tectonically driven quakes of magnitude MW > 4.4 from the vicinity of Olympus Mons during the nominal 2 Earth-year InSight prime mission. This is a conservative lower bound that does not consider contributions from numerous potential volcano-tectonic sources in Tharsis and elsewhere on Mars.

Measurements of the flow directions of 80 long lava flows in the flexural basin that surrounds Olympus Mons reveal that most are no longer aligned with the regional downslope direction but diverge by an average of 15.4° ± 9.8° in the counterclockwise direction. This misalignment is consistent with topographic change in the basin after the flows were emplaced due to subsidence centered on nearby Olympus Mons. The observed tilting indicates subsidence of 0.62 to 2.13 km at the center of Olympus Mons induced by the addition of 1.33 × 10⁵ to 1.35 × 10⁶ km³ of volcanic material or about 1–10% of the total volume of the present‐day edifice. New crater size‐frequency measurements for selected areas within the flexural basin indicate that the subsidence took place in the past 210 ± 40 Ma. These findings indicate that previously recognized Late Amazonian volcanic and tectonic events at Olympus Mons signify an important period of magmatic activity during which a substantial portion of the present‐day edifice may have been added.