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Caldera Subsidence and Magma Chamber Depth of the Olympus Mons Volcano, Mars

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Caldera Subsidence and Magma Chamber Depth of the Olympus Mons Volcano, Mars JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. Ell, PAGES 18,295-18,307, NOVEMBER 25, 1992 Caldera Subsidence and Magma Chamber Depth of the Olympus Mons Volcano, Mars M. T. ZUBER1 Geodynamics Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland P. J. MOUGINIS-MARK Planetary GeosciencesDivision, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu Observations of the distribution of tectonic features and their relationship to local surface topography indicate that the floor of the oldest and largest crater of the summit caldera of Mars' Olympus Mens volcano may have undergone subsidence in response to depressurizatien of a subsurface magma chamber. In this study, we construct an axisymmetric finite element model to calculate elastic stressesin a volcanic edifice to investigatethe relationshipbetween surface tectonism, caldera subsidence, and the physical characteristics of Olympus Mens' magmatic reservoir. Con- straints on the model are provided by the stressfield within the crater indicated by the distribution of radial ridges and circumferential graben that we hypothesize formed due to a postcollapse and pestresurfacing phase of subsidenceof the caldera floor. Model results show that the surface stress state is not strongly sensitiveto the aspect ratio or pressure distribution of the magma chamber, or to the contrast in stiffnessbetween the magma chamber and surroundings,but is strongly dependent on the width and depth of the chamber. For a range of plausible model parameters, we find the maximum depth to the top of the magma chamber to have been -<16 km, which indicates that the chamber, at the time of crater floor subsidence, was positioned within the Olympus Mens edifice, at a level much shallower than the estimated source depth of Martian magmas (--•140-200 km). The allowable range of solutionsindicates that the vertical position of the magma chamber may have been controlled by the neutral buoyancy level of ascending magma. Our results suggest a gross similarity between the configurations of the magmatic plumbing systems of Olympus Mens and several well-studied terrestrial volcanoes such as the Hawaiian shields. INTRODUCTION [Wilson and Head, 1988; Thomas et al., 1990]. In the vicinity of the summits of Martian volcanoes that exhibit a clear Studies of the eruptive characteristics of terrestrial basal- record of tectonism, however, models of the stress field may tic volcanoes indicate that magma is generally transported represent an alternative and arguably more direct means of from a deep, upper mantle source region to the surface via a understandingthe nature of magma accumulation and trans- shallow holding chamber [Eaton and Murata, 1960]. The port at shallow subsurface levels. configuration and depth of this chamber have implications The summit caldera of the Olympus Mons volcano, shown for the nature of magmatic transport as well as for the in Figure 1, displays one of the clearest examples of tectonic processes which initiate and sustain eruptions. In closely processesassociated with shield volcanism on Mars. Within monitored shields such as Hawaii's Kilauea volcano, the the -90 x 60 km structure are six nested craters that geometry and depth of the magma chamber have been indicate that the summit has undergone multiple collapse constrained from seismic and gravity data [Crosson and episodes [Mouginis-Mark, 1981]. Observations of the Endo, 1981; Karpin and Thurber, 1987; Klein et at., 1987; caldera include high-resolution Viking images and local Thurber, 1987;Ryan, 1988], as well as from geodetic leveling topography derived from stereophotogrammetry [Wu et at., and triangulation measurements [Dieterich and Decker, 1984] and photoclinometry [Mouginis-Mark and Robinson, 1975; Ryan et at., 1983; Delaney et at., 1990; Yang et at., 1992]. Comparison of these data sets shows that the central 1992]. portion of the largest and oldest crater (hereafter, crater 1), Shield volcanism is a common process on Mars, and the contains radial and circumferential ridges and represents a major shields have been the focus of numerous morpholog- topographic low. In contrast, the outer part of the crater is ical studies [cf. Cart, 1973; Greeley and Spudis, 1981; characterized by circumferential graben which are found on Mouginis-Mark et al., 1992]. Unfortunately, due to the a topographically elevated region along the perimeter of the absence of seismic data, high spatial resolution gravity crater floor. The total change in relief from the central ridges measurements, and measured ground displacements, analy- to peripheral graben is -•1300 m [Mouginis-Mark and Rob- ses of magma chamber positions within Martian shieldshave inson, 1992]. Tectonic features within the interior of the been limited to gravitational scaling of terrestrial volcanoes caldera as a whole are most abundant in areas with the greatest surface slopes [Watters and Chadwick, 1990]. Pho- 1Also at Departmentof Earthand Planetary Sciences, The Johns togeologicevidence for basalt-like re surfacingof the caldera Hopkins University, Baltimore, Maryland. floor [Greeley and Spudis, 1981], in combination with the Copyright 1992 by the American Geophysical Union. observed topography, has been interpreted to indicate that a Paper number 92JE01770. large lava lake within the crater has subsided in its central 0148-0227/92/92JE-01770505.00 region due to pressure reduction in the underlying magma 18,295 18,296 ZUBER AND MOUGINIS-MARK: OLYMPUS MONS CALDERA SUBSIDENCEAND MAGMA CHAMBER DEPTH .;-";':¾?i Fig. 1. Image of the Olympus Mons caldera showing six nested craters. The largest and oldest crater (referred to as crater 1 in text) has a radius of--•32 km. The tectonic features used in this study as constraints on magma chamber depthare located within the box. The resolution of theimage is approximately156 m pixel-• . Northis at thetop of the image. Illumination is from the right. (Viking Orbiter image 890A68.) chamber [Mouginis-Mark, 1981; Mouginis-Mark and Robin- possible relationship between tectonic features within the son, 1992]. Olympus Mons caldera and the nature of the magma cham- On the basis of geologic mapping and numerical modeling ber. To accomplish this, we begin by reviewing the geologic of the stress regime of the Olympus Mons caldera complex, and structural context of the caldera, then describe a simple we suggestthat certain tectonic features within crater 1 may model of magma chamber depressurization and surface preserve a record of the proposed subsidenceevent. If this is subsidencethat can explain the distribution and timing of a the case, then the stress field inferred from the features may specific assemblageof tectonic features within crater 1. We contain information about the magma chamber at the time of apply the model, with constraints provided by the observed subsidence. The objective of this study is to explore the spatial distribution of the tectonic features, to investigate the ZUBERAND MOUGINIS-MARK:OLYMPUS MONS CALDERASUBSIDENCE AND MAGMA CHAMBERDEPTH 18,297 ;""• RADIALWRINKLE RIDGE -•- CIRCUMFERENTIAL RIDGE • CIRCUMFERENTIAL GRABEN 5 km N Fig. 2a. Sketch map of tectonic features within the area of crater 1 outlined in Figure 1. The center of the crater is in the direction of the top of the image. This area was mapped from Viking Orbiter images 473S27-29 and 474S25-30. extent to which the size, shape, depth, and pressurization the radial wrinkle ridges and are often superimposedupon state of the magmatic reservoir can be determined. We find the radial ridges. Unlike superficially similar features on the that a wide range of magma chamber aspect ratios and volcano Alba Patera, which were interpreted by Catterrnole pressure distributions can produce surface stressesconsis- [1986] to be spatter ridges, we see no evidence of compara- tent with the observed pattern of tectonism and conclude ble individual constructional cones or small flows within the that neither the relative dimensions of the magma chamber Olympus Mons caldera. We interpret the Olympus Mons nor the details of its pressurization can be confidently circumferential ridges, which are distributed from the center constrained from the tectonics alone. We also find that the of the crater out to approximatelyhalf its radius (r -• 0.53Rc distribution of surface stress is a sensitive indicator of the width and depth of the magma chamber. From our model results and observations,we find the depth to the top of the magma chamber at the time of subsidence to have been within the volcanic edifice, possibly at the level of neutral buoyancy, as is the case for several well-studied terrestrial volcanic shields. CALDERA TECTONICS Structural Context Figure 2a shows a structural sketch map of crater 1. The area was mapped from high-resolution Viking Orbiter im- ages, such as Figure 2b, which have a maximum spatial resolutionof 15-20m pixel-• [Mouginis-Markand Robin- son, 1992]. The map shows three types of tectonic features. The first consists of prominent wrinkle ridges oriented radially from the center of the crater. These features are morphologically similar to wrinkle ridges that occur on spatially extensive plains units elsewhere on Mars and on the lunar maria [Greeley and Spudis, 1981; Plescia and Golombek, 1986; Watters, 1988]. The features have topo- graphic amplitudesof---300 m, average widths of---2 km, and lengths of 5-15 km. The orientation of these features indi- cates that they formed as a consequenceof circumferentially directed horizontal compression
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