State of stress, faulting, and eruption characteristics of large volcanoes on Mars
Authors: McGovern Patrick J. , Solomon Sean C. 
Published: February 2008

The formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting. In turn, lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport. Modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes. We develop models of time‐dependent stress and deformation for the Tharsis volcanoes on Mars. By means of a finite element code, we calculate stresses and displacements due to a volcano‐shaped load emplaced on an elastic plate overlying a viscoelastic mantle. Models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere. The models illustrate the manner in which time‐dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories, and the derived stress fields can be related to tectonic features on and surrounding Martian volcanoes. As a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting, according to the Mohr‐Coulomb criterion: at the surface of the plate just outward of the volcano, near the base of the elastic lithosphere beneath the center of the volcano, and on the upper flanks of the volcano early in its growth history. Normal faulting is the dominant mode of failure predicted for the first region, consistent with circumferential graben observed around the Tharsis Montes and with the scarp at the base of Olympus Mons, interpreted as a large‐offset, listric normal fault. Normal faulting, mostly radially oriented, is predicted for the second region. Failure in the third region is predicted to consist of thrust faulting, circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks. In models simulating a growing volcano, this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail; this volume of early failure remains the most highly stressed area in the edifice. Concentric terraces, interpreted by some workers as thrust faults, on the upper flanks of Olympus Mons may correspond to the predicted circumferential thrust features, if the most recent increments of volcano growth were relatively large, or in the presence of local material property or stress field variations. For volcanoes detached from the plate, predicted failure in the edifice takes the form of radial normal faulting near the volcano base. The addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the Tharsis Montes, including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben. This stress state is also consistent with an interpretation of the aureole deposits of Olympus Mons as the result of gravity sliding along a basal detachment. Our models also suggest an explanation for the lack of strike‐slip features, predicted by previously published flexural models, around the Tharsis volcanoes. For a given load increment, the first mode of near‐surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation. As the volcano grows and the flexural response to the increasing load proceeds, the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike‐slip faulting. Because normal faulting has been predicted to have taken place earlier, however, it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike‐slip faults.