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CONCENTRIC DIKE SWARM and INTERNAL STRUCTURE of PAVONIS MONS (MARS). L. G. J. Montési, Massachusetts Institute of Technology, 5

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CONCENTRIC DIKE SWARM and INTERNAL STRUCTURE of PAVONIS MONS (MARS). L. G. J. Montési, Massachusetts Institute of Technology, 5 Lunar and Planetary Science XXX 1251.pdf CONCENTRIC DIKE SWARM AND INTERNAL STRUCTURE OF PAVONIS MONS (MARS). L. G. J. Montési, Massachusetts Institute of Technology, 54-521, Cambridge, MA 02536; [email protected] Introduction: The Tharsis Montes and Olympus with the summit caldera of Pavonis (at least the Mons are the most recent major volcanic shields on smaller, more recent one), which is likely above the Mars [e.g., 1]. They are thought to represent several level of the plains (Fig 4) stages of evolution of Martian shields, according to State of stress of Tharsis Volcanoes: Most dikes the degree of embayment of the flanks by late rift trace a concentric pattern around the volcano, which zones [2] and the development of a lobate unit, possi- implies radial least compressive stress. Possible ble flank slumping [3]. This analysis treats another sources for that state of stress include: 1) gravitational feature of the volcanoes, the system of concentric gra- relaxation of the flanks e.g., [6]; 2) topographic bens and fracture, using Viking Orbiter images mostly stresses associated with caldera walls [7]; 3) regional of Pavonis Mons. We address the morphological ori- stresses e.g., [8]; 4) magma chamber inflation (uplift) gin of these fractures and their implications for the [9]; 5) underplating [10]; 6) burial of the lower flanks. structure and state of stress of the edifice. Comparison We consider that a combination of mechanisms (4) with other volcanoes indicates that Olympus Mons and (6) best explains the intrusions. Mechanism (3) is and Arsia Mons followed different evolutionary paths probably at the origin of the rift zones them selves, rather than being end-members of the same. and may have influenced the emplacement of more Evidence for concentric dike swarms: The linear swarms at the western edge of Pavonis Mons, structures usually described as graben and fractures Arsia Mons, and Biblis Patera. The concentric swarm display a variety of morphologic structures such as: a) of Biblis Patera is better explained by a combination of simple graben, with flat floored and antithetic bound- mechanisms (2) and (6), as for the Galápagos on ing faults; b) linear U-shaped troughs, usually deeper Earth [7]. Mechanism (5) has been invoked to explain and narrower than the grabens; c) pit chains; d) coa- the fracturing of Alba Patera [10]. lescing pit chains, resembling linear troughs (Fig Flank terraces are seen as alternative to concentric 1,2,3). Transitions in style along trends or within the dike swarms on Olympus Mons and Ascræus Mons. same general pattern indicates a similar origin for all They may represent thrust [11] or normal [12] faults. the above morphologic characteristics. The interaction Both cases can be explained from the absence of burial of an underlying dike and the Martian permafrost [4] of the last layer and substitution of mechanism 6) by can provide the required mechanism: dike intrusion either the natural compression of the edifice [11] or produces extension at the surface, and possible graben mechanism 1). Normal faults on many terrestrial vol- formation [5], and the volatilization of a water or ice canoes, in particular Kilauea, seem to result from layer can produce collapse features (pits) tracing the gravitational spreading. dike, possibly guided by surface fractures (troughs). General evolution: Arsia Mons again appears to Phreato-magmatism is an alternative explanation of have evolved further than the other volcanoes in terms the pits. Arsia Mons also displays a set of concentric of the rift zone and dike system being more exten- graben, Ascræus Mons troughs at the base of the edi- sively developed. However, Olympus Mons is a more fice, and Biblis Patera concentric troughs near the evolved edifice in at least two aspects: the terraces and summit. the caldera. Terraces indicate activity posterior to the Internal structure: Troughs near the base of the plain emplacement, independent of the favored mode volcano often terminate as pit chains in the direction of formation of the terraces. Terraces on Ascræus of the linear rift zones (Fig 2), requiring rapid arrival Mons formed within a unit emplaced above the of the dike, and sinuous rilles at the other end, which troughed shield, and not covered by the regional implies magma eruption. Also, the structures are nar- plains, although it is partially embayed by the flank rower near the base of the volcano, indicating deeper rifts. Arsia Mons also had a period of resurfacing to dikes [5]. It is possible to reconstruct the trajectory of the east posterior to the graben formation. The Olym- dikes inside the edifice: they originate at depth near pus-type caldera of Pavonis Mons is within the Arsia- the rift zone, propagate upward, and then curve to type caldera [13], and the associated magma chamber wrap along the flanks. seems shallower, indicating a late transition from Ar- The dike trajectories and depths imply a magma sia-like to Olympus-like structures. chamber beneath the rift zones below the level of the In summary, Arsia Mons and Olympus Mons do surrounding plains. Such a magmatic plumbing sys- not appear to be end-members of a single trend of tem configuration differs from the chamber associated evolution of shield morphology, but rather followed Lunar and Planetary Science XXX 1251.pdf Concentric Dikes on Pavonis Mons: L. G. J. Montési two divergent path. A less evolved volcano may look Dieterich J. H. (1995) J. Volcanol. Geotherm. Res., similar to Biblis Patera. The evolution as the volcanic 66, 37-52. [8] McGuire, W. J. and Pullen, A. D. province ages would be towards shallower magma (1989), J. Volcanol. Geotherm. Res., 38, 325-344; chambers, Olympus-type calderas, late resurfacing of Russo G. et al. (1996) in Volcano instability on the the edifice and formation of terraces, whereas the high Earth and Other Planets, Geol. Soc. Spe. Pub. 110, regional stress around the Tharsis Montes may lead to 65-75. [9] Gudmunsson A. et al. (1997) GRL., 23, a different evolution with rifting of the flanks and 1559-1562; Gudmunsson A. (1998) JGR., 103, 7401- possibly short-circuiting of the shallower chamber. 7412. [10] McGovern et al. (1998) EOS Suppl., 79, [1] Plescia J. B. and Saunders R. S. (1979) Proc. F526 (abstr.) [11] Thomas, P.J. et al. (1990) JGR., 95, LPSC X, 2841-2859; Greeley R. and Spudis P. D. 14345-14355; McGovern P. J. and Solomon S. C. (1981) Rev. Geophys. Space Phys., 19, 13-41; Mougi- (1993) JGR., 98, 23553-23579. [12] Cipa A. et al. nis-Mark P. J. et al. (1992) in Mars, 424-452 (1996) Compte-Rendus Acad. Sci. Paris II, 315, 369- [2] Crumpler L. S. and Aubele J. C. (1978) Icarus, 376. [13] Crumpler L. S. et al. (1996) in Volcano in- 34, 496-511. [3] Zimbelman, J. R. and Edgett, K. S. stability on the Earth and Other Planets, Geol. Soc. (1992) Proc. LPSC XXII, 31-44. [4] Mège D. and Spe. Pub. 110, 307-348. Masson P. (1996) Planet. Space Sci., 44, 1499-1546. Acknowledgments: This work was initiated in [5] Pollard D. D. and Holzhausen, G. (1979), Tecto- the Laboratoire de Pétrologie-Volcanologie of the nophysics, 53, 27-57; Pollard D. D. et al. (1983), University of Paris-Sud under the direction of J. Bé- Tectonophysics, 94, 541-584; Mastin, L. G. and Pol- bien and B. Bonin as a stage de DEA Géodynamique lard, D. D. (1988), JGR, 93, 13221-13235; Rubin, A. et Physique de la Terre. I thank D. Mège for signifi- M. (1992), JGR, 92, 1839-1858. [6] Borgia, A. (1994) cant discussion and M. T. Zuber for support during JGR, 99, 17791-17804. [7] Chadwick, W. W. and the later part of this project. Figure 1: Simple grabens on the upper west flank of Figure 2: Concentric troughs on the eastern flank of Pavonis Pavonis Mons. Mons. The southern rift zone is towards the right. The southern termination merge into pit chains. Sinuous rilles are presents at the northern termination. Figure 3: Interpretation of the observed morphological sequence (redrawn after [4]). A dike at depth produce surface extension (grabens). If ascent it rapid enough, it may interact actively with the permafrost (phreato-magmatic pit) or, if the level of interac- tion is lower, volatilize the permafrost and form collapse pits and troughs, possibly guided by the fracturation. Figure 4: Schematic of the structure of Pavonis Mons. Black: dikes; Grey: magma chambers. Arrows denote magma propagation. The late caldera probably resulted from a different chamber than the dikes and rift zones..
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