Lunar and Planetary Science XXXI 1854.pdf

COLLAPSE FEATURES AND NARROW GRABENS ON MARS AND VENUS: DIKE EMPLACEMENT AND DEFLATION OF UNDERLYING MAGMA CHAMBER. D. Mege1, Y. Lagabrielle2, E. Garel3, M.-H. Cormier4 and A. C. Cook5, 1Laboratoire de Tectonique, ESA 7072, Universite Pierre et Marie Curie, case 129, 75252 Paris cedex 05, France, e-mail: [email protected], 2Institut de Recherche pour le Developpement, Geosciences, BPA5, Noumea, New Caledonia, e-mail: [email protected], 3Institut Europeen de la Mer, UMR 6538, Universite de Bretagne Occidentale, Place Nicolas Copernic, 29280 Plouzane, France, e-mail: [email protected], 4Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, e-mail: [email protected], 5Center for Earth and Planetary Studies, National Air and Space Museum, Washington, D. C. 20560, e-mail: [email protected]

Summary: Grabens in the Tharsis region of Mars interpretation. However, the mechanism of deflation for a displaying aligned pit craters, and elongated troughs, cannot single event, resulting in a single collapse event, does not be explained by regional tectonic extension only. Similar necessarily imply spreading, and therefore may be compared grabens and troughs are observed at the summit of fast to a collapse event at a Martian graben. Crustal and lithosphere spreading ridges, such as the East Pacific Rise (EPR), where rheology used in experiment models must merely be adapted bathymetric and seismic data, as well as experimental to the Martian rheological conditions. Below we describe the modelling, suggest that they result from deflation and rheological conditions that we find the most likely, report compaction of an elongated magma body underneath. Scaled results from scaled experimental modelling, and compare to experimental modelling confirms analogy between observations. mechanisms of graben, pit, and trough formation at Tharsis Rheology of the shallow Martian crust: Brittle crust at and the EPR, and suggests a sequence of volcanic-tectonic the EPR, 2-3 km thick, is made of volcanic flows overlying events that might also apply to similar features observed at dikes. It is underlain by a ductile layer of deformable gabbros some Venusian grabens, e. g. at coronae. located below the 750-800°C isotherm. At the axis itself, the Introduction: Some narrow grabens on Mars display semi-liquid crystal mush is overlain by a thin magma lens in inner pits, coalescing pits, and linear closed depressions [1-3]. turn overlain by a 2-km thick brittle crust [10]. Models of formation include maar explosion [3, 4], collapse in Geophysical models suggest that the elastic Martian crust subsurface tension fractures [2, 5], compaction induced by is tens of km thick [12, 13]. How much thick the brittle crust permafrost heating and removal [3, 6-8], and pressure drop at was while the grabens, pits and troughs formed is hard to upper dike tip [3, 9]. For reason extensively discussed determine. The geotherm should have been higher, due to elsewhere [e. g., 3], we have found that the pits and troughs in higher mantle temperature and to the Tharsis plume thermal the Tharsis region are better explained by some mechanism anomaly. Existence of a ductile crustal layer is likely, but involving collapse above dikes forming giant swarms where the brittle-ductile transition was compared to the depth radiating from major magma centers. of the postulated magma reservoir remains conjectural. Large summit troughs of seemingly similar dimension Permafrost may have been the main rheological contrast level ratios, although one order of magnitude smaller, have been in the upper crust, however it should not affect its brittle reported at the EPR [10]. Analysis of bathymetric and seismic behavior. The crustal rheology model we need therefore to data suggests that they represent elongated collapsed calderas consider includes either a single brittle layer or a brittle layer that form when the melt supply to formerly inflated axial underlain by a ductile layer in contact with the magma magma chambers wanes or nearly ceases. Dike emplacement reservoir. The latter crustal structure is similar to the crustal in the overlying crust favors hydrothermal circulation and structure at the EPR. The main uncertainty is therefore on gradual deepening of the magma reservoir roof, and dike layer Martian crustal thickness and depth of the brittle-ductile thickening and gravitational collapse of the brittle crust along transition. the fissures. Steady state seafloor spreading during a Scaled analog modelling: Three series of scaled prolonged interval of waning magma supply causes the whole experimental modelling of caldera collapse were carried out, magma reservoir (a magma lens overlying a semi-solid crystal with scale ratio 1/40k [11, 14]. In the first series, brittle crust mush, i. e. future gabbros) to stretch, resulting in further was modelled using fine-grained dry Fontainebleau sand caldera collapse following the geometry of the magma having Mohr-Coulomb rheology. The sand layer was given a reservoir at depth [10]. Scaled experimental modelling [11] dome shape that approximates the morphology of fast has confirmed the plausibility of this model, and allows to spreading ridges. An inflatable elongated latex balloon filled better understand the role of deep processes in the generation with water accounted for low viscosity magma reservoir and sequence of development of grabens and collapse troughs. having variable volume. Magma reservoir deflation was Extrapolation to pits and troughs on Mars is not accounted for by balloon withdrawal. In the second series, the straightforward, as morphological and structural analysis of apparatus was modified by replacing the fixed walls the Martian grabens is hardly consistent with a spreading ridge perpendicular to the balloon orientation by motor-driven Lunar and Planetary Science XXXI 1854.pdf

DIKE EMPLACEMENT AND DEFLATION OF UNDERLYING MAGMA CHAMBER. D. Mege, et al.

mobile walls. The apparatus in the third series of experiments as follows: (1) high extension rate compared to deflation rate is similar, but a layer of silicone putty of selected density and to the South, (2) waning extension and deflation rates in the viscosity was taken as a viscous analog to the rocks located at central area, and (3) magma eruption to the North, forming the the brittle-ductile transition between the highly deformable ridge. Ridge formation would express a single episode of crystal mush and the solid roof. lateral crustal accretion, similar to a single episode of magma Layer thicknesses were adjusted to the EPR case between accretion at dome-shaped fast-spreading ridges on Earth [10]. latitudes 17°56'S and 18°35'S. Variation of magma reservoir depth along the EPR was accounted for by varying sand layer thickness. Due to the absence of reliable data on layer thicknesses for Mars, the resulting crustal models were taken as several of many possible crustal models. Results: The best approximation to shallow graben formation, and central pit crater or elongated trough formation, is obtained in the experiments in which the whole crust is brittle. Sequence of events. (1) 15-35% withdrawal: formation of an inner collapse trough. The collapse trough width is either smaller or similar to the lateral extent of the balloon. The trough is bounded by reverse, outward dipping faults and is immediately subject to landsliding. Reverse faults curve inward at depth and become normal faults near the reservoir. (2) 45-65% withdrawal: formation of an external graben bounded by normal faults having greater offset than the reverse fault offsets. (3) Final stage: development of small Figure 1 – Graben and troughs at Noctis Labyrinthus. VO 47A20, 100 lateral normal faults parallel to the graben border faults m/px, area displayed 85x60 km between the collapse trough and the graben. Future research: We plan to (1) create high height and In the models containing a ductile layer, the sequence of spatial resolution DEMs from Viking/MOC stereo imagery events is pretty much similar. The main difference is that those fitted to MOLA profiles [15] in order to better characterize the models predict formation of narrow grabens on both side of morphometry of grabens, pits, and troughs, (2) perform further the collapse trough instead of single normal faults, which experimental, as well as three-dimensional boundary element provides a good approximation to tectonics at the EPR, but modelling of volcanic-tectonic events to better constrain the does not reflect the structural patterns usually observed on mechanisms involved at depth. Mars. Additional observations of interest. (1) Balloon depth is References: [1] Tanaka K. L. and Golombek M. P. (1989) Proc. proportional to border fault spacing and vertical offsets, and 19th LPSC., 383-396. [2] Davis P. A., Tanaka K. L. and Golombek inversely proportional to the width of the collapse trough. (2) M. P. (1995) Icarus, 114, 403-422. [3] Mege D. and Masson P. (1996) Remote extension has no visible effect on the morphology and Planet. Space Sci., 44, 1499-1546. [4] McGetchin T.R. and Ullrich G. tectonics induced by the quicker collapse process. That is, W. (1973) JGR, 78, 1833-1853. [5] Battistini R. (1985) in Ices in the graben formation occurs even if remote extension is Solar System, Reidel, 156, 607-617. [6] Montesi, L. (2000) in negligible. In the absence of remote extension, vertical offsets Locating pre-Mesozoic Mantle Plumes, GSA Sp. Pap. (in press). [7] of border faults are observed to be ca. 2 times larger than Scott E. D. and Wilson L. (1999) JGR, 104, 27079-27089. [8] Mege reverse fault offsets. (3) If extension is applied after a first D. and Masson P. (1997) LPSC XXVIII, 929-930. [9] Liu S. Y. and stage of single withdrawal, the preexisting collapse structures Wilson L (1998) LPSC XXIX (CD-ROM), #1602 [10] Lagabrielle, will widen, but no new structure is generated. (4) When the Y. and Cormier M.-H (1999) JGR, 104, 12,971-12,988. [11] extension rate is high compared to the deflation rate, a central Lagabrielle Y., Garel E., Dauteuil O. and Cormier M. H., JGR horst develops that is surrounded by two lateral grabens. (submitted). [12] Banerdt W. B., Golombek M. P. and Tanaka K. L. Application to Mars: Figure 1 shows an example of (1992) in Mars, Univ. Arizona Press, 249-297. [13] Mege D. and graben, pit, and trough interpretation at Noctis Labyrinthus. Masson P. (1996) Planet. Space Sci., 44, 1471-1497. [14] Garel, E. Graben spacing, fault offsets, width of collapse troughs, are (1998) Univ. Bretagne Occidentale, DEA Rept., 78 p. [J] Schultz R. A. used to infer subsurface structure. (1995) Rock Mech. Rock Engng., 28, 1-15. [15] Cook A. C., Mege D. Another example is observation of a northern domal ridge and Craddock R. A. (this CD-ROM). in the prolongation of a graben-collapse trough displaying an inner linear topographic ridge (MGS-MOC image 41305). [Abstract published in Lunar and Planetary Science, XXXI, CD- Ridge topography tapers from South to North. Interpretation is ROM, Lunar and Planetary Institute, Houston (2000).]