Very recent and wide spread basaltic volcanism on Mars

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 38,, doi: /2011gl047310, 2011 Very recent and wide spread basaltic volcanism on Mars E. Hauber, 1 P. Brož, 2 F. Jagert, 3 P. Jodłowski, 4 and T. Platz 4 Received 1 March 2011; revised 9 April 2011; accepted 14 April 2011; published 17 May [1] New spacecraft data provide increasing evidence for a dynamic environment on present day Mars. Exogenic processes such as impact cratering, mass wasting processes, and active dune migration have all been observed to modify the surface. No traces of current endogenic activity have been found yet, but some studies point to very localized volcanism in the last few millions of years. However, no systematic of young volcanic surfaces had been performed so far. We present absolute model age determinations of plains volcanism on Mars as derived from impact crater sizefrequency distributions. Extended areas in Tharsis, the largest volcano tectonic region on Mars, have been resurfaced by lava flows in the last few tens of millions of years. We also present results on the rheologic properties of these lava flows, inferred from morphometric measurements. Yield strengths are in the range of Pa, and viscosities reach values of 10 2 to 10 3 Pa s, indicating basaltic compositions. The results imply that Mars retained until recently, and probably still retains, enough internal heat to produce wide spread plain style volcanism, producing low viscosity lava flows throughout large parts of Tharsis. Citation: Hauber, E., P. Brož, F. Jagert, P. Jodłowski, and T. Platz (2011), Very recent and wide spread basaltic volcanism on Mars, Geophys. Res. Lett., 38,, doi: /2011gl Introduction [2] The mass of Mars is smaller than that of Earth by a factor of 10, and its current internal heat production should therefore be much lower. Indeed, thermal evolution models of Mars suggest that volcanism should have stopped at least some hundred million years ago [Hauck and Phillips, 2002; Breuer and Spohn, 2003]. These models are in contradiction to several studies, however, which seem to indicate ongoing volcanic activity, perhaps as recent as a few million years ago [Hartmann, 2005; Hartmann and Berman, 2000; Hartmann et al., 1999,2008;Neukum et al., 2004;Vaucher et al., 2009a]. To account for discrepancy, it has been suggested that volcanism in Tharsis and Elysium, the most longlived volcanic provinces on Mars, could have been caused by a mantle plume originating at the core mantle boundary [e.g., Harder and Christensen, 1996; Kiefer and Li, 2009]. Alternatively, Schumacher and Breuer [2007] proposed that the insulating effect of a crust with low thermal conductivity could generate a thermal anomaly in the upper mantle beneath 1 Institut fu r Planetenforschung, DLR, Berlin, Germany. 2 Institute of Geophysics, ASCR, v.v.i., Prague, Czech Republic. 3 Geographisches Institut, Ruhr Universität Bochum, Bochum, Germany. 4 Institut fu r Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany. Copyright 2011 by the American Geophysical Union /11/2011GL Tharsis, which would be sufficiently large to induce partial melting. Recently, Grott and Breuer [2010] calculated the heat flow for Mars based on a chondritic bulk composition, taking into account local variations of the crustal thickness and the concentration of heat producing elements as well as variations of the strain rate. They obtained an average surface heat flow of 21 mw m 2 and an elastic lithospheric thickness at the North Pole which is significantly less than the modeled value of 300 km [Phillips et al., 2008]. To reconcile their average heat flow with the lower heat flow at the North Pole implied by the results of Phillips et al. [2008], Grott and Breuer [2010] postulated one or more presently active weak mantle plumes. Such plumes would compensate the low heat flow at the North Pole with a locally enhanced heat flow elsewhere, e.g., underneath Tharsis. While recent volcanism seems to be consistent with models predicting zones of partial melt underneath or an enhanced heat flow at Tharsis [e.g., Schumacher and Breuer, 2007; Grott and Breuer, 2010; Morschhauser et al., 2011], the observed young volcanic ages have been reported for very localized phenomena only, e.g., the platy terrain of Cerberus Palus, commonly interpreted as lava plains, and some lava flows on large Tharsis volcanoes. To test whether young volcanism is spatially and, perhaps, volumetrically more significant than previously acknowledged, we investigate the surface ages of a large number of low volcanic shields, which are distributed in several clusters over wide portions of Tharsis [Hauber et al., 2009]. These low shields and the associated volcanic landforms, collectively classified as plain style volcanism [Greeley, 1982], are well known with respect to their morphology and topography [Hauber et al., 2009]. With few exceptions [e.g., Vaucher et al., 2009a], however, no comprehensive of their ages has ever been performed. The of the ages is complemented by an investigation of the lava rheologies. Due to the ubiquitous dust mantling on Tharsis [e.g., Christensen, 1986], only few spectroscopic studies provide compositional information on volcanic surfaces, thus morphometry is currently the only means to constrain the composition of Tharsis lava flows. 2. Data and Methods [3] We use images of the Context Camera Investigation (CTX) on Mars Reconnaissance Orbiter [Malin et al., 2007], which are ideally suited for purpose due to their good contrast, high resolution (5 6 m/pixel), and wide coverage (swath width 30 km). Our counts are, therefore, based on a homogeneous data set. Representative surface areas for age determinations were mapped and craters counted on CTX images (Figure 1a) utilizing the software tool cratertools [Kneissl et al., 2010]. Absolute crater model ages were derived with the software craterstats [Michael and Neukum, 2010] by analysis of crater size frequency distributions (CSFD) applying the production function coefficients of Ivanov [2001] 1 of 5

2 Figure 1. Example of a low shield and crater count measurements. (a) High resolution images with favorable illumination conditions display the radial patterns of lava flows emanating from the summit that are characteristic for the morphology of low shields. The example shown here is SEOlymp_07 on CTX image P13_006192_1972 (see auxiliary material). (b) Mapped outline of lava flow and counted craters (red circles). (c) Cumulative histogram of crater size frequency distribution with derived model age. The largest crater (350 m) was excluded from the fit as it is a partially flooded crater at the shield s margin. and the impact cratering chronology model coefficients of Hartmann and Neukum [2001]. The entire exposed area of low shield volcanoes (with one exception) was counted. Where possible, craters were counted down to about 20 m in diameter, however, to avoid problems that might arise from the target property influence [Dundas et al., 2010], only craters with more than 30 m were used for data fitting. Secondary craters are frequently observed at low shield volcanoes and the areas they cover were excluded. Eleven volcanoes showed a steepening CSFD curve suggesting that small isolated secondary craters had been counted. Despite close re examination, those volcanoes were excluded from surface age determinations to minimize complications from the inclusion of small secondary craters in the analyzed crater population, although Werner et al. [2009] and Hartmann et al. [2010] showed that issue does not compromise absolute age dating as severely as predicted by earlier assertions [McEwen et al., 2005]. Figure 2. Determination of lava flow height. (a) Image shown in Figure 1a with superposed locations of single MOLA shots. Note selected MOLA track shown in red. (b) Topographic profile of selected MOLA track, with highlighted lava flow. [4] Morphometric properties of lava flows were determined from CTX images (Figure 1b) and single shots of the Mars Orbiter Laser Altimeter (MOLA) [Zuber et al., 1992], which were superposed on CTX images in a GIS environment (Figure 2). We analyzed lava flow rheologies by applying established methods [Hulme, 1974; Moore et al., 1978]. A Bingham flow model was used to derive the yield strength, and a Newtonian fluid behaviour was assumed to derive the viscosity via Jeffrey`s equation. Assumptions on certain parameters (e.g., magma density, Grätz number, thermal diffusivity) followed earlier studies [Hiesinger et al., 2007, and references therein] for better comparison. 3. Results 3.1. Chronology [5] Several low shield volcanoes in each shield cluster were dated by crater counts. The goal was to derive absolute model ages for a given shield, but also to determine whether the shields within one shield cluster formed at roughly the same time or over a prolonged period, and whether the clusters have comparable ages or not. An example of a low shield is shown in Figure 1a. The CSFD of shield with an absolute model age of 52 Ma is presented in Figure 1b. [6] We inspected 94 small and low shield volcanoes in Tharsis of which 83 were suitable for determining surface model ages (Table S1 of the auxiliary material). 1 The ages within a given shield cluster are similar, with a smaller age variation in the younger clusters. This smaller age range within younger clusters was expected, since the inherent systematic error of the method is within a factor of about two for an assigned absolute age, and there is an average statistical error in the range of 20% 30% for ages <3 Ga [Neukum et al., 2004]. The spatial distribution of ages is shown in Figure 3. Most clusters have ages <100 Ma. Shields in Tempe Terra 1 Auxiliary materials are available in the HTML. doi: / 2011GL of 5

3 Figure 3. Ages of low shields in Tharsis. Background is MOLA shaded topography. Details for all counts are given in Table S1 (auxiliary material). Table 1. Comparison of Rheologic Properties of Lava Flows on Mars Name Yield Strength (Pa) Effusion Rate (m3 s 1) Viscosity (Pa s) a1 ( E, N) a2 ( E, N) b ( E, N) c ( E, N) d1 ( E, N) d2 ( E, N) e ( E, 0.6 S) f ( E, 0.49 S) Arsia Mons, Mars Elysium Mons, Mars Olympus Mons, Mars near Ascraeus Mons, Mars North of Pavonis Mons Large flows at central Elysium Planitia, Mars Small flows at central Elysium Planitia, Mars East of Jovis Tholus, Mars to to to to to to to to to < Sourcea a 1, Warner and Gregg [2003]; 2, Wilson and Mouginis Mark [2001]; 3, Hulme [1976]; 4, Hiesinger et al. [2007]; 5, Baloga and Glaze [2008]; 6, Vaucher et al. [2009b]; 7, Wilson et al. [2009]. 3 of 5

4 are older, with ages of a few hundred million years. The oldest cluster is located in Syria Planum (0.3 Ga 2.9 Ga) Rheology [7] The rheology of Martian lava flows has commonly been determined either for flows on large shield volcanoes [e.g., Zimbelman, 1985; Baloga et al., 2003; Hiesinger et al., 2007] or for very long lava flows on volcanic plains [e.g., Zimbelman, 1998; Baloga and Glaze, 2008]. The very gentle flank slopes of small and low shields (1 ) indicate lowviscosity lavas [Hauber et al., 2009], but there are only few studies that tried to quantify the lava rheology of Martian plain style volcanism [Vaucher et al., 2009b;Wilson et al., 2009]. [8] We analyzed eight lava flows in three shield clusters (southeast of Olympus Mons, Ceraunius Fossae, southeast of Pavonis Mons). The lava flows are located on the flanks of low shields or are associated with fissure eruptions within the plains. Average yield strengths range between Pa and Pa for lava densities of 2500 kg m 3 and 2800 kg m 3, respectively. Effusion rates vary between a few hundred m 3 s 1 and 2500 m 3 s 1. Viscosity values range from Pa s (2500 kg m 3 ) to Pa s (2800 kg m 3 ). These results apply to lava flows both on the flanks of the low shields and on the adjacent plains, possibly suggesting a similar composition and a genetic link. The rheology of the lava flows is consistent with a basaltic composition [cf. Murase and McBirney, 1973] (Table 1). 4. Discussion and Conclusions [9] Landforms of plain style volcanism cover significant areas in Tharsis. Their ages span a wide part of Martian history, beginning in the Early Amazonian and extending into the geologically very recent past. Most analyzed clusters of low shields have ages of only a few tens of millions of years. This observation does not imply, however, that the relative importance of type of volcanism increased with time. Instead, it suggests that plain style volcanism was active throughout the Amazonian, and probably even before, but that more recent volcanic deposits bury older ones. Consequently, the current surface is dominated by the exposures of the youngest clusters. [10] The thickness of the deposits is unknown, but the general lack of obvious kinks in the crater curves suggests that they are not only skin deep. In that case, they would not have buried the raised rims of large, older impact craters, which would show up in the CSFD as distinct older populations [De Hon, 1974;Hiesinger et al., 2002;Platz et al., 2010]. While we cannot currently constrain the volume of material erupted during the activity of very late stage plainstyle volcanism on Mars, we suggest that it is not a negligible amount, given the wide areas covered by the products. This finding of an ongoing wide spread volcanic activity throughout the Amazonian casts doubts on previous assertions of the magmatic history of Mars, which proposed that most of Tharsis was already in place at the end of the Noachian [Phillips et al., 2001]. Based on the results shown here [see also Werner, 2009] it appears possible that the magmatic activity in Tharsis declined slower than predicted by these studies, although it is difficult to quantify the volume of igneous material emplaced in the Amazonian. [11] Our analysis of the rheologic properties of plains style lava flows confirms previous reports that their yield strengths and viscosities are low [Vaucher et al., 2009b; Wilson et al., 2009]. The values (Table 1) are consistent with basaltic compositions. Thus, it can be concluded even without spectral measurements that the volcanic plains between the large shields in Tharsis are predominantly basaltic. We propose that the origin of these basalts is related to either a zone of partial melting below a thickened crust in Tharsis [Schumacher and Breuer, 2007], or to a mantle plume [Kiefer and Li, 2009; Grott and Breuer, 2010]. [12] Volcanic eruptions release large amounts of volatiles into the atmosphere and have a significant impact on the climate [Robock, 2000]. The analysis of Martian meteorites has shown that the Martian mantle, the likely source region for the Tharsis basalts, might be wetter than previously thought (up to ppm H 2 O[McCubbin et al., 2010]). The recent volcanism, therefore, should have outgassed more water and CO 2 (and also SO 2 ; Gaillard and Scaillet [2009]) than assumed in the past. This is consistent with the work of Niles et al. [2010], who studied stable isotopes of atmospheric CO 2 at the Phoenix landing site and concluded that the modern atmosphere of Mars must have recently been modified by volcanic degassing. Although searches for thermal signatures [Christensen et al., 2003] and surface changes within the last decade [Edgett et al., 2010] have not yielded any positive evidence for active volcanism, it appears possible that Mars is not yet volcanically extinct. [13] Acknowledgments. This benefitted from discussions of EH with an International Team lead by O. Popova and supported by the International Space Science Institute (ISSI) in Bern. We would like to thank T. Kneissl and G. Michael for making available their software tools Crater- Tools and Craterstats, and G. Neukum for discussions. Reviews by J. Lanz and D. Williams helped to improve the manuscript. This research has been partly funded by the Helmholtz Association through the research alliance Planetary Evolution and Life and by the student exchange programme Erasmus. [14] The Editor thanks David Williams and Julia Lanz for their assistance in evaluating paper. References Baloga, S. M., and L. S. 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