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Lunar and Planetary Science XXXIII (2002) 1565.pdf

LAVA FLOW FIELD AT , : ANALOG FOR EXTRATERRESTRIAL ? 1Gregg, T.K.P., 2Bulmer, M.R., 3Warner, N.H, 1Dept. of Geology, 876 NSC, University at Buffalo, Buffalo, NY 14260; (716) 645-6800; [email protected]; 2JCET, University of Maryland, Baltimore County, MD 21250.

Introduction: Sabancaya volcano, Peru, is located ~75 air photos we propose that the flow field emanated km northwest of and displays a large (~60 from Sabancaya, though some of the lobes on western km2) lava flow field comprising 39 individual lobes. side of the volcano originated from . Of the These lobes are channeled, and exhibit surface total number of flows within the flow field, 38% are <1 structures oriented with their long axes perpendicular km in length, while 40% are between 2 and 4 km. The to the flow direction. We interpret these structures to longest flow was previously calculated to be 11 km but be surface folds [e.g., 1, 2, 3]. The spacing and our analysis shows that there are two superposed flows amplitude of these folds are similar to those observed and the longest traveled 5 km. Flows that traveled on lava flows on Mars and Venus (Table 1). These initially northwest were diverted by -aged morphologic similarities, coupled with the evidence glacial moraines causing them to follow a course to the found by Pathfinder for martian [4], suggest southwest. Similarly, flows from Sabancaya that that significant volumes of evolved lavas may exist on moved north were constrained by the break of slope at Mars and Venus. Detailed examination of the the southern flanks of a nearby volcano. Flows to the Sabancaya lava flow field provides us with quantitative northeast also appear to have had their paths topographic and geochemical data that can be used for constrained by a break in slope. The longest flows comparison with extraterrestrial flow fields. Our traveled to the southeast, corresponding to the location understanding of the behavior of evolved lava flows of more open topography. The spread of these flows and coulees on Earth is currently limited, and we must laterally was constrained only by the conditions within understand the terrestrial analogs before we can hope the flow at the time of emplacement. to understand their behavior on Mars and Venus. Field results: We collected detailed topographic data Location Wavelength Amplitude along 3 flow lobes of Sabancaya volcano [6] and (m) (m) conducted preliminary geochemical analyses on Sabancaya SPOT, 59 - samples collected along the topographic transects. TM, Radarsat Flow 2a is 3.5 km long and has flow-margin and flow- Sabancaya Air photos Av. 55 3-5 front slopes of 36°-38°. In the distal zone the flow is 108 m thick. Flow 3a is adjacent to 2a, 2.2 km long (1:55,000 scale) and 0.5 km wide with similar slope angles at the Air photos Break of 110 8 margins as 2a. This flow is 173 m thick and displays slope larger (greater wavelengths and amplitudes) surface Sabancaya GPS 67 5 folds than 2a. Flow 6, which we interpret to be one of Chao [2,4] 200 30 the youngest from the dome vent, is 4.2 km long, 0.65 Glass Mtn rhyolite[2] 39 - km wide at the distal end and 85 m thick. These flows Mahuea Tholus, 686-820 <56 are 0.35 km3, 0.19 km3 and 0.23 km3 in volume, Venus[5] respectively. The flow surfaces at Sabacaya are Arsia Mons, Mars [6] 100 27 characterized by large (£6 m) blocks. These blocks tend to be angular, making the flow appear similar to a Table 1. Fold wavelengths and amplitudes for lava blocky basaltic flow, except for the large flows on Earth, Mars and Venus. block size. We propose the term "megablocky" to describe this flow texture. The blocks are sufficiently Sabancaya volcano, Peru: Sabancaya is part of the large that this flow surface actually appears smooth at volcano complex that includes Nevados Ampato and radar wavelengths <60 cm [8]. Hualca Hualca. The last eruptive event at Sabancaya The lavas are andesitic to trachyandesitic in 1990 deposited ash up to 20 km away and minor, (Figure 1) Small (< 0.5 – 20 cm) inclusions of basaltic sporadic explosive activity continues to this day. andesite (52 wt% SiO2) exist throughout the flows and Limited age dating, geochemical analysis and small- are most likely indicative of mixing. scale large area mapping has been conducted on Geochemical analysis shows that very little H2O (< 1.7 Sabancaya [7] but no detailed geological history of the wt%) is present within the lavas. The lavas contain volcano has been compiled. Based on mapping from abundant large (<1.0 cm) plagioclase phenocrysts, Lunar and Planetary Science XXXIII (2002) 1565.pdf

Gregg et al. SABANCAYA – ANALOG FOR EXTRATERRESTRIAL LAVAS

pyroxene, biotite, and sanidine with rare horneblendes believe that investigations of morphology alone are not surrounded by a glassy matrix; microlites are rare. sufficient to fully characterize the behavior of There are no vesicles. Basaltic inclusions contain rare terrestrial or extraterrestrial lava flows, for the skeletal , biotite and sanidine as well, also following reasons. First, there may be fundamental suggesting intimate magma mingling. differences in lava composition between Earth and Mars, for example, causing us to model a Martian flow as a basaltic andesite when it may in fact be a phonolite (and little work has been done on the rheology of phonolites, for instance). Second, small 7.3 amounts of materials that cannot be detected via 7.2 remote sensing (e.g., dissolved water, CO2, or flourine) can stronlgy affect flow behavior. Finally, lava flows 7.1 on Earth tend to be orders of magnitude smaller than

7 those on other planets, and the underlying slopes on O 2 Mars tend to be much shallower than those typically O+K 2 6.9 associated with volcanic terrains on Earth. These Na ambient parameters may create unique flow kinematics 6.8 on Mars. Therefore, we believe that it is imperative

6.7 that more attention be focused on obtaining measurements of in-situ lava flows during future 6.6 missions to Venus or Mars.

6.5 58 58.5 59 59.5 60 60.5 61 61.5 62 References: [1] Fink and Fletcher, 1978. [2] Fink, SiO 2 J.H., 1980, Geology 8:250-254. [3] Gregg, T.K.P., J.H. Fink and R.W. Griffiths, 1998, J. Volcanol. Figure 1. Na20 + K2O wt % vs. SiO2 wt. % for samples taken from lava flow lobes 2a, 3 and 6. Geotherm. Res. 80:281-292. [4] Guest, J. and J. Sample with <53.5 wt% is from an inclusion. Sanchez, 1969, Bull. Volcanol. 33:778-790. [5] Moore, H.J. et al., 1992, J. Geophys. Res. 97:13,479- Discussion: 13,494. [6] Warner, N.H., T.K.P. Gregg and M.R. The emplacement style for these flows at Bulmer, 2002, this volume. [7] Bulmer, M.H. F.C. Sabancaya is enigmatic. It difficult to reconcile the Engle, and A.K. Johnston, 1999 RADARSAT ADRO geochemistry and the flow morphology. Based on the Program Summary. [8] Bulmer, M.H. B.A. Campbell dimensions and geochemistry (particularly the low and J. Byrnes, 2001 LPSC XXXII, 1850. [9] water content) of the lava flow morphologies at Christiansen, E.H., M.F. Sheridan and D.M. Burt, Sabancaya, we propose the following scenario. A 1986, GSA Special Paper 205, 82 pp. differentiated (dacitic or trachyandesitic) was injected with hot, basaltic magma. This triggered devolatilization of the differentiated magma, and predicts an immediately prior to the emplacement of the lava flows. After the volatiles were evacuated from the chamber, the hot, relatively dry lava flows were emplaced. Flow morphology suggests that these lavas were emplaced quickly at high temperatures; the absence of microlites and vesicles are consistent with this scenario. We have begun to test this scenario using a more detailed analysis of the melt temperatures. Currently, we are investigating the role of flourine in these lavas. Christiansen and others [9] demonstrated that minor amounts of flourine in evolved lavas can drastically lower flow viscosities, thereby enhancing flow mobility and a tendency to fold.

Planetary implications: Our continuing studies of the flow field at Sabancaya volcano, Peru, lead us to