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Lunar and Planetary Science XXX 1227.pdf

TERRESTRIAL ANALOGS AND THERMAL MODELS FOR FLOOD LAVAS. L Keszthelyi1 and A S McEwen1, Th. Thordarson2 1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 ([email protected]) 2CSIRO, Division of Exploration and Mining, P.O. Wembley, WA, 6014 Australia

Introduction: The Orbital Camera (MOC) Based on the surface morphology of pahoehoe sheet onboard the (MGS) spacecraft flows in Hawai’i, New Mexico, Australia, and has returned spectacular new images of flood lavas elsewhere [7,8,9,10], we expect the surface of the on Mars [1, 2]. These flows are relatively recent and CFB’s consist of a convoluted set of lobes, tumuli, well preserved in eastern (the and plateaus, all inflated to about the same level. The Cerberus Formation of Plescia [3]) and in a portion of resulting surface has a distinctive mottled Amazonis Planitia centered at 30 N, 160 W. These appearance, especially when partially infilled by two regions are connected by , which is eolian deposits [8]. These flows are generally fed by also filled by flood lavas. Well-preserved flood lavas tubes and sheets, not open channels. Except along of similar appearance (although probably older) are some flow margins, the flood lavas on Mars do not also present high on the Bulge, near Tharsis have a surface morphology consistent with an . inflated pahoehoe sheet flow. The lavas have a distinctive surface morphology Instead, the pressure ridges and channels are more consisting of plates and pressure ridges. These flows consistent with very rapid emplacement. In Hawai’i, are remarkably flat, with less than 50 m of vertical similar structures are largely confined to the near- elevation change per 100 km of lateral distance [4]. vent facies of lava flows. In particular, ponded However, the region is radar-bright, implying a rough flows, open channels, and rafted pieces of crust can surface on the decimeter scale [5]. From Viking data, be found along the Southwest Rift Zone of Kilauea Plescia [3] mapped the Cerberus Formation as and in the summit caldera (Mokuaweoweo) of Mauna covering approximately 2 x 106 km2, and the lavas in Loa. These near-vent flows are characterized by high Amazonis Planitia probably cover a larger area. The (local) effusion rates and hot, gas-rich lavas (with cratering is sparse, indicating that the flows are relatively low viscosities). In the case where the among the youngest large-scale units on Mars. Based flows have ponded, they are also characterized by on partially buried craters the set of flows is very shallow slopes. estimated to be at least 400 m thick in the central The closest terrestrial analog to the Martian flood region of the Cerberus Formation, but probably much lavas that we have found to date is the 1783-1784 thicker [3]. Individual flow fronts appear to be about Laki flow field in Iceland. One section of the distal 10 m thick [3], but better measurements will be end of the flow field is comprised of arcuate pressure possible with MOC images and MOLA profiles. At ridges and rafted crustal plates. The pressure ridges high spatial resolution (2 to 20 m/pixel), these flows formed when crustal plates were banked up against exhibit a complex surface morphology interpreted to obstacles - in this case rows of rootless cones in the include channels, rafted crustal slabs, pressure ridges, underlying Eldgja flow field. and marginal lobes with inflation structures [1,2]. Understanding the formation of these voluminous There are recorded observations of the emplacement outpourings of lava late in Mars' geologic history is of this part of the Laki flow field. It appears that the of great importance for understanding the evolution flows initially moved in rapidly, fed by an open of the interior of the body and perhaps its climate. channel confined within the Skafta River gorge. The crust that formed was broken up by surges of lava Terrestrial Analogs: It is difficult to find direct traveling from the vents to the flow front. It is terrestrial analogs to these immense Martian flows. estimated that these surges increased flow velocities The terrestrial flows most similar in scale are by a factor of 2-5. However, based on limited continental flood basalts (CFBs). The overall outcrops in cross-section, it appears that these flows dimensions and flat general topography is also were able to inflate after the broken up crust froze similar to flood basalt provinces such as the into a single coherent layer during more quiescent Columbia River Plateau. However, CFBs are not times. This mode of emplacement has not been preserved/exposed well enough for their surface observed in Hawaii, but may apply to the ~20% of morphology to be directly observed. Instead, it must the flows in the Columbia River flood basalt province be inferred from observations made in cross-sections that have rubbly flow tops and smooth pahoehoe-like along river valleys and road cuts. Such studies flow bases. indicate that the majority of the flows are inflated pahoehoe sheet flows [6]. Modeling: The fact that the best terrestrial analog is emplaced in a manner that has not been previously Lunar and Planetary Science XXX 1227.pdf

MODELS FOR MARTIAN FLOOD LAVAS: L. Keszthelyi et al.

described makes modeling the emplacement of these Earth. This allows larger, more highly pressurized flow uncertain. However, some constraints can be magma chambers to form. We note that the trend produced. There are two basic methods by which to toward larger volumes and higher sustained eruption move lava a long distance before it cools: rapidly or rates as a function of lithospheric thickness is via insulation [11]. Inflated pahoehoe sheet flows observed on the Earth. Eruptions through normal and tube-fed flows epitomize the latter. Producing oceanic crust (6-10 km thick) typically have eruption Martian flows hundreds of kilometers long in the rates on the order of 1-100 m3/s and total volumes insulated mode is very plausible even on these usually less than 1 km3. Iceland, with a 15-20 km shallow slopes. If the flow is tube-fed, the model of thick lithosphere has had several eruptions with Keszthelyi [12] predicts that effusion rates of 3-25 average effusion rates >500 m3/s and total volumes of m3/s would suffice to produce a flow 100-1000 km 15-20 km3. And continental flood basalt provinces long on a slope of 0.05% given a lava of ~100Pa s erupted through lithosphere ~30 km thick have viscosity. If the flow is sheet-fed, the most critical estimated eruption rates of 1000-5000 m3/s and parameter is the thickness of the upper crust. For volumes exceeding 1000 km3 [10]. upper crust thicknesses of 1-10 m, the sheet flow will REFERENCES balance heat loss with viscous dissipation with flow velocities of only 0.25-3 m/s and total flow [1] McEwen, AS, et al. (1998) GSA abstract 50268. thicknesses of 9-12 m. If the sheets are assumed to [2] McEwen, AS, et al. (1999) LPSC 30. [3] Plescia, be 1-10 km wide, this requires effusion rates of 500- J (1990), Icarus 88: 465-490. [4] Zuber, MT et al. 2.4 x106 m3/s. Since these flows are thought to be a (1998) GSA abstract 50577. [5] Harmon, JK, et al few tens of meters thick, upper crust thicknesses >10 (1992) Icarus 95: 153-156. [6] Self, S, et al. (1997) m and effusion rates <500 m3/s are reasonable. Large Igneous Provinces, AGU Geophys. Monog. 100: 381-410. [7] Hon et al. (1994) GSA Bull. 106: The rapid mode of emplacement, via open channels 351-370. [8] Keszthelyi, L, & D Pieri (1993) J. with a disrupted crust has been modeled [11] using a Geotherm. Volcanol. Res. 59: 59-75. [9] Stephenson modified version of the thermal model of Crisp and PJ, et al. (1998) JGR 103: 27359-27370. [10] Self, S, Baloga [13]. This model is applicable to both et al. (1998) Ann. Rev. Earth Planet. Sci. 26: 81-110. laminar and turbulent flow. Table 1 lists the output [11] Keszthelyi, L, & S. Self (1998) JGR 103: 27447- from the model for inputs appropriate for lavas 27464. [12] Keszthelyi, L (1995) JGR 100: 20411- ranging from ultramafic to basaltic andesite and a 20420. [13] Crisp, J, & S Baloga (1994) JGR 99: slope of 0.25%. If the channels are assumed to be 1- 11819-11831. 10 km wide, then minimum effusion rates range from 1.5x106 to 3.5x107 m3/s. Also note that the flow Table 1: Results from Rapid Emplacement Model of thickness is controlled just as much by the nature of Keszthelyi and Self [11] modified to Mars. the crust as by the composition of the lava. In other lava type crust minimum minimum words, there is little hope for estimating lava type thickness velocity composition from the thickness of the flows. ultramafic thin 62 m 5.6 m/s Conclusions: While these flows could be produced ultramafic thick 38 m 4.0 m/s by very long-lived, moderate effusion rate eruption, basalt thin 73 m 4.7 m/s the observed morphology is far more consistent with basalt thick 46 m 3.3 m/s the channel-fed, rapidly emplaced model. However, basaltic andesite thin 89 m 3.7 m/s until the emplacement of flows with rubbly tops and basaltic andesite thick 62 m 2.4 m/s smooth bottoms is better understood and modeled, this conclusion will be open to debate. The ultramafic: T=1300 °C, r =3000 kg/m3, h=10 Pa s implications for eruption duration and style of basalt: T=1200 °C, r =2500 kg/m3, h=100 Pa s eruption are enormous. If the flows were emplaced bas. and.: T=1100 °C, r =2000 kg/m3, h=1000 Pa s in the insulated manner, they could have taken thin crust: t =100s, f=50%, Hcrust=5cm, Tcrust=T-100C thousands of years. If our favored, rapid, thick crust: t =1000s, f=1%, Hcrust=10cm, emplacement is correct, the eruption duration was T =T-300C probably on the order of a few months or several crust weeks. While the rapid eruption rates we prefer are at least 2-3 orders of magnitude higher than the fastest observed terrestrial eruptions, they may be quite plausible on Mars. The lithosphere on Mars is presumed to be significantly thicker than on the