Rheologies and ages of lava flows on Elysium Mons, Mars

Icarus 219 (2012) 443–457

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Rheologies and ages of lava flows on Elysium Mons, Mars Jan Hendrik Pasckert ⇑, Harald Hiesinger, Dennis Reiss Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, Münster 48149, Germany

a r t i c l e

i n f o

Article history: Received 22 August 2011 Revised 21 February 2012 Accepted 15 March 2012 Available online 23 March 2012 Keywords: Mars, Surface Volcanism Cratering Geological processes

a b s t r a c t We present results of our study of the rheologies and ages of lava flows in the Elysium Mons region of Mars. Previous studies have shown that the geometric dimensions of lava flows reflect rheological properties such as yield strength, effusion rate and viscosity. In this study the rheological properties of lava flows in the Elysium Mons region were determined and compared to the rheologies of the Ascraeus Mons lava flows. We also derived new crater size-frequency distribution measurements (CSFDs) for the Elysium lava flows to identify possible changes in the rheological properties with time. In addition, possible changes in the rheological properties with the distance from the caldera of Elysium Mons were analyzed. In total, 35 lava flows on and around Elysium Mons were mapped, and divided into three groups, lava flows on the flanks of Elysium Mons, in the plains between the three volcanoes Elysium Mons, Hecates and Albor Tholus and lava flows south of Albor Tholus. The rheological properties of 32 of these flows could be determined. Based on our morphometric measurements of each individual lava flow, estimates for the yield strengths, effusion rates, viscosities, and eruption duration of the studied lava flows were made. The yield strengths of the investigated lava flows range from 3.8  102 Pa to 1.5  104 Pa, with an average of 3.0  103 Pa. These yield strengths are in good agreement with estimates for terrestrial basaltic lava flows. The effusion rates are on average 747 m3 s1, ranging from 99 to 4450 m3 s1. The viscosities are on average 4.1  106 Pa s, with a range of 1.2  105 Pa s to 3.1  107 Pa s. The eruption durations of the flows were calculated to be between 6 and 183 days, with an average of 51 days. The determined rheological properties are generally very similar to those of other volcanic regions on Mars, such as on Ascraeus Mons in the Tharsis region. Calculated yield strengths and viscosities point to a basaltic/andesitic composition of the lava flows, similar to basaltic or andesitic a’a lava flows on Earth. Absolute model ages of all 35 lava flows on Elysium Mons were derived from crater size-frequency distribution measurements (CSFD). The derived model ages show a wide variation from about 632 Ma to 3460 Ma. Crater size-frequency distribution measurements of the Elysium Mons caldera show an age of 1640 Ma, which is consistent with the resurfacing age of Werner (2009). Significant changes of the rheologies with time could not be observed. Similarly, we did not observe systematic changes in ages with increasing distances of lava flows from the Elysium Mons caldera. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction 1.1. Geological context The Elysium Mons region is the second largest volcanic region on Mars and contains three volcanoes: Hecates Tholus in the north, Elysium Mons in the center and Albor Tholus in the south (Fig. 1). The Elysium Mons region is located in the northern lowlands between 15° and 35° northern latitude and 135° and 155° eastern longitude. The entire region is characterized by a broad asymmetric topography, of which Elysium Mons forms an essential part and contains one of the youngest volcanic surfaces on Mars (Plescia, 2007). ⇑ Corresponding author. E-mail address: [email protected] (J.H. Pasckert). 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.03.014

With a summit elevation of about 14 km and a volume of 20  1013 m3, Elysium Mons is the largest volcano of the three named above (Plescia, 2007). With elevations of 4.1 km of Albor Tholus and 4.8 km of Hecates Tholus, the other two volcanoes are much lower in elevation (Plescia, 2007). The caldera of Elysium Mons has a diameter of about 14 km and does not show lava flows starting directly at the caldera like, for example, Ascraeus Mons. However, the western part of the caldera shows a significant scarp of about 400 m, whereas the eastern flank shows evidence of a broad flooding event caused by overflowing lavas from the caldera. These lavas cover nearly the whole eastern flank, but individual lava flows could not be recognized. This has also been observed by Plescia (2007) and is shown in Fig. 2. The flanks of Elysium Mons show numerous lava flows changing in size and shape, some of them have been mapped during this investigation (Fig. 2). The flanks of Albor Tholus have a radial,

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hummocky morphology (Plescia, 2007). This volcano has two calderas, a small one at the northern margin and a larger one in the south. With depths of 5.9–7.5 km for the large caldera and depths of 6–6.5 km for the smaller caldera, both calderas are very deep in contrast to the caldera of Elysium Mons (Plescia, 2007). While Elysium Mons has only one caldera, Hecates Tholus shows a much more complex summit. It consists of four calderas varying in size and getting younger to the south (Plescia, 2007). Hecates Tholus also shows evidence of an explosive eruption on the northwestern flank that occurred 350 Ma ago (Mouginis-Mark et al., 1982; Hauber et al., 2005). Morphologically clearly defined lava flows were observed on Elysium Mons and in the plains between Albor and Hecates Tholus (Plescia, 2007). Tectonic deformation seems to be very rare. Plescia (2001) identified only some concentric graben and wrinkle ridges near the caldera of Elysium Mons and morphological evidence of a thrust fault dipping toward the summit. Similarly, the other two volcanoes show concentric graben-like structures, for example, on the southern flank of Albor Tholus. This tectonic activity might be related to the rise of the whole region during volcanically active phases (Plescia, 2001). The so-formed bulge presumably is the result of a large mantle plume, located directly underneath the entire region (e.g., Kiefer and Hager, 1989). An analogue example on Earth is the hot spot below Hawaii. However, in contrast to Earth, Mars has no plate tectonics, so the hot spot does not move with respect to the surface. Another example for this kind of hot spot volcanism is the Tharsis Region, which is also thought to have formed by a similar, but even larger mantle plume (e.g., Kiefer and Hager, 1989). In addition, Pedersen et al. (2010) observed hundreds of narrow, linear ridge segments in the transition zone between the Elysium rise and the Utopia basin north-east of Hecates Tholus, which are interpreted to be dikes and dike swarms related to the volcanic activity of the Elysium Mons region.

region, because the lava flows on the flanks of Elysium Mons are much smaller than the extensive flows in Elysium Planitia. Most of the previous studies were based on Mariner and Viking images with spatial resolutions of several tens of meters. With HRSC and CTX images that cover large areas of Mars, we now are able to map and measure such lava flows in great detail at 5–25 m/pixel spatial resolution. These high resolution images also allow us to perform crater size-frequency distribution (CSFD) measurements of the lava flows in the Elysium Mons region to determine their surface model ages. Our CSFD measurements complement previous age determinations of lava flows in Elysium Planitia by Hartmann and Berman (2000). To our knowledge there are no published ages for individual lava flows on the flanks of Elysium Mons. The combination of the surface ages with the rheologies and location of the lava flows on Elysium Mons allows us to investigate changes in the rheologies over space and time, which has not been studied before. With our study we address the following questions: (1) What are the rheological properties of the lava flows on Elysium Mons? (2) Is there a difference in the rheologies to other volcanic regions on Mars and elsewhere in the Solar System? (3) Is there a change of the rheological properties over time and distance to the caldera of

1.2. Motivation The rheological properties of the lava flows of Elysium Mons are not well known. Although there have been several studies on lava flows in Elysium Planitia and around the Elysium Montes (e.g., Mouginis-Mark et al., 1984; Mouginis-Mark and Yoshioka, 1998; Plescia, 1990; Glaze et al., 2003; Glaze and Baloga, 1998, 2007; Vaucher et al., 2009; Hurwitz et al., 2010), there are no studies of lava flows directly located on the flanks of Elysium Mons and in the plains between the three volcanoes. In the past such studies have been hindered by the lack of high resolution images of this

Fig. 1. Context map of the Elysium Mons region. The black box outlines the study area.

Fig. 2. MOLA shaded relief map of the studied lava flows (red) in the Elysium Mons region.

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Elysium Mons? (4) Is there a change in age with distance to the caldera of Elysium Mons? 2. Data Images obtained by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express spacecraft (Neukum et al., 2004a,b; Jaumann et al., 2007), in combination with images of the Mars Reconnaissance Orbiter Context camera (CTX) (Malin et al., 2007) and data from the Mars Orbiter Laser Altimeter (MOLA) (Smith et al., 1998, 2001) were used to identify, map, and measure the dimensions and slopes of lava flows to constrain their rheological properties. The HRSC coverage of Elysium Mons is 100%, with a resolution between 12.5 m/pixel and 25 m/pixel. The HRSC is a line scanner and is orbiting Mars since 2003. In addition, the Elysium Mons region is covered up to 50% by CTX images with spatial resolutions of 5–6 m/pixel. The CTX camera is orbiting Mars since 2006. It was designed, built, and is operated by Malin Space Science Systems (Malin et al., 2007). Orbiting Mars from 1997 to 2001, the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor (MGS) spacecraft delivered three types of data sets: topography, surface roughness, and reflectivity of the martian surface (Smith et al., 1998, 2001). MOLA transmitted infrared laser pulses with a wavelength of 1064 nm towards the martian surface at a rate of 10 Hz and measured the time it took the beam to reach the surface and be reflected to the spacecraft (Smith et al., 2001). The vertical accuracy of each individual MOLA point is 1 m (Smith et al., 2001). The diameter of one single measurement point at the surface is 160 m (Smith et al., 2001). The distance between the measurement points is in the range of 300 and 400 m. As a result, the surface resolution in flight direction is between 300 and 400 m (Smith et al., 1998). The relative vertical accuracy between two subsequent measurement points is 30 cm (Smith et al., 1998, 2001). We used individual MOLA measurements to determine the heights of the lava flows and their slopes. In order to determine the heights, 5–20 MOLA profiles were selected for each flow, depending on the dimensions of the flows. 3. Methods To investigate the lava flows on Elysium Mons two major methods were used. First, 35 lava flows were mapped and the morphometric dimensions such as height, length, width, and slope were measured. The dimensions were utilized to infer the rheological properties of the flows (e.g., Wilson and Head, 1983; Moore et al., 1978; Mouginis-Mark and Yoshioka, 1998; Hiesinger et al., 2007). To get information about the ages of the mapped lava flows, crater size-frequency measurements were performed (e.g., Hartmann and Berman, 2000; Neukum et al., 2004a,b). 3.1. Rheology To investigate the rheological properties of the lava flows with remote sensing techniques, it is important to map and measure the dimensions of the flows as accurately as possible. It is known from previous studies (e.g., Wilson and Head, 1983; Moore et al., 1978; Mouginis-Mark and Yoshioka, 1998; Hiesinger et al., 2007) that lava flow morphologies reflect the rheological properties of the erupted lavas. However, the determination of the rheological properties from remote sensing data requires some important assumptions: 1. Lava flows behave as Bingham fluids (e.g., Hulme, 1974; Wilson and Head, 1983).

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2. Lava flows in a laminar fashion (Moore et al., 1978; Wilson and Head, 1983). 3. No inflation of lava flows has occurred. 4. The density q of martian lava flows is on average 2500 kg m3. 5. The Graetz number Gz is 300. 6. The thermal diffusivity j is 3  107 m2 s1. In the following paragraphs we describe these assumptions in more detail: 1. A major assumption in this study is that flowing lava behaves as a non-Newtonian liquid. If lava was an ideal Newtonian liquid, it would flow downhill and would not stop flowing even after the supply at the vent has ceased until it reaches a topographic depression. Furthermore, the flow would spread laterally until it was restricted by topography or until surface tension prevented further spreading. Observations, for example by Hulme (1974), showed that lava does not behave like a Newtonian liquid. Commonly it comes to rest on a slope as soon as the supply of fresh lava ceases and many flow fronts are high and steep although unconfined by topographic features. It is clear that there is some process which limits the flow of lava, brings it to rest on slopes, and prevents its lateral spreading. From measurements of stationary lavas, Shaw (1968) determined that lava flows behave similar to Bingham liquids. In general, the length of a lava flow is limited by cooling or by the supply of material. In simple channelized eruptions, lava stops flowing due to heat loss, resulting in the growth of the internal yield strength. Thus, these flows are cooling-limited (Guest et al., 1987; Pinkerton and Wilson, 1994; Harris and Rowland, 2009). In contrast, a volumelimited eruption stops flowing when the supply from the vent ceases before the full cooling-limited length has been reached (Guest et al., 1987; Harris and Rowland, 2009). For example, individual pahoehoe lava flows are generally cooling-limited, whereas tube-fed pahoehoe flow fields, as a whole, are usually volume-limited (Harris and Rowland, 2009). As all lava flows determined in this study appear as single flows and no lava tubes have been observed, we propose that these are cooling-limited lava flows. However, 3 out of 35 lava flows show signs of possible central channels and levees, which might indicate that these flows are volume-limited. 2. The used equations for yield strength and viscosity, listed below, are valid only for lava that moves in laminar fashion, although turbulent motion can appear (Wilson and Head, 1983). Turbulent motion occurs at mass eruption rates of 3  107 kg s1 for lunar and terrestrial basalts (Wilson and Head, 1983). Terrestrial basaltic eruptions do not reach such rates, but lunar basalts show eruption rates of 108–109 kg s1, which led in some cases to the formation of sinuous rilles by thermal erosion (Wilson and Head, 1983). Features like these could not be observed for the investigated lava flows on Elysium Mons. Hence, the lava on Elysium Mons appears to flow in laminar fashion. 3. Besides erosion, inflation can change the flow dimensions, especially the flow heights. Inflation of lava flows can be caused by different processes. Depending on the flow type, inflation can be caused by changing effusion rates and injection of liquid lava into a cooling-limited flow underneath the solid crust. This is typical for terrestrial subaerial and submarine pahoehoe lava flows (e.g., Walker, 1991; Hon et al., 1994; Zimbelman et al., 2011). Also the release of volcanic gases from inside the flow and variation of the surface slope can result in inflation. However, the observed lava flows do not show changes in shape since their emplacement, such as inflated sheet lobes and inflation-rise pits (e.g., Walker, 1991; Hon et al., 1994). Therefore, we assume that the observed flows still have their original shape and have not been inflated.

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4. A very important factor for the determination of rheological properties is the density of the erupted lavas. Since we do not know the density of martian lava flows, because of the lack of samples, we have to assume a certain density. The density of lava depends on the composition, the water content, and the porosity of the erupted material (Wilson and Head, 1994). However, as mentioned before, we do not have samples from known regions on Mars and until now there is only few remote sensing data available, concerning the composition of the lava flows of Elysium Mons (e.g., Bandfield et al., 2000; Bibring et al., 2005). There are spectral data sets such as the Thermal Emission Spectrometer (TES), the Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) and the Compact Reconnaissance Imaging Spectrometer (CRISM) available for Mars, but they do not cover the lava flows in the study area (CRISM) or do not have the resolution to resolve individual lava flows (TES/OMEGA). However, on a global scale, TES and OMEGA spectral data show that large parts of the surface are composed of basalts and andesites (Bandfield et al., 2000; Bibring et al., 2005). Analyses of the characteristics of lava flows on martian shield volcanoes by Hulme (1976), Moore et al. (1978) and Cattermole (1987) showed lava flows with rheological properties reflecting mafic basalts and basaltic andesites. Characteristics of the SNC meteorites interpreted to be from Mars show similarities to terrestrial basaltic and ultramafic rocks (e.g., Baird and Clark, 1981). The melt densities of these materials were calculated to vary between 2750 and 2960 kg m3 (Longhi, 1990). On the basis of previous work, the densities of martian lava flows vary in a relative wide range from 725 to 2960 kg m3. For example, Moore et al. (1978) used densities between 2500 and 2900 kg m3 for their calculation. Baloga and Glaze (2008) used a value of 2600 kg m3, whereas Zimbelman (1985), Warner and Gregg (2003), and Hiesinger et al. (2007) assumed the density to be 2500 kg m3. For our study we use a density of 2500 kg m3. 5. The Graetz number is a dimensionless number that relates the rate of heat loss from a flow to the rate of heat advection within a flow along its length (Gregg and Fink, 1996). When a lava flow has stopped flowing, the Graetz number has decreased from an initially large value of about 1000 to about 300 (Wilson and Head, 1983). Consequently, for the following calculation of the viscosity a value of 300 was chosen. Such a value is consistent with other publications about the viscosity of martian lava flows (e.g., Wilson and Head, 1983; Gregg and Fink, 1996; Vaucher et al., 2009). 6. The thermal diffusivity describes the rate at which heat is conducted through a medium. Literature values range between 104 (Gregg and Fink, 1996) and 108 m2 s1 (Gregg and Zimbelman, 2000). Gregg and Zimbelman (2000) published a list of thermal diffusivities of lavas depending on their compositions. In this list, rhyolites have the lowest value with 1.4  106 m2 s1 followed by dacites with 2.0  107 m2 s1. The lavas with the highest thermal diffusivity are basalts with 5.0  107 m2 s1 and andesites with 3.0  107 m2 s1. On the basis of the basaltic and andesitic nature of the martian surface derived from spectroscopy (Bandfield et al., 2000; Bibring et al., 2005), we assume a thermal conductivity of 3.0  107 m2 s1. 3.1.1. Yield strength As discussed above, lava is likely to behave like a Bingham liquid, which is controlled by two parameters, the yield strength and the viscosity of the erupted material (Wilson and Head, 1983; Hiesinger et al., 2007). Moore et al. (1978) and Hulme (1974) related the yield strength, s (Pa), of lava flows to the flow dimensions by the following equations:

s ¼ qg sin ah

ð1Þ

s ¼ qgh2 =wf

ð2Þ

s ¼ qg2wb sin2 a

ð3Þ 3

where q is the density (kg m ), g is the acceleration of gravity (m s2), a is the slope angle (degree), h is the flow height (m), wf is the flow width (m) and wb is the width of the levee (m) (Moore et al., 1978). Eq. (3) was not used for the Elysium Mons lava flows because the levees of the 3 leveed lava flows investigated in the study area were too small to be measured with the available data sets. Thus, we used Eqs. (1) and (2) to estimate the yield strengths of theses flows. In summary, there are three ways of calculating the yield strength of a lava flow from its morphology, all require an assumed value for the density of the flow as discussed before and the acceleration of gravity. Eq. (1) relates the yield strength to the topographic slope and the thickness of the lava flow. Compared to Eq. (1), the slope angle is not needed in the calculation using Eq. (2). Eq. (2) is very similar to the equation used for terrestrial glaciers by Orowan (1949) and relates the yield strength to the thickness and the width of a lava flow. Eq. (3) can only be used for the calculation of the yield strength of leveed flows. 3.1.2. Effusion rate The effusion rate, Q (m3 s1), can be calculated by using the equation of Wilson and Head (1983) or Zimbelman (1985):

Q ¼ Gz jxw=h

ð4Þ

where Gz is the dimensionless Graetz number, j is the thermal diffusivity (m2 s1), x is the flow length (m), and w and h are the width and the height of the flow. 3.1.3. Viscosity To calculate the viscosity g (Pa s), equations given, for example by Zimbelman (1985), Glaze and Baloga (2007), and Vaucher et al. (2009) were used:

h ¼ ðQ g=qgÞ1=4

ð5Þ

In addition, Jeffrey’s equation relates the viscosity of a flow to its effusion rate and its dimensions (Glaze and Baloga, 2007; Hiesinger et al., 2007):

g ¼ ðqgh3 w sin aÞ=nQ

ð6Þ

In this equation, n is a constant equal to 3 for broad flows and 4 for narrow flows, w is the flow width and a is the slope. In both equations, the viscosity is related to the effusion rate Q, the flow height h, and the rock density q. Applying Eqs. (5) and (6), we have to be aware of the fact that these models assume lava flows to behave like Newtonian fluids (e.g., Gregg and Fink, 1996; Warner and Gregg, 2003; Hiesinger et al., 2007; Vaucher et al., 2009). However, lava flows are thought to behave as Bingham plastics and consequently Eqs. (5) and (6) are simplified models to describe the viscosities of lava flows (Gregg and Fink, 1996; Hiesinger et al., 2007; Vaucher et al., 2009). 3.1.4. Mean flow velocity The mean flow velocity u (m s1) can be calculated by using Jeffrey’s equation for laminar flow (Jeffreys, 1925), which is: 2

u ¼ ðg qh sin aÞ=3g

ð7Þ

with all variables defined as above. Converting this equation leads to the following equation, which relates the velocity to the effusion rate:

u ¼ Q =ðhxÞ

ð8Þ

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Consequently, the mean flow velocity depends on the effusion rate, the flow length and flow height. 3.1.5. Eruption duration The eruption duration T (days) can be calculated in two different ways. One way is to divide the length of the flow through the mean flow velocity:

T ¼ x=u

ð9Þ

In this equation u is the mean flow velocity described in Section 3.1.4 and x is the flow length. Another method is to divide the flow volume by the effusion rate:

T ¼ V=Q

ð10Þ

where V is the flow volume and Q is the effusion rate. 3.1.6. Flow dimension measurements To determine the flow dimensions, such as flow length, flow width, and flow thickness, different methods can be applied. The flow lengths and widths were measured in ArcGIS (developed by ESRI) on HRSC- and CTX-images as shown in Fig. 3. The flow widths were determined several times along the lava flows orthogonal to the flow direction (Fig. 3). All these width measurements of one flow were used to calculate an average width for each individual lava flow. As described by Glaze et al. (2003), the thicknesses of the lava flows were determined by using individual MOLA profiles crossing the flows (Fig. 4). To determine the slope angle of a lava flow, single MOLA shots on and beside the flow were utilized (Fig. 3). Knowing the absolute height and the position of each MOLA point, the slope between each point can be calculated by:

s ¼ ðh1  h2 Þ=x where h1 is the elevation of one MOLA point, h2 is the elevation of another MOLA point, and x is the distance between the MOLA points. To verify the results, the slope was measured in several ways. First of all, the slope between the first and the last MOLA point was calculated. This slope was compared to the slope of the bestfit line of all measured slope points of the flow. In most cases both slopes were nearly identical and the slope of the best-fit line was chosen for the calculations of the yield strengths. In addition, the slope on and beside the lava flows were calculated to check a possible flattening of the slopes on the flows, by lava ponding at the flow front (Fig. 3). 3.2. Age dating To derive absolute model ages of the lava flows, crater size-frequency distribution measurements were carried out by using the production function from Ivanov (2001) and the chronology

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function from Hartmann and Neukum (2001). For reliable crater counts it is important to exclude endogenic and secondary craters, i.e., craters that were formed by the impact of material ejected by primary craters (McEwen, 2003). At CTX resolution (5–6 m/pixel), in some cases, separating small secondary from primary craters was difficult because of the generally rough appearance of the flow surfaces and possible subsequent modification of the craters. The technique to derive relative and absolute model ages of planetary surfaces from crater size-frequency distribution measurements has been described in great detail by various authors (e.g., Neukum and Ivanov, 1994; Hiesinger et al., 2000a,b; Stöffler and Ryder, 2001; Neukum et al., 2001; Ivanov, 2001; Hartmann and Neukum, 2001; Stöffler et al., 2006). This method has been developed for the Moon by Shoemaker and Hackman (1962), Baldwin (1964), Hartmann (1966) and Neukum (1971). With appropriate modifications (scaling laws), it can be applied to all bodies in the Solar System (Ivanov, 2001). To determine relative ages of homogenous units, the surface area of these units has to be calculated and all craters apart from secondary and endogenic craters are counted and their diameters measured, in order to derive the cumulative crater density. This cumulative crater density reflects the time the unit has been exposed to the meteoritic bombardment. In order to obtain absolute model ages these crater size-frequency distribution measurements have to be linked to radiometric ages from samples of the respective planetary body. Due to the lack of samples from other bodies than the Earth and the Moon, the lunar chronology of the Moon has to be extrapolated to these other bodies. Radiometric ages of martian meteorites cannot be correlated to crater size-frequency distributions, because the source areas and their associations to individual geologic units are unknown. Hartmann (1977) and Ivanov (2001) determined the catering rate of Mars relative to the Moon and showed that the absolute model ages of martian surface units can be determined with an uncertainty factor of about two or three (Hartmann and Neukum, 2001).

4. Results For this investigation, 35 lava flows on and around Elysium Mons (Fig. 2) with distances from the caldera between 32 and 642 km were mapped. Considering the surface slope and the distance to the caldera of Elysium Mons, these flows can be subdivided into 3 major groups: flows on the flanks of Elysium Mons (Flows 1–3, 5, 7, 8, 23, 32), flows in the plains between Elysium Mons, Albor Tholus and Hecates Tholus (Flows 4, 6, 9–22, 24–29, 34), and flows south of Albor Tholus (Flows 30, 31, 33, 35). The rheological properties of 32 of these flows were calculated. Three of the flows were too small to yield reliable thicknesses from the MOLA profiles. Consequently, we did not calculate the rheological properties of Flow 3, 7 and 8. In addition, Flow 23 for example, is located in a depression so that only few MOLA profiles show the lava flow as an elevated feature. Because of this geometric setting

Fig. 3. Example of the measurements of flow dimensions (Flow 1). The slope in flow direction on and beside the flow has been measured between the blue dots. The red lines indicate the locations of lengths and widths measurements and the solid black lines are sections of individual MOLA profiles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4. Example of a MOLA profile across a lava flow. The blue points are individual MOLA points. The two red points indicate the first points of the lava flow, i.e., REF 1 and REF 2. The black line shows a fitted line between REF 1 and REF 2. This fit line is used for the calculation of the flow thickness. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of lava Flow 23, MOLA points beside the flow show higher elevations than on the flow and the flow does not appear to have a significant thickness in the MOLA profiles. Therefore, calculated rheologic characteristics of this flow should be treated with caution. In other cases, impact craters on or beside the flows complicated the determination of flow heights in some MOLA profiles. The absolute model ages of all 35 lava flows were determined by crater counting on HRSC and CTX images. In addition, the age of the Elysium Mons caldera was estimated by crater counting on CTX images. 4.1. Rheology The lengths of the investigated Elysium flows vary between 9.9 and 118 km, with widths on the order of 428 m to 13.670 km. The MOLA profiles of the investigated flows show heights from 7 to 34 m with an average of 18 m. All three flow dimensions, length, width, and height are larger than those of the Ascraeus Mons flows investigated by Hiesinger et al. (2007). The slopes determined from individual MOLA points vary between 0.06° and 8.2° with an average of 2.1° and a standard deviation (SD) of 1.6°. Whereas the slope of the 10 lava flows directly located on the flanks of Elysium Mons show relatively high values, varying from 2.1° to 8.2° (avg. 4.9°, SD: 1.7°), all other flows are located in much flatter regions, showing slopes from 0.1° to 1.6° (avg. 1.0°, SD: 0.5°) in the plains, and slopes from 0.2° to 0.4° (avg. 0.3°, SD: 0.1°) for the flows south of Albor Tholus. Using these measured dimensions, we calculated the rheological properties of individual flows. 4.1.1. Yield strengths The yield strengths of the Elysium Mons lava flows have been calculated using Eqs. (1) and (2) with assumed values for the rock density (2500 kg m3) and the gravity (3.73 m s1). All other required input parameters such as the slope, the flow length, width, and height could be directly measured as described in Section 3.1. The yield strengths calculated with Eq. (1) range from 1.1  102 Pa (Flow 17) to 2.7  104 Pa (Flow 2), with an average

of 4.7  103 Pa (SD: 1.6  103 Pa). Using Eq. (2) we calculated yield strengths ranging from 3.3  102 Pa (Flow 26) to 3.9  103 Pa (Flow 1) with an average of  1.3  103 Pa (SD: 8.9  102 Pa). Taking the average yield strength derived from both equations results in a yield strength of 3.0  103 Pa (SD: 3.3  103 Pa). The yield strengths of the flows on the flanks of Elysium Mons vary between 1.4  103 Pa and 1.5  104 Pa (avg. 7.2  103 Pa, SD: 4.8  103 Pa), whereas the flows in the plains show yield strengths ranging from 3.8  102 Pa to 5.5  103 Pa (avg. 2.0  103 Pa, SD: 1.2  103 Pa). The yield strengths of the flows south of Albor Tholus range from 7  102 Pa to 1.2  103 Pa (avg. 9.6  102 Pa, SD: 1.8  102 Pa). Thus, the yield strengths of the lava flows in this study are generally very similar to the lava flows of the Tharsis volcano Ascraeus Mons (2.1  103 Pa; Hiesinger et al., 2007), but much higher than the lava flows in central Elysium Planitia (avg. 156 Pa) investigated by Vaucher et al. (2009). The results of our yield strength calculations for each individual lava flow are shown in Table 1.

4.1.2. Effusion rate The effusion rates were calculated by utilizing Eq. (4). As described in Section 3.1, the Graetz number and the thermal diffusivity have to be assumed by analogy with terrestrial lava flows. All other parameters were directly derived from remote sensing data. Effusion rates were found to be 747 m3 s1 on average (SD: 894 m3 s1), ranging from 99 m3 s1 (Flows 9, 23, 32) to 4452 m3 s1 (Flow 33) (Table 1). The effusion rates of the flows on the flanks of Elysium Mons vary between 99 m3 s1 and 219 m3 s1 (avg. 139 m3 s1, SD: 41 m3 s1) and the flows in the plains have effusion rates from 126 m3 s1 to 1834 m3 s1 (avg. 663 m3 s1, SD: 471 m3 s1). The flows south of Albor Tholus have effusion rates between 612 m3 s1 and 4452 m3 s1 (avg. 2253 m3 s1, SD: 1506 m3 s1). In general, the flows of Elysium Mons show slightly higher effusion rates than the lava flows on Ascraeus Mons (23–404 m3 s1, Hiesinger et al., 2007), but are similar (20–9800 m3 s1) to the lava flows studied by Vaucher et al. (2009). The effusion rates of lava flows in the Elysium region,

Table 1 Calculated rheologies and results of crater size-frequency measurements of the investigated lava flows on Elysium Mons. Flow number

Flow height (m)

Flow length (m)

Flow width (m)

Slope (°)

Yield strength (Pa)

Effusion rate (m3 s1)

Viscosity (Pa s)

Eruption duration (days)

Area (km2)

Crater retention age, N(1)

Absolute Model Ages (AMA) (Ma)

1a

22.9

30,260

1238

6.1

1.3  104

147

2.1  107

66

3610

3.97  104

813

2a

24.2

29,552

1998

6.7

1.5  104

219

3.1  107

97

9310

6.23  104

1280

3920

4

1120

4

1050

3

a

4

a

6.7

15,300

793

5a

21.6

28,500

6b

17.1

43,127

–

–

–

8.2

–

–

2.1

1.4  10

3

1236

3.8

2993

1.4

–

–

5.45  10

164

1.2  10

5

6

1180

5.12  10

8.5  103

147

1.3  107

56

4110

3.99  104

818

2.4  103

680

1.2  106

37

12,600

8.54  104

1750

3

2620

–

–

–

7.2

–

–

–

–

4770

1.28  10

8a

–

–

–

5.0

–

–

–

–

1420

4.71  104

967

9a

13.2

2.5

3.5  103

1330

9.24  104

1890

1.1

3

5320

4

1400

4

890

7

b

10

19.9

14,522 33,518

1005 1876

b

11

18.0

20,419

1294

12b

16.3

29,885

13b

11.4

35,617

2.8  10

99

2.6  106

285

3.7  10

6

6

46

1710

4.34  10

21 51

6.84  10

1.1

3

3.0  10

126

6.4  10

2081

1.6

2.7  103

344

1.8  106

36

6170

6.58  104

1350

1487

1.5

1.8  103

419

3.5  105

16

4320

5.96  104

1220

2

820

1.8  10

6

76

26,800

4

8.99  10

1840

b

14

22.7

37,374

5516

0.2

8.4  10

15b

14.9

26,987

1928

1.3

2.2  103

314

1.3  106

29

5460

2.52  103

3460

16b

19.0

31,686

2373

1.6

3.1  103

357

3.1  106

47

7800

6.27  104

1290

0.1

3.8  10

2

317

2.5  10

5

3400

4

1970

3

928

1.2  10

6

49

18,700

4

5.55  10

1140

b

17

11.3

21,775

1828

17

9.61  10

b

18

19.9

60,038

3425

0.6

1.6  10

19b

12.0

30,621

1804

1.4

1.8  103

414

4.5  105

19

5910

8.37  104

1720

20b

12.9

48,633

2431

0.7

1.0  103

822

2.5  105

26

15,600

8.00  104

1640

3

778

4.6  10

5

35

18,100

4

5.61  10

1150

b

21

15.0

45,465

2846

0.6

1.1  10

22b

21.3

23,788

1811

1.4

3.7  103

182

8.1  106

58

4190

8.72  104

1790

23a

11.4

16,250

782

4.4

4.9  103

99

2.9  106

21

2580

6.99  104

1430

0.9

3

320

5

4610

3

2630

4

1740

b

24

12.4

26,434

1663

b

25

20.2

71,720

5742

26b

12.1

42,492

27b

34.5

91,981

1.3  10

0.3

2

9.0  10

4090

0.8

3984

1.4

5.2  10

21

1.29  10

1834

6.0  10

5

54

42,700

8.51  10

9.1  102

1295

1.9  105

28

33,900

3.08  104

632

5.5  103

956

1.2  107

183

51,000

1.22  103

2500

+120 130 +120 120 +380 400 +310 360 +130 130 +310 330 +330 390 +300 350 +540 560 +230 250 +200 210 +260 310 +330 350 +320 380 +150 1500 +300 320 +310 340 +140 160 +190 210 +390 480 +220 260 +700 880 +290 330 +260 290 +640 760 +200 200 +260 310

Distance to EM caldera (km) 53 40 32 97 81 204 42 64 100 113 112 102 166 349 109 163 252

J.H. Pasckert et al. / Icarus 219 (2012) 443–457

a

Error AMA (Ma)

277 143 265 270 180 92 121 272 146 258 449

(continued on next page)

J.H. Pasckert et al. / Icarus 219 (2012) 443–457

380

7.97  104 10,600

14,000

–

43 –

c

b

a

– caldera EM –

These lava flows are located directly on the flanks of EM. Lava flows located in the plains between EM, Hecates and Albor Tholus. Lava flows located south of Albor Tholus.

– – –

0.4 3136 37,624 17.4 c

35

18.5 34b

21,216

–

1.3 2176

1.0  10

612

9.2  10

5 3

4.1  106 224 2.7  103

0.3 13,669 32.5 33c

117,737

–

1660 4

4060

8.11  10

880 4.29  104

12,900 4452 1.2  103

1.8  106

41

1810 8.84  104

1380 21 3.2 32

12.8

9915

1431

3.9  10

99

3.2  10

77

127

863 4

34,900

6

2807

3

0.2 8592 24.6

a

31

c

89,379

0.4 16.7 30c

54,071

3911

7.0  10

2

7.8  10

5

4.21  10

1560 4

7.59  10

1980 9.67  104 22,700 1142 9.2  102

4.5  105

37

1150 5.59  104 13,800 34 6.0  105 782 1.1  103 0.5 2997 16.7 29b

47,628

0.3 7941 32.7 28b

78,738

4.1.3. Viscosity The viscosities were calculated using Eqs. (5) and (6). The viscosities calculated with Eq. (5) range from 2.0  104 Pa s (Flow 17) to 4.8  107 Pa s (Flow 2) with an average of 4.3  106 (SD: 9.3  106 Pa s). Using Eq. (6) we calculated viscosities ranging from 1.1  105 Pa s (Flow 4) to 1.7  107 Pa s (Flow 1) with an average of 4.0  106 Pa s (SD: 4.8  106 Pa s). The average values of both equations range from 1.2  105 Pa s (Flow 4) to 3.1  107 Pa s (Flow 1) with an average of 4.1  106 Pa s and a standard deviation of 6.7  106 Pa s (Table 1). The viscosities of the flows on the flanks of Elysium Mons range between 1.2  105 Pa s to 3.1  107 Pa s (avg. 1.1  107 Pa s, SD: 1.1  107 Pa s), while the flows in the plains show viscosities from 1.9  105 Pa s to 1.2  107 Pa s (avg. 2.5  106 Pa s, SD: 3.0  106 Pa s). The flows south of Albor Tholus have viscosities in the range of 4.5  105 Pa s to 1.8  106 Pa s (avg. 1.0  106 Pa s, SD: 5.1  105 Pa s). These results for the lava flows of the Elysium Mons region are in the range of the viscosities of lava flows on Ascraeus Mons, for which the viscosity was calculated to be on average 4.1  106 Pa s (Hiesinger et al., 2007). The viscosities of the lava flows determined by Vaucher et al. (2009) are lower, ranging from 6.9  102 to 2.5  105 Pa s with an average of 4.9  104 Pa s.

+320

413

107

642

89

497

541

160

380

+360 450 +430 550 +530 570 +150 160 +200 230 +230 240 +290 360 +160 170 1640 1510 7.38  104 61,100 1724 1.5  103

4.2  106

140

Absolute Model Ages (AMA) (Ma) Crater retention age, N(1) Eruption duration (days) Viscosity (Pa s) Effusion rate (m3 s1) Yield strength (Pa) Slope (°) Flow width (m) Flow length (m) Flow height (m) Flow number

Table 1 (continued)

calculated by Keszthelyi (1995) (20 m3 s1) are at the lower end of values calculated by Hiesinger et al. (2007) and Vaucher et al. (2009), and much lower than in this study.

Area (km2)

Error AMA (Ma)

Distance to EM caldera (km)

450

4.1.4. Eruption duration On the basis of the calculated mean flow velocities, the flow lengths, the effusion rates and the flow volumes, the eruption durations of the flows were calculated using Eqs. (9) and (10). The average values of these two equations are between 6 (Flow 4) and 183 (Flow 27) days, with an average of 51 and a standard deviation of 38 days (Table 1). We found variations of the eruption durations of the flows on the flanks of Elysium Mons from 6 to 97 days (avg. 42 days, SD: 31 days), whereas the eruption durations of the flows in the plains vary between 16 and 183 days (avg. 50 days, SD: 40 days). The flows south of Albor Tholus have eruption durations from 37 to 127 days (avg. 71 days, SD: 36 days). The minimum eruption duration of 6 days is similar to those of flows on Ascraeus Mons, calculated by Hiesinger et al. (2007) to be on the order of 2 days. The maximum value of 183 days is twice as large as those of Ascraeus Mons flows (80 days), which might be a consequence of the larger flow lengths on Elysium Mons. Taking the flow dimensions determined by Vaucher et al. (2009) for lava flows in Elysium Planitia, we calculated the eruption duration with Eqs. (9) and (10). They range from  1 to 155 days with an average of 32 days, thus being consistent with the results for the Elysium Mons lava flows of this study. Consequently, the flows in the Elysium Mons region show generally similar eruption durations independent from the distance to the caldera of Elysium Mons. 4.1.5. Error discussion As already discussed in Section 3, there are some uncertainties associated with calculating the rheological properties of lava flows from remote sensing data. The errors caused by these uncertainties will be estimated and discussed in this chapter. For this purpose, we determined how reliable our results are with respect to variations of the input parameters. For example, for our error estimation, we varied the rock density and the thermal diffusivity by +10%, +70%, 10%, and 70%. The wide error range of ±70% reflects the wide variability of rock densities and thermal diffusivities found in literature. Our literature search revealed variations of +20% to 71% for the rock density of 2500 kg m3 and a variation of +66% to 33% for the thermal diffusivity (Gregg and Fink,

J.H. Pasckert et al. / Icarus 219 (2012) 443–457

1996; Gregg and Zimbelman, 2000). With the available data sets, flow heights, widths, lengths, and slopes can likely be estimated with higher accuracy. Consequently, for our error estimates we assumed that these parameters have errors of ±10%, respectively. Table 2 shows the effects on the rheological properties when the input parameters are varied by +10%, +70%, 10%, and 70%. For example, if the density is changed by +10%, the viscosity (using Eq. (5)) will change by 10%; if the height is changed by 10%, the viscosity will change by 61%. From this it becomes clear that the flow height is the most sensitive parameter for the calculation of rheological properties. The measurement of the flow heights is directly linked to the quality of the MOLA data. Considering the resolution of the MOLA points illustrates that not the vertical resolution of 30 cm of each MOLA point is the limiting factor, but the horizontal resolution. The elevation of every MOLA point is averaged over an area of 160 m in diameter and MOLA points are taken every 300 m along the orbital track. Consequently, it becomes increasingly difficult to determine accurate heights with decreasing flow size. Compared to changes in flow heights, variations in rock densities, thermal diffusivities, flow widths, lengths, and slopes have a much smaller impact on the rheological properties (Table 2). A similar error discussion is also provided by Hiesinger et al. (2007). 4.1.6. Terrestrial and extraterrestrial analogues The rheological properties of lavas show wide variations throughout the Solar System. Table 3 shows results for the yield strengths, the viscosities, and the effusion rates of lava flows on Mars, Earth, Venus and the Moon. The yield strengths of terrestrial basalt flows vary between 102 and 106 Pa depending on their compositions (e.g., Moore, 1987; Hulme, 1976; Moore et al., 1978; Warner and Gregg, 2003). Lunar mare basalts have yield strengths from 102 to 104 Pa (e.g., Moore and Schaber, 1975; Wilson and Head, 2003; Moore et al., 1978). The lava flows on the Tharsis volcanoes on Mars show variations of 102–105 Pa (e.g., Moore et al., 1978; Hiesinger et al., 2007). Lavas on Venus investigated by McColley and Head (2004) have yield strengths of 104–105 Pa. The yield strengths of the lava flows in the Elysium Mons region are on the order of 102–104 Pa, which is in the range of flows on Ascraeus Mons, thus showing similarities to terrestrial basalts as described by Hiesinger et al. (2007). Compared to the viscosities of lavas on Venus (106–109 Pa s) calculated by McColley and Head (2004), the lava flows on Elysium Mons show significantly lower values. Lunar lava flows have much lower viscosities on the order of several Pa s, which allowed them to spread over very long distances (e.g., Hiesinger and Head, 2006; Hörz et al., 1991). With viscosities ranging from 105 to 107 Pa s, the Elysium Mons lava flows are more similar to the Ascraeus Mons lava flows and terrestrial basalts, which have viscosities ranging from 103 to 107 Pa s. From Earth it is known that lava flows show a wide range of effusion rates (1–103 m3 s1). Lunar lava flows show only relatively low values ranging from 5 to 120 m3 s1, whereas lavas on Venus show relatively high values (102–104 m3 s1). Baratoux et al. (2009) modeled relative effusion rates of small martian shield volcanoes in Syria Planum, Central Elysium Planitia, and on Pavonis Mons and Arsia Mons. The relative effusion rates are obtained by dividing the modeled effusion rates by the area of the particular volcano. The relative effusion rates of the volcanoes investigated by Baratoux et al. (2009) in Central Elysium Planitia vary between 1 and 15 m3 s1 km2, whereas shield volcanoes in Syria Planum, on Pavonis Mons, and Arsia Mons have values of 0.02 to 0.8 m3 s1 km2. To compare these results with the effusion rates of our study, we also divided our effusion rates by the areas of the particular lava flows and we derived relative effusion rates between 2 and 14 m3 s1 km2. These values are comparable with the

451

modeled relative effusion rates of shield volcanoes in Central Elysium Planitia determined by Baratoux et al. (2009). Like yield strengths and viscosities, the results for the effusion rates of the flows on the Elysium Mons region (99–103 m3 s1) are similar to terrestrial basaltic a’a lava flows and the Ascraeus Mons lava flows (18–104 m3 s1). 4.2. Absolute model ages Crater counts for all 35 lava flows were either performed on CTX or HRSC images, depending on the image quality and the coverage of the respective flow. Absolute model ages of 21 lava flows have been derived from CTX images and 24 lava flows have been dated with HRSC images. Hence, the ages of 10 lava flows have been determined on both HRSC and CTX, to analyze potential differences in age derived from the two data sets. Whereas, Flows 5, 9, 10 and 11 showed the same absolute model ages on CTX and HRSC, Flows 6, 13, 14, 19, 20 and 31 display slightly different ages on CTX and HRSC images. However, the differences in age are within the error ranges for all these flows. A reason for the differences in ages might be the lower quality of the two HRSC images, due to dust in the atmosphere and a lower spatial resolution compared to the CTX images. Secondary cratering also has to be taken into account, especially for small craters on the CTX images. Because Flows 13, 4, and 20 showed evidence of secondary cratering of craters <150 m on CTX images, we chose the HRSC ages, based on craters >150 m, for our statistics. The absolute model ages of all lava flows range from 632 Ma (N(1) = 3.39  104) for Flow 26 to 3460 Ma (N(1) = 2.52  103) for Flow 15 (Table 1). Looking at the three regions defined above (flanks, plains, and south of Albor Tholus) we found that the model ages of lava flows on the flanks of Elysium Mons range from 663 Ma to 2620 Ma. The flows in the plains show ages between 632 Ma and 3460 Ma, and the flows south of Albor Tholus have ages from 1660 Ma to 2360 Ma. Thus, in general the lava flows on the flanks and in the plains have similar minimum and maximum ages, whereas the lava flows south of Albor Tholus have higher minimum ages. We also determined the absolute model age of the Elysium Mons caldera floor with crater counts. With an absolute model age of 1640 Ma (N(1) = 7.97  104), the caldera of Elysium Mons is generally similar in age to the calderas of Albor Tholus (500 Ma, 600 Ma, 1600 Ma, 2200 Ma) and Hecates Tholus (100 Ma, 300 Ma, 1000 Ma) (Neukum et al., 2004a,b), all of these being in the range of ages of the lava flows in the Elysium Mons region. Crater counts of the Elysium Mons caldera on HRSC images by Werner (2009) indicate an age of 3500 Ma with a resurfacing event at 1600 Ma. Robbins et al. (2011) divided the Elysium Mons caldera into three smaller calderas and derived absolute model ages of 2800 Ma, 3100 Ma and 3200 Ma. Compared to the calderas of the Tharsis volcanoes, which show ages between 100 and 400 Ma (Neukum et al., 2004a,b), the calderas of the Elysium Mons region show young and old ages (100–2200 Ma) (Neukum et al., 2004b). 4.3. Correlations between rheologies, ages, and distances to the caldera One objective of this study was to search for potential changes in the rheological properties with time and distance to the caldera. Furthermore, studying any potential relationships between the ages of the lava flows and their distance to the caldera has been a point of interest (Table 1). For the following discussion, we have to keep in mind that we only investigated a relatively small number of flows. Consequently, our statistics most likely are influenced by effects that occur when dealing with small numbers. In addition, not all of the studied flows are necessarily associated with eruptions from Elysium Mons.

452

J.H. Pasckert et al. / Icarus 219 (2012) 443–457

Table 2 Estimation of errors for the calculated rheological properties. Relevant input parameters such as density, thermal diffusivity, height, width, length, and surface slope were varied by +10%, +70%, 10% and 70%. Columns show the effects of these variations on the rheological properties.a Parameter and assumed errors

Error YS I(1) (%)

Error YS II(2) (%)

Error ER(4) (%)

Error EDU(5,6) (%)

Error VI I(7) (%)

Error VI II(8) (%)

Density +10% +70% 10% 70%

10 70 10 70

10 70 10 70

n.a. n.a. n.a. n.a.

n.a. n.a. n.a. n.a.

10 70 10 70

10 70 10 70

Thermal diffusivity +10% +70% 10% 70%

n.a. n.a. n.a. n.a.

n.a. n.a. n.a. n.a.

10 70 10 70

9 41 11 233

9 41 11 233

9 41 11 233

Flow height +10% 10%

10 10

21 19

9 11

21 19

61 41

45 34

Flow width +10% 10%

n.a. n.a.

9 11

10 10

n.a. n.a.

9 11

n.a. n.a.

Flow length +10% 10%

n.a. n.a.

n.a. n.a.

10 10

n.a. n.a.

9 11

Slope +10% 10%

10 10

n.a. n.a.

n.a. n.a.

n.a. n.a.

n.a. n.a.

9 11 10 10

a Notes: ‘‘n.a.’’ indicates that theses parameters do not affect the used equations. YS I/II = Yield Strength I/II; ER = Effusion Rate; VI I/II = Viscosity I/II; EDU = Eruption Duration; (x) used equations.

Table 3 Comparison of rheological properties of lava flows on Earth, Mars, Moon, and Venus. Location

Yield strength (Pa)

Viscosity (Pa s)

Effusion rate (m3 s1)

Elysium Monsa Ascraeus Monsb Venusc Earthd Moone

102–105 102–105 104–105 102–106 102–104

104–107 104–108 106–109 103–107 1–109

69–103 18–104 102–104 1–103 5–120

a

This study and Vaucher et al. (2009). Hiesinger et al. (2007). McColley and Head (2004). d Moore (1987), Hulme (1976), Moore et al. (1978), and Warner and Gregg (2003). e Mare Basalts from Moore and Schaber (1975) and Moore et al. (1978), viscosities of domes from Wilson and Head (2003), and low viscous lava flows from Hiesinger and Head (2006). b

c

Fig. 5 plots the yield strengths of the studied flows and their distances to the caldera. The figure shows that the flows on the flanks of Elysium Mons have the widest range in yield strengths, whereas the flows south of Albor Tholus have a narrow range. The flows in the plains, which might be the most distant flows still associated with Elysium Mons show a broader range in yield strengths than the flows of Albor Tholus, but not as broad as the flows on the flanks of Elysium Mons. Although the flows on the flanks have a higher average yield strength (7.2  103 Pa) than the flows in the plains (2.0  103 Pa) and south of Albor Tholus (9.6  102 Pa), we did not find a statistically significant change of the yield strengths with the distance to the caldera of Elysium Mons (R2 = 0.54). The narrow range of the yield strengths for the flows south of Albor Tholus might simply be a consequence of the small number of studied flows. Including the results for the yield strength estimated by Vaucher et al. (2009), who investigated the rheological properties of lava flows in Elysium Planitia, about 1400 km away from the caldera of Elysium Mons, shows that their maximum value of 316 Pa is even lower than our minimum value of 379 Pa for Elysium Mons. However, the flows investigated by Vaucher et al. (2009) might have different sources and cannot be confidently linked to Elysium Mons.

Correlating the effusion rates with the distance to the caldera shows that the lava flows on the flanks of Elysium Mons show a lower average effusion rate (139 m3 s1) than the flows in the plains (663 m3 s1). The flows south of Albor Tholus show the highest average effusion rate of 2253 m3 s1. However, the coefficient of determination R2 for all studied lava flows is low (0.67). Including the lava flows investigated by Vaucher et al. (2009) illustrates that the effusion rates of the Elysium Planitia flows are similar to the Elysium Mons lava flows (Fig. 5). Correlating the viscosities of the flows with their distance to the caldera shows that the lava flows on the flanks of Elysium Mons (Fig. 5) show wider variations in their viscosities (1.2  105 to 3.1  107 Pa s) than the flows in the plains (1.9  105 Pa s to 1.2  107 Pa s). The flows south of Albor Tholus have an even narrower range in their viscosities (4.5  105 Pa s to 1.8  106 Pa s). Although, the three flows on the flanks closest to the caldera have the highest viscosities compared to the other flows, the R2 value is low (R2 = 0.17), indicating a statistically insignificant correlation of the two parameters. Including the lava flows investigated by Vaucher et al. (2009) shows that these flows have much lower viscosities (6.8  102 to 2.5  105 Pa s) than the flows in our study. Changes in rheology with time have not been observed for the studied flows. We did observe changes in the yield strengths, effusion rates and viscosities, but they occur among lava flows of the same age. In general, with the exception of the three flows closest to the Elysium Mons caldera, young lava flows show similar yield strengths and viscosities as old lava flows (Fig. 6). We also did not find a correlation between the effusion rates and the ages of the studied flows. We also investigated potential changes in the ages of the lava flows with their distances to the caldera. The lava flows on the flanks of Elysium Mons and in the plains show a wide variation in age, ranging from 632 Ma to 3.5 Ga, whereas the lava flows south of Albor Tholus show a narrower range from 1500 Ma to 2000 Ma. Fig. 7 shows that the lava flows in Elysium Planitia investigated by Hartmann and Berman (2000) are much younger than our investigated lava flows in the Elysium Mons region. Hartmann and Berman (2000) detected lava flows with ages of 10 Ma to

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453

Fig. 5. Rheologies (yield strength (top), effusion rate (center), viscosity (bottom)) of flows versus their distances to the caldera of Elysium Mons (EM).

300 Ma. We did not observe lava flows younger than 600 Ma, neither close to the caldera, nor farther away from the caldera. 5. Discussion The investigation of lava flows on Elysium Mons has shown that their rheological properties are very similar to those of other major volcanic regions on Mars, such as Ascraeus Mons in the Tharsis re-

gion. The calculated values of the yield strengths and viscosities point to a basaltic/andesitic composition of the lava flows, similar to basaltic or andesitic a’a flows on Earth and other planetary bodies (Table 3). The effusion rates calculated in this work are similar to those of Vaucher et al. (2009) for flows in Elysium Planitia and higher than those calculated by Keszthelyi (1995). Model ages of the lava flows, derived from crater size-frequency distribution measurements, range from 632 to 3500 Ma. Including

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Fig. 6. Rheologies (yield strength (top), effusion rate (center), viscosity (bottom)) versus the absolute model ages of the flows on Elysium Mons.

the crater counts of Hartmann and Berman (2000) of lava flows in Elysium Planitia, the range becomes even larger varying between 10 and 3500 Ma. Consequently, we have evidence that the Elysium Mons region was volcanically active throughout most of the martian history. Our crater counts on CTX images of the Elysium Mons caldera show that the youngest eruption occurred 1640 Ma ago, which is different to the ages of the youngest calderas of the other two volcanoes in this region, Hecates (100 Ma) and Albor Tholus

(500 Ma) investigated by Neukum et al. (2004b). However, Neukum et al. (2004b) showed that the oldest caldera on Hecates Tholus is 1000 Ma old and that the oldest caldera on Albor Tholus is about 2200 Ma old. Using HRSC images, Werner (2009) also counted craters of the caldera of Elysium Mons, determining an age of 3500 Ma with a resurfacing event 1600 Ma ago. They interpreted this resurfacing event to be of eolian origin. However, on the basis of CTX images, we did not find evidence for eolian

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455

Fig. 7. Absolute model ages of flows versus their distances to the caldera of Elysium Mons (EM).

processes, such as dunes or yardangs that could have covered or eroded older craters in the caldera. Consequently, we interpret the age of 1640 Ma to represent the last volcanic activity in the Elysium Mons caldera. Such young volcanic activity can also be observed in form of lava flows that erupted on the flanks of Elysium Mons. The three absolute model ages (2800 Ma, 3100 Ma, 3200 Ma) of the Elysium Mons caldera investigated by Robbins et al. (2011) are significantly different to our result. This might have several reasons. First, Robbins et al. (2011) counted craters on the entire caldera floor, including areas with clear evidence for secondary cratering such as crater clusters, crater chains, and elongated craters. Second, Robbins et al. (2011) derived their model ages by fitting the production function only to craters with diameters between 100 m and 250 m. Our crater size-frequency distribution measurements were performed on carefully mapped count areas excluding obvious secondary craters. Despite these precautions, we still see a steepening of the cumulative crater frequency at smaller crater diameters. Consequently, we fitted the production function over a wider diameter range and to larger craters, which we believe provide a more reliable age compared to the ages of Robbins et al. (2011). Comparing the ages of the lava flows to the rheological properties suggests that the rheologies have been relatively constant during an extended period of time. This might indicate that the compositions of lavas have not changed significantly since the initial formation of Elysium Mons. Comparing absolute model ages of the flows and their distances to the caldera shows that lava flows close to the caldera have a wide variation in ages from 632 to 3500 Ma. The lava flows investigated by Hartmann and Berman (2000) in Elysium Planitia, are much younger than those on the flanks of Elysium Mons, the surrounding plains, and south of Albor Tholus. 6. Conclusion To our knowledge, this study is the first effort in combining absolute model ages with the rheological properties of lava flows of Elysium Mons. There have been numerous studies of the rheological properties (e.g., Moore et al., 1978; Mouginis-Mark and Yoshioka, 1998; Glaze and Baloga, 2007) in the past. However, these studies were limited to a much smaller number of flows and did not address the temporal aspect. In addition, the main focus of these previous studies was on the morphology and rheology of lava flows located

in Elysium Planitia, but not directly on the flanks of Elysium Mons. Hartmann and Berman (2000) performed crater size-frequency measurements on MOC images of lava flows in Elysium Planitia, but their ages could not be combined with the previously published rheological properties, due to the fact that they have studied different lava flows in Elysium Planitia. The combination of the two data sets offers the opportunity to investigate the evolution of rheologic properties with time. On the basis of measurements of the flow dimensions in HRSC and CTX images, the rheological properties of 32 lava flows on and around the flanks of Elysium Mons and south of Albor Tholus, with distances to the Elysium Mons caldera from 32 to 642 km, were calculated. Flow thicknesses measured with individual MOLA profiles are 18 m on average, ranging from 7 to 34 m. Flow lengths range from 9.9 to 118 km, with an average length of 40 km. The derived flow widths range between 430 m and 13.7 km. The surface slopes of the flows show wide variations between 0.06° and 8.2°. Based on these measurements and a few assumptions, calculated yield strengths are on the order of 3.0  103 Pa, ranging from 3.8  102 to 1.5  104 Pa. Viscosities were calculated to be on average 4.1  102 Pa s, with a range of 1.2  105 to 3.1  107 Pa s. The obtained effusion rates of the flows range from 99 to 4450 m3 s1, averaging at 747 m3 s1. The lava flows have been emplaced between less than a week (very small flows) to up to half a year (6–183 days). The calculated rheological properties are very similar to those of other volcanic regions on Mars, such as the Tharsis Montes volcanoes. The absolute model ages of the flows, derived from crater sizefrequency measurements, range from 632 to 3460 Ma, and are much older than the lava flows investigated by Hartmann and Berman (2000) in Elysium Planitia. Hartmann and Berman (2000) determined flow ages from 10 to 300 Ma. Our results indicate that the latest volcanic activity in the caldera of Elysium Mons occurred approximately 1640 Ma ago. Compared to other large volcanoes in Elysium Planitia (Hecates Tholus and Albor Tholus), the Elysium caldera is rather old. The youngest eruptions of Hecates Tholus and Albor Tholus are in the range of 100 and 500 Ma, based on crater counts from Neukum et al. (2004b). Such young volcanic activity could only be observed in form of lava flows erupted on the flanks of Elysium Mons and not in the caldera. Lava flows younger than 632 Ma could not be observed. Our study combines calculations of rheologies, ages, and distances to the Elysium Mons caldera in order to study potential changes in rheologies with distance to the caldera and time. On

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