Lunar and Planetary Science XXXIII (2002) 2024.pdf INTERPRETATIONS OF GRAVITY ANOMALIES AT OLYMPUS MONS, MARS: INTRUSIONS, IMPACT BASINS, AND TROUGHS. P. J. McGovern, Lunar and Planetary Institute, Houston TX 77058-1113, USA, ([email protected]). Summary. New high-resolution gravity and topography We model the response of the lithosphere to topographic loads data from the Mars Global Surveyor (MGS) mission allow a re- via a thin spherical-shell flexure formulation [9, 12], obtain- ¡g examination of compensation and subsurface structure models ing a model Bouguer gravity anomaly ( bÑ ). The resid- ¡g ¡g ¡g bÓ bÑ in the vicinity of Olympus Mons. ual Bouguer anomaly bÖ (equal to - ) can be Introduction. Olympus Mons is a shield volcano of enor- mapped to topographic relief on a subsurface density interface, using a downward-continuation filter [11]. To account for the mous height (> 20 km) and lateral extent (600-800 km), lo- cated northwest of the Tharsis rise. A scarp with height up presence of a buried basin, we expand the topography of a hole Ö h h ¼ ¼ to 10 km defines the base of the edifice. Lobes of material with radius and depth into spherical harmonics iÐÑ up h with blocky to lineated morphology surround the edifice [1-2]. to degree and order 60. We treat iÐÑ as the initial surface re- Such deposits, known as the Olympus Mons aureole deposits lief, which is compensated by initial relief on the crust mantle =´ µh c Ñ c (hereinafter abbreviated as OMAD), are of greatest extent to boundary of magnitude iÐÑ . These interfaces the north and west of the edifice. are subsequently deformed by the load of the observed topog- raphy, accounting for filling of the basin by material with load The free-air gravity field near Olympus Mons [3] exhibits a Ö ¼ density Ð . We calculate a suite of models, varying and large anomaly centered over the Olympus Mons edifice (Figure h ¼ to determine the dimensions of the compensated basin that 1a). However, a lower-magnitude positive anomaly extends best reduces the residual Bouguer anomaly in the vicinity of outward from the edifice toward the north, partially cover- the OMAD. ing several of the OMAD lobes. The existence of a positive gravity anomaly associated with the northern OMAD was first 0 mGal [100] 0 mGal [50] discussed by [4], who attributed it to a dense intrusive body at 2 depth. Under this hypothesis, the intrusive body was the source 30˚ 30˚ of the OMAD, emplaced as pyroclastic flows. We use the grav- ity and topography data to constrain the dimensions of such a body. However, we also consider an alternative hypothesis, 20˚ 20˚ 1000 300 00 14 inspired by several observations about the nature of Martian 1800200016001200 northern-hemisphere crust. First, Olympus Mons is located 10˚ ((a)a) OObs.bs. FFree-airree-air GGravityravity 10˚ ((b)b) OObs.bs. BouguerBouguer GravityGravity near the boundary between the northern hemisphere (nearly constant-thickness crust) and southern hemisphere (generally 210˚ 220˚ 230˚ 240˚ 210˚ 220˚ 230˚ 240˚ thickening toward the south pole) crustal provinces [5]. Sec- mGal [50] mGal [50] ond, Mars Orbiter Laser Altimeter (MOLA) topography data 30˚ 30˚ from MGS have revealed a large population of partially buried 200 impact craters and basins in the northern plains of Mars [6], 0 indicating an ancient age for northern hemisphere basement 300 units. If Olympus Mons was constructed on such basement, 20˚ 20˚ 100 ancient impact basins adjacent to the edifice may have been 100 10 filled and buried by volcanic flows or mass movements derived 100 0 from the flanks [e.g., 7, 8]. Large ancient basins are likely to 10˚ ((c)c) RResidualesidual BouguerBouguer Mod.Mod. 1 10˚ ((d)d) RResidualesidual BouguerBouguer Mod.Mod. 2 be compensated by an Airy isostatic mechanism (as observed 210˚ 220˚ 230˚ 240˚ 210˚ 220˚ 230˚ 240˚ elsewhere on Mars [9]). If such a basin is filled and covered m m by volcanic materials during a later time when thick elastic -8000 -4000 0 4000 8000 -8000 -4000 0 4000 8000 lithosphere conditions prevailed [e.g., 9], the basin becomes Topography Topography overcompensated, yielding a roughly circular positive Bouguer Fig. 1. Contours of gravity (harmonic expansion to degree anomaly like that seen near the OMAD (see below). and order 60; contour intervals given in white box in upper right), over color image of topography (1x1 degree grid; color Method. We use harmonic expansions of Martian shape scale bar in meters given at bottom). (a) Contours of Free-air (radius from center of mass [10]), topography (referenced to gravity. Contours were cut off at 2000 mGal, the maximum an equipotential surface [10]) and gravity [3] fields to degree anomaly over Olympus mons is 4052 mGal. (b) Contours of Ñ (Ð ) and order ( ) 60 (the limit of adequate gravity resolution). Bouguer gravity, for surface density of 2900 kg/m¿ . (c) The importance of the distinction between shape and topog- Contours of residual Bouguer gravity, for flexure model with raphy is discussed in [9]. We calculate the Bouguer gravity Ì e = 200 km. (d) As in (c), for model with initial topography ¡g anomaly b in the following manner. First, we calculate the Ö containing a buried, fully compensated basin ( ¼ = 300 km ¡g gravity signal × produced by the observed shape (for an Æ Æ h ¼ ¿ and = 3.5 km, centered at 27.5 N 222 E). assumed density Ð = 2900 kg/m ), using a finite-amplitude ¡g formulation [11]. We then subtract × from the observed Results. The Bouguer anomaly for the Olympus Mons ¡g ¡g bÓ free-air gravity faÓ to obtain the Bouguer anomaly . region is shown in Figure 1b. In contrast with the free-air grav- Lunar and Planetary Science XXXIII (2002) 2024.pdf OLYMPUS MONS GRAVITY: McGovern ity (Figure 1a), the highest-magnitude feature in the Bouguer unique event is required. Furthermore, Acheron Fossae, a frac- gravity is a roughly circular high (> 500 mGal) centered tured feature with a half-annular topographic profile located northwest of the Olympus Mons edifice between two OMAD to the north of Olympus Mons and OMAD, is of Noachian lobes (slightly northwest of the highest free-air OMAD-related age, implying a similar age for the basement terrain under- Ì anomaly). A flexure model with elastic lithosphere thickness lying the northern OMAD. For plausible values of e [9], Ì e = 200 km [9] yields a peak residual Bouguer anomaly Acheron Fossae lies on the flexural arch produced by the load ¡g > bÖ 400 mGal, again centered near the boundary of two of the Olympus Mons edifice. The resulting uplift likely al- OMAD lobes (Figure 1c). This anomaly corresponds to peak lowed Acheron Fossae to escape burial by OMAD materials Ù ¡ uplift a = 18 km on an interface with density contrast and flows from Olympus Mons and Alba Patera. In contrast, ¿ d = 500 kg/m , centered at depth i = 50 km. For shallower the proposed basin lies within the flexural moat of Olympus Mons, subjecting it to burial [e.g., 8]. d Ù Ù ÑaÜ a i , decreases. The minimum value of is constrained by the lack of evidence for exposures of dense intrusive vol- As noted above, no other volcanic regions exhibit resid- canic material at the surface of the northern OMAD, yielding ual Bouguer anomalies comparable in magnitude to that seen d Ø Ø Ù in the northern OMAD (Figures 1c,d). However, comparable i the condition ÑiÒ = , where is the topography at Ù anomalies are evident to the southwest of Olympus Mons, as- the location of peak uplift. The resulting ÑiÒ is about 10 km. For flexure models that include the effects of a subsurface sociated with northwest-trending valleys whose formation was Ì recently attributed to catastrophic floods originating in central compensated basin (again, with e = 200 km), we attempt to g ¡ Tharsis (the northwestern slope valleys, or NSVs, of [14]). minimize bÖ in the vicinity of the OMAD anomaly. The Ö These valleys exhibit gravity anomalies that roughly follow the best fit is for a basin with ¼ = 300 km (600 km diameter) and h linear valley trends: free-air gravity lows but Bouguer gravity ¼ = 3.5 km (Figure 1d). Discussion. The gravity and topography data from MGS highs, the latter implying subsurface compensation. A simi- place new constraints on subsurface structures in the vicinity lar linear residual Bouguer high extends from northern Tharsis of Olympus Mons. A gravity high northwest of the Olympus westward though the Olympus Mons region (roughly along the Mons edifice has been cited as evidence for a dense magmatic 20Æ N parallel in Figures 1b-d). We propose an analogy to the body at depth [4]. MGS gravity data reveal that such a body buried basin hypothesis presented above: the linear Bouguer must be at least 10 km thick and have a large density con- anomaly beneath Olympus Mons may reflect the presence of trast with the crust (lowering ¡ will increase the required an at least partially compensated trough, subsequently buried Ì thickness). While an association of thick intrusive bodies and during an era characterized by high e . The NSVs [14] may large volcanic edifices is plausible, no other anomalies of mag- be the remaining surface manifestation of troughs analogous nitudes similar to that in the OMAD are seen near any other to the proposed sub-Olympus trough (although the former are large Martian edifice or volcanic province. Such a model there- less well-buried than the latter). If the NSV troughs and the fore requires a structure apparently unique on Mars. While the sub-Olympus trough originated at the same time, the latter OMAD deposits themselves are often considered to comprise a must be no younger than the Late Noachian-Early Hesperian unique structure, we note that there is no apparent correlation age attributed to the carving of the valleys [14].