Earth and Planetary Science Letters 230 (2005) 241–254 www.elsevier.com/locate/epsl Evolution of the Tharsis region of Mars: insights from magnetic field observations Catherine L. Johnsona,*, Roger J. Phillipsb aInstitute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0225, United States bDepartment of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, MO 63130, United States Received 15 March 2004; received in revised form 15 September 2004; accepted 13 October 2004 Available online 8 January 2005 Editor: A.N. Halliday Abstract Mars Global Surveyor (MGS) observations of crustal magnetic fields over Tharsis provide new constraints on models for the thermal and magmatic evolution of this region. We analyze the distribution of magnetic field anomalies over Tharsis surface units of Noachian, Hesperian and Amazonian age. These data suggest that early Noachian crust underlies the Tharsis province, and formed contemporaneously with the existence of a martian dynamo. This crust either pre-dates the formation of Tharsis, or formed during the earlier phases of Tharsis volcanism. The preservation of strong magnetic field anomalies over some of the earliest Noachian and topographically high units, together with the observation of magnetic field anomalies over Hesperian- and Amazonian-age surface units, indicate that a large fraction of the magnetized crust has remained cool (below the blocking temperature of the magnetic carrier) throughout the construction of Tharsis. Moreover, the distributions of magnetic anomaly amplitudes over Noachian, Hesperian, and Amazonian surface units suggest that the youngest units overlie sites of prolonged intrusion and have undergone a greater extent of thermal demagnetization. The absence of magnetic anomalies around the Tharsis Montes and Olympus Mons argues for strong, localized heating, as would be expected at volcanic centers. We show that end-member models for progressive thermal demagnetization of a Noachian magnetized crustal layer are consistent with the anomaly amplitude distributions. We integrate the magnetic field observations with constraints from tectonics, gravity, and topography, and present a revised scenario for the evolution of the Tharsis region. D 2004 Elsevier B.V. All rights reserved. Keywords: Tharsis; Mars; magnetism; volcanism; magmatism 1. Introduction * Corresponding author. Tel.: +1 858 822 4077; fax: +1 858 534 5332. The Tharsis volcanic province dominates the west- E-mail addresses: [email protected] (C.L. Johnson)8 ern hemisphere of Mars; understanding the evolution of [email protected] (R.J. Phillips). this region is critical to understanding planet-scale 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.10.038 242 C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 mantle dynamics, magmatism, tectonics, volatile Tharsis. Later concentric contractional deformation— inventories, and climate history. The regional top- the Hesperian ridged plains that are proposed to form a ography comprises a long-wavelength elevation rise of global circum-Tharsis system [10] for which the type several kilometers elevation (Fig. 1). Superposed on locale is Lunae Planum [11]—is of a shorter wave- this are the Tharsis Montes, Olympus Mons, and Alba length and lower amplitude than the contractional Patera. To the southeast, the Tharsis rise includes Solis, deformation in the South Tharsis ridge belt and the Syria, Sinai, and Thaumasia Planae, bounded by Valles Coprates rise. Marineris, Claritas Fossae, and the Coprates rise. The A variety of explanations for Tharsis have been region also dominates the gravity field of the western proposed that have attempted to explain the gravity, hemisphere, with typical free air anomalies of several topography, and tectonic deformation of the region. hundred milligals, reaching over 1000 mgal at Olym- Models include dynamic support of topography by a pus Mons [1]. large mantle plume [12,13], regional uplift due to Tectonic deformation is pervasive throughout the underplating of crustal material derived from the Tharsis region and provides constraints on the history northern hemisphere [14], uplift, either due solely to of uplift, loading, and volcanic construction. Several mantle anomalies—thermal and/or compositional major episodes of radial fracturing occurred from the [15]—or from crustal thickening by intrusion [7], and Noachian onwards [2–5]. Concentric contractional flexural loading from volcanic construction [4,16,17]. deformation of mid-Noachian units of the Coprates AplumeoriginforTharsishasbeenexplored rise and South Tharsis ridge belt has been reported [6]. extensively; however, several difficulties are still Observations of concentric extensional fractures in the associated with this hypothesis. Numerical models in oldest (early Noachian) units of Claritas Fossae first which single plume structures have been generated are noted by Phillips et al. [7], and supported by analyses limited by the timescale required for plume develop- [8] of more recent tectonic mapping [9] provide ment (8 Gyr [18,19]), the necessity of the spinel to additional information on the earliest history of perovskite phase transition in the lowermost mantle Fig. 1. MOLA-derived topography (Hammer-Aitoff projection) for the Tharsis region [68]. The area extends from 458S to 458N and from 1808E to 08E. Color bar gives scale in meters above mean elevation. Referenced in the text are the Tharsis Montes — Arsia (ArM), Pavonis (PM) and Ascreaues (AsM), Olympus Mons (OM), Valles Marineris (VM), Solis Syria, and Sinai Planum (SP collectively), Thaumasia Planum (TP), Claritas Fossae (CF), Tempe Fossae (TF), Ceraunius Fossae (CeF), Nectaris Fossae (NF), and Daedalia Planum (DP). Alba Patera is to the north of Ceraunius Fossae and the Coprates rise is the ridge to the east of Nectaris Fossae. C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 243 [18,19], or by the model geometry (spherical axisym- relationship to the pre-Tharsis expression of the metric) [20]. Furthermore, the required maintenance of dichotomy boundary? How have the magmatic flux a persistent plume for the 4 Gyr history of Tharsis [17] and thermal environment varied over time? Answering is challenging. Inversions of geoid and topography for these questions is crucial to constructing realistic load structure for a viscoelastic body indicate that less models for the evolution of Tharsis. Geology, gravity, than 25% of the present-day geoid can be attributed to and topography provide information on the loading an upper mantle plume [21]. An alternative proposes history of the region and on the history of extrusive that early in Tharsis’ history, the lithosphere was weak volcanics. In this paper, we show that magnetic field and thin, overlying a regionally warmer than average data provide a complimentary and rich source of upper mantle, permitting localized melting and con- information on the subsurface crustal structure and on sequent intrusion and extrusion [4].Kiefer[22] the thermal history of the region. In the following proposes a hybrid model consisting of a warm region section, we summarize previous work and outstanding of broad mantle upwelling beneath Tharsis that is host questions regarding Mars’ magnetic field, to set the to a series of core–mantle boundary plumes that feed stage for our magnetic studies of Tharsis. individual volcanic centers (and overcome the issue of long development time for a single plume structure). While viable for early Tharsis, support of the rise 2. Mars’ magnetic field history throughout its history solely by mantle thermal and compositional anomalies is not favored since it requires One of the most dramatic discoveries of the Mars the maintenance of large lateral variations in density Global Surveyor (MGS) mission was that of crustal over billions of years. Thus, the earliest history of magnetic field sources of multiple scales, strength and Tharsis is still not well understood from a dynamical geometry [29,30]. Strong magnetic fields are observed perspective. A key to the formation of Tharsis may be over the Noachian-age Terra Cimmeria southern hemi- pre-existing hemispherical differences in crustal thick- sphere region, with weaker isolated anomalies over ness—that permit either the formation of a persistent, northern hemisphere terrain and other southern hemi- localized plume [23] or regional-scale, nonplume- sphere areas. A general paucity of strong magnetic related, transient melting [24]. fields exists over the major impact basins. Present-day gravity and topography are consistent Mars’ magnetic field history, specifically the timing with flexural loading and crustal thickening at of a core dynamo implied by the crustal magnet- Tharsis [16,17,25]. Estimates for lithospheric thick- izations, is difficult to unravel from the MGS observa- ness in the region range from 70 to 150 km [26,27]. tions, whose major challenges are the spatial Crustal thickness in the region varies by almost distribution of the anomalies and their strength. 100%, and, while absolute values are unconstrained, Interpretation of the crustal magnetic field is difficult reasonable assumptions lead to estimates of 50 km to since areas lacking anomalies may reflect crustal over 90 km [25,28].Flexurallyinducedglobal formation during a period when Mars did not possess topography