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Home , Ceraunius Fossae, Mars, Noachian, Olympus Mons, Tempe Fossae, Tharsis

and Planetary Letters 230 (2005) 241–254 www.elsevier.com/locate/epsl

Evolution of the region of : insights from observations

Catherine L. Johnsona,*, Roger J. Phillipsb

aInstitute of 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 , and . These data suggest that early Noachian underlies the Tharsis province, and formed contemporaneously with the existence of a dynamo. This crust either pre-dates the formation of Tharsis, or formed during the earlier phases of Tharsis . 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 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 and 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 , 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 -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 Letters 230 (2005) 241–254 dynamics, magmatism, tectonics, volatile Tharsis. Later concentric contractional deformation— inventories, and climate history. The regional top- the Hesperian ridged that are proposed to form a ography comprises a long-wavelength rise of global circum-Tharsis [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 Tharsis ridge belt and the Syria, Sinai, and Thaumasia Planae, bounded by Valles Coprates rise. Marineris, , 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 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), (VM), Solis Syria, and Sinai Planum (SP collectively), (TP), Claritas Fossae (CF), (TF), (CeF), Nectaris Fossae (NF), and (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 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- 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 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 of core–mantle boundary plumes that feed 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 Global Surveyor (MGS) mission was that of crustal over billions of . 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 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 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 due to the Tharsis load may have a global field (i.e., either predating or postdating a strongly influenced the orientations of many subse- martian dynamo), magnetized crust formed during a quent late Noachian networks [17], implying dynamo and subsequently demagnetized, or regions that any transition from either plume or buoyant where the wavelength structure of crustal fields is mantle support of Tharsis to flexural support beyond the currently available magnetic field resolu- occurred during the Noachian. Furthermore, tion. Three different interpretations lead to the follow- models based on present-day gravity and topography ing hypotheses for the timing of a dynamo: (1) Dynamo predict the type and orientation of Noachian tectonic onset that postdated the youngest observed impact structures, indicating that the Tharsis distribu- basins on Mars [31], based on the rationale that the tion has changed little since Noachian times [16]. absence of anomalies in Hellas and Argyre imply basin Despite the extensive studies of Tharsis to-date, formation prior to a dynamo. The inferred magnet- fundamental questions still remain. How and when did ization of carbonates at least 3.9 Ga in this province start to form? What is its geographical ALH84001 [32],suggeststhat,inthisscenario, 244 C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 dynamo onset occurred between the Hellas/Argyre across the Central Anomaly Magnetic High reflect real events (N3.9 Ga) and 3.9 Ga. Such a dynamo could be paleointensity variations of the Earth’s magnetic field, driven by inner core solidification, but current models not alteration. Although a viable hypothesis, the details [33] indicate that such a dynamo would persist for 3 of hydrothermal alteration are difficult to model since Gyr, from the early Hesperian to the late Amazonian. its magnetic expression is poorly understood even for (2) A short-lived early dynamo, with onset in the early terrestrial oceanic [43–47] or continental [48] crust. Noachian (supporting evidence being strongly magne- Additionally, ubiquitous circulation of fluids through- tized early Noachian crust and inferred remanence in out the entire magnetized crustal column over nearly ALH84001), and cessation prior to the formation of the the whole of the northern hemisphere is required, major impact basins of Argyre and Hellas [29,34,35]. whereas in the southern hemisphere geographical Impact demagnetization [36,37] without an extant field restriction of to the crust in available for remagnetization may also explain the the regions of the Argyre and Hellas basins (low absence of magnetic anomalies over the Isidis and relative to the geoid) and their rims (high relative to the Argyre basins and provides further constraints on geoid) is needed. Thus, on scale lengths of several dynamo timing [34,35,38]. (3) Dynamo onset in the hundred kilometers or more, impact demagnetization early Noachian, but with a duration that could extend or heating of the lower crust from below is more likely a into the late Noachian or even early Hesperian [39]. mechanism for regional demagnetization, with hydro- This scenario, in concert with evidence for early- thermal alteration being a potentially important, but Noachian crust underlying in the northern more localized, effect. lowlands [35,40], suggests crustal formation globally Magnetic anomalies over the Terra Cimmeria during a dynamo era, followed by selective regional region are an order of stronger than crustal modification rendering little to no detectable terrestrial crustal anomalies observed at equivalent magnetic signal at altitudes. Possible mech- spacecraft altitudes. These require intense crustal anisms include impact demagnetization [36,37], ther- magnetizations [30,49–51]. Assuming mal demagnetization, or chemical alteration of crustal crustal remanence to be primarily of thermal origin, material. Solomon et al. [39] suggest that hydrothermal magnetic experiments suggest that single-domain demagnetization decorrelates magnetization over sev- magnetite most easily explains the intense magnetic eral hundred-kilometer-length scales and/or demagnet- anomalies [52], but other potential magnetic carriers izes crust. The paucity of magnetic anomalies over are pyrrhotite [53], or multidomain and major impact basins may then be unrelated to the titanohematite [52]. Establishing the magnetic carrier timing of formation of the basins. Furthermore, the is important to understanding the conditions under hypothesis permits observed isolated weak fields over which a magnetic remanence can be acquired and some northern hemisphere terrain. Hydrothermal alter- subsequently modified. ation has been proposed to explain the reduction in amplitude of terrestrial marine magnetic anomalies away from mid-ocean ridges [41,42]. However, this 3. Magnetic field observations at Tharsis clean interpretation is complicated by studies of zero- age mid-ocean ridge samples, indicating that The radial component of four Mars magnetic field such alteration occurs almost instantaneously [43,44], models [49,54–56] is compared with elevation and rather than gradually over 20–30 Myr as required by the surface geology (Fig. 2), focusing on the geographical magnetic anomalies [42]. Furthermore, a near-seafloor region encompassing the Tharsis rise (1508W–458W marine magnetic anomaly study by Gee et al. [45] and 458S–458N). The four models shown use different shows that decreases in magnetic anomaly amplitude data sets and modeling techniques. Purucker et al. [49]

Fig. 2. Topography, geology and magnetic field models over the Tharsis Rise ( 1808E, 08E, 458S, 458N). (a) Topography as in Fig. 1; (b) À Geological map simplified from [10]. Noachian units are denoted by brown colors; Hesperian units by purple, blue, and colors; and

Amazonian units by , , and yellow and white colors. (c)–(f) Br at 200 km from (c) Langlais et al. [56]; (d) Cain et al. [55], (e) Purucker et al. [49], (f) Arkani-Hamed [54]. The color bar gives the scale for Br in nT. C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 245 246 C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 select only the lower altitude data from the aerobrak- non-unique, we proceed by first analyzing structure in ing (AB) and science phasing (SPO), and use an the magnetic field itself, then confirming that our equivalent source technique to generate an altitude inferences are supported by a recent magnetization normalized map at 200 km. They assume radially model. oriented dipole sources (i.e., radial magnetization) Figs. 1 and 2 indicate that radial magnetic field with a spacing of 1.898 (110 km). Arkani-Hamed [54] values greater than 25 nT do not occur at and Cain et al. [55] use AB, SPO, and mapping above about 8 km (topography associated with the data (MO—higher altitude but improved geographical volcanic constructs of Olympus Mons and the Tharsis coverage and ability to minimize external field Montes). Significant magnetic fields occur at all contamination by selecting only night-time data) to positive elevations below 8 km, although the peak produce spherical harmonic models with spatial field magnitude decreases with increasing elevation. resolutions of 530 km and 235 km respectively. Comparison of Br for all four models [49,54–56] with Langlais et al. [56] use AB, SPO, and night-time mapped geological units is useful. Anomalies of over MO data and equivalent source modeling to generate 50 nT amplitude are observed above the 7-km-high a three-component magnetization model (no restric- early Noachian basement complex (Nf) at Claritis tion on magnetization direction, unlike the model of Fossae (unit Nb [11]; see Fig. 1 for labeled features). [49]) with a spatial resolution of 2.928 (175 km). Similar magnitude anomalies are seen above the Nf Good agreement between the major features in the units of Nectaris Fossae [11]. In contrast, no magnetic radial field of the four models is seen. For example, field anomalies are associated with the Noachian units anomalies are visible on the southwest flanks of the (Nf) of Tempe Fossae and Ceraunius Fossae. Negli- Tharsis rise. They are observed in the low-altitude data gible anomalies are observed over Sinai Planum, Syria alone (Fig. 2e) and their presence, long-wavelength Planum, Solis Planum, Thaumasia Planum, and west- structure and amplitude, is confirmed by the more ern Valles Marineris. Anomalies are associated with extensive, but higher altitude MO data set (Fig. 2c,d,f). deformed Noachian terrain southwest (Nplr, Npl1) of Shorter wavelength structure, e.g., across Valles Daedalia Planum; further to the southwest these Marineris, and lower-amplitude anomalies, e.g., close anomalies increase in magnitude to their maximum to the Tharsis Montes, vary among the models as values planet-wide over the Terra Cimmeria region. would be expected on the basis of differing spatial Significant, although of lower amplitude, anomalies resolution and contributing data. are observed at eastern Daedalia Planum, which rises Here we focus on variations in the radial compo- to about 4 km elevation on the Tharsis rise. Surface nent of the magnetic field, resolved by all four models units here are Amazonian in age. Weak (less than 25 over the Tharsis rise. We use the radial component of nT) anomalies close to the Tharsis Montes and the magnetic field (Br), since it is the least contami- Olympus Mons have variable structure and amplitude nated by ionospheric fields. We compare Br with in the different field models, suggesting that resolv- large-scale variations in surface geology and eleva- able magnetic fields are absent at, and immediately tion. It is important to note that an observation of Br at around, Olympus Mons and the Tharsis Montes, agivenspacecraftpositionreflectsthevolume- except over Amazonian surface units on the lower- integrated crustal magnetization. Radial field anoma- most western flanks of Olympus Mons. lies maximally sample the radial component of The highest spatial resolution three-component magnetization directly beneath the spacecraft location; magnetization model available to date [56] is shown the same is not true, however, of sampling of the in Fig. 3. The radial field in Fig. 2c is derived from horizontal component of magnetization, resulting in this magnetization model. As expected, the radial an offset between the magnetized body and its component of magnetization shows similar associated radial magnetic field anomaly. Ideally, spatial structure to that seen in the radial field. Many one would like to correlate crustal magnetization of the observations above hold also for the horizontal directly with geology. Magnetization models for Mars magnetization components (although longitudinal have been produced (e.g., [49,56,57]). As the inver- variations, Mu (Fig. 3d), are more noisy than sion of magnetic field observations for such models is latitudinal variations, Mh (Fig. 3c)), and consequently C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 247

Fig. 3. Magnetization model of Langlais et al. [56]. (a) |M|; (b) Mr; (c) Mh; (d) Mu. Color bar gives the scale in A/m, assuming a 40-km-thick source layer [56]. for the magnetization magnitude (Fig. 3a)—notably: tion with decreasing surface unit age. We investigate locally higher amplitude magnetization at Claritas this further through the distributions of radial field Fossae, significant magnetizations are associated with amplitude and magnetization magnitude (|M|) over units of Amazonian and Hesperian age on Daedalia Amazonian-, Hesperian-, and Noachian-age units for Planum, and increasing toward the southwest. two magnetic field models [49,56]. The model of Significant crustal field anomalies (Br greater than Purucker et al. [49] (Fig. 2e) is of interest as it uses 10 nT) exist over units of all surface ages in the only the lowest altitude and thus highest spatial Tharsis region (Fig. 2). Areally, significant anomalies resolution data. The model resolution is 1.898, likely occur over 56% of Noachian, 34% of Hesperian, and representative of the true resolution in the observa- 28% of Amazonian units. Figs. 2 and 3 also suggest, tions along-track, but interpolated across-track due to on average, decreasing magnetic field anomaly more limited longitudinal coverage. For improved amplitude, and decreasing magnitude of magnetiza- spatial data coverage (but reduced overall resolution 248 C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 from the dominance of higher altitude data), we use bin. The region examined extends from 458S to 458N the model of Langlais et al. [56] (Fig. 2c). This has and from 2108E to 3158E. similar resolution and structure to the spherical The peak anomaly, , and variance of harmonic degree and order 90 model of Cain et al. the distributions of Br and |M|, along with the areal [55] (Fig. 2d), and similar structure to the lower percentage of magnetized terrain, decrease with resolution model of Arkani-Hamed [54] (Fig. 2f). We decreasing surface unit age (Fig. 4 and Table 1). also investigate the corresponding Langlais et al. [56] The results are robust with respect to different models, magnetization model (Fig. 3). different choices of model noise level (10–25 nT, as For each field model, we use a grid of Br or |M| at reported in [42,56]), and even interpolation of the the maximum resolution of the model (28 for [49] and radial field grid (18 grid, Table 1) onto a finer grid for 38 for [56]). The digitized geological map has a comparison with the geological map. Values of Br and resolution of 0.1258 and so we calculate the age |M| over Noachian surface units can be large, but do distribution of surface units in each 28 or 38 bin, not approach the several hundred nano-Tesla field corresponding to the Br or |M| grid. Each value of Br anomalies and correspondingly high magnetizations or |M| is assigned the modal age for the corresponding of the Terra Cimmeria region.

Fig. 4. Histograms (left) and empirical cumulative distribution functions (right) for the absolute value of Br over Amazonian (green), Hesperian (blue) and Noachian (red) surface units. Values of Br less than 10 nT are excluded. Upper figures are for Purucker et al. [49] using a 28 by 28 grid, lower figures for Langlais et al. [56] using a 38 by 38 grid. See text and Table 1 for details. C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 249

Table 1

Model/Ref Grid Cut-off NB(M)ave rB(M) B(M)max

Br [49] 18 18 10 1173/1316/915 45/35/31 33/29/27 191/193/158 Â 25 764/593/352 61/58/56 31/30/28 191/193/158 Br [49] 28 28 0 497/973/847 26/15/11 31/24/19 171/155/155 Â 10 281/332/237 43/37/31 32/31/27 171/155/155 25 175/161/93 60/59/56 31/30/28 171/155/155 Br [56] 38 38 0 225/449/376 27/14/12 33/23/22 188/172/152 Â 10 130/137/96 44/36/35 35/31/32 188/172/152 25 78/62/46 62/60/57 34/33/35 187/172/152 |M| [56] 38 38 0 206/430/374 1.0/0.6/0.5 0.8/0.6/0.5 4.8/4.4/3.8 Â 0.5 133/138/91 1.4/1.2/1.2 0.9/0.8/0.7 4.8/4.4/3.8

N—number of observations. B(M)ave, rB(M), and B(M)max—mean value, standard deviation, and maximum value for field component (Br—radial field) or magnetization, respectively. Values below the cut-off value are excluded. Units are nT for field component and A/m for magnetization. Values for N, B(M)ave, rB(M), and B(M)max are given in following format: Noachian/Hesperian/Amazonian. References: Purucker et al. [49] and Langlais et al. [56].

Kolmogorov–Smirnov tests [58] demonstrate that, small number of measurable, independent field/mag- for the radial field models [49,56] and the magnet- netization estimates over Amazonian-age terrain. ization amplitudes [56], the Noachian cumulative distribution functions are statistically significantly different from the Amazonian and Hesperian distribu- 4. Discussion tion functions at the 95% confidence level (Fig. 3 and Table 2). Furthermore, the decrease in Br amplitudes Comparison of the magnetic field, topography, and from Hesperian to Amazonian units is statistically geology at Tharsis indicates that, in addition to significant for the Purucker et al. model (Table 2), but constraints from gravity and tectonics, the following not for the Langlais et al. model (Table 2) due to the observations must be explained by any model for crustal evolution of the region:

Table 2 (1) the presence of large (~50 nT) anomalies over Kolmogorov–Smirnov (K–S) tests on cumulative distribution early Noachian units at high elevation (~7 km), functions for the radial field or magnetization magnitude associated with Claritas Fossae; Model Data sets K–S Probability, P (2) the presence of significant magnetic anomalies compared statistic 5 over surface units of all ages and corresponding 28 grid, Br N10 Noachian/ 0.187 3.7 10À  crustal magnetization signatures; nT [49] Hesperian 2 Hesperian/ 0.117 4.3 10À (3) the decreasing mean amplitude and variance of  Amazonian anomalies and the decreasing percentage of 2 38 grid, Br N10 Noachian/ 0.161 4.4 10À  magnetized crust with decreasing surface unit nT [56] Hesperian 2 age; Hesperian/ 7.63 10À 0.89  (4) anomalies greater than 50 nT amplitude over Amazonian 2 38 grid, |M| N0.5 Noachian/ 0.166 4.3 10À Amazonian units (Daedalia Planum) high (~4  A/m [56] Hesperian km) on the Tharsis rise; and 2 Hesperian/ 8.87 10À 0.76  (5) the paucity of anomalies at and around Olympus Amazonian Mons, the Tharsis Montes and Tempe Fossae. The K–S statistic is the maximum absolute difference between the cumulative distribution functions. The probability, P,isthe Noachian-age magnetized crust is required to probability of obtaining a K–S statistic this large or larger if the two samples are drawn from the same underlying statistical explain the radial magnetic field [49,54–56] and distribution. Small values of P show that the two distributions are crustal magnetizations [56] observed at Claritas significantly different at the level (1 P) 100%. Fossae, and at Nectaris Fossae. The presence of À  250 C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 magnetic anomalies over surface units of all ages, will only have a small demagnetization effect on the along with the decrease in amplitude of such underlying crust, unless the magnetic carriers are anomalies could be interpreted as evidence for a strongly concentrated upwards. In the second calcu- long-lived dynamo, perhaps waning through the lation, we use the results of the nominal thermal Hesperian and Amazonian. However, the absence of model of Hauck and Phillips [62], (their Fig. 2 and magnetic anomalies and associated crustal magnet- Table 3), to estimate a lower bound on the temperature ization over the Hesperian-age units of Syria, Sinai, gradient in the martian lithosphere at ~4 Ga (12.58/ Solis, and Thaumasia Planai is inconsistent with this km-estimated using 48 mW/m2 for the heat flux into scenario [34].Thissuggestsanearlydynamo, the of the crust). With surface temperature of operative only during the Noachian, but requires us 50 8C, about the present-day equatorial value, and still to explain observations (2)–(5). possiblyÀ low for Noachian Mars, this predicts a depth We propose that the observed magnetic field to the isotherm of ~50 km, approximately the anomalies and crustal magnetization signatures asso- base of the initial crustal layer. Persistent intrusions ciated with Hesperian and Amazonian surface age and underplating of crustal material are permitted, units reflect underlying magnetized Noachian base- such that the temperature at the base of the crust is ment. The weaker anomalies and larger percentage of held close to its melting temperature for a time unmagnetized areas associated with younger surface interval sufficient to reach steady state (~10–100 units reflect the combination of post-dynamo crustal Myr). This provides a crude upper limit on the vertical formation, and regional variations in the extent of extent of demagnetization of the crust by intrusions. demagnetization of buried crust. We focus on thermal For single-domain magnetite, the lower 50% of the demagnetization as the causative agent for two crust is raised above the Curie temperature. The reasons: (1) local increases in crustal thermal gra- thermal results are not significantly different for dients in the Tharsis region would have been hematite (Curie temperature 670 8C); however, the associated with the magmatism that continued through low Curie temperature of titanomagnetite (below to the Amazonian, (2) the physics of thermal 200 8C) renders the retention of significant magnet- demagnetization is well understood. We later address ization difficult. thermal and hydrothermal demagnetization around From these calculations, it is clear that it is volcanic constructs and along fracture belts likely possible to retain magnetization signatures during associated with more localized upper crustal - construction of Tharsis, explaining the magnetic tism and related volatile release [17,59]. anomalies observed at Claritas Fossae and over We investigate thermal demagnetization of a surface units of all ages. For magnetic carriers magnetic source layer for two end member cases: distributed uniformly throughout the crust, the demagnetization by extruded flows (fast cooling) and thermal calculations predict a reduction in anomaly demagnetization from sustained intrusion or a long- amplitudes on the order of 50%. Anomalies over lived magma body. We assume an initial crustal southern hemisphere crust to the SW of Tharsis have thickness of 50–60 km (typical of southern hemi- peak amplitudes of several hundred nT, and average sphere crust [25,28]), uniform magnetization of the at least 100 nT. Thus, on scale lengths of hundreds crust, and a Curie temperature corresponding to of kilometers, magnetic field observations over single-domain magnetite (580 8C). Emplacement of Tharsis are consistent with regional thermal demag- surface flows heats the underlying crust and the netization of crust with an initial magnetic signature maximum depth to the Curie isotherm scales linearly similar to that SW of Tharsis. The lower peak and with the flow thickness [60]. Terrestrial basaltic and mean anomaly amplitudes associated with the young- andesitic lava flows typically have thicknesses of a est surface units are consistent with these units few meters to a few tens of meters [61]. Even for 1- forming after dynamo cessation, and overlying km-thick flows, only the upper few hundred meters of previously magnetized crust that has experienced a the underlying crust are raised above the Curie greater cumulative magmatic flux and enhanced temperature. Thus, as noted in other studies [34,50], thermal demagnetization. Furthermore, focusing of volcanic extrusions after the cessation of a dynamo post-dynamo demagnetization (thermal and/or hydro- C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 251 thermal) along fracture belts (such as that proposed The observed magnetic anomalies, along with to underlie the Tharsis Montes [13]) and at centers of current understanding of Mars’ magnetic field history, magmatic activity is supported by the lack of suggest that much of the Tharsis region is underlain magnetic anomalies at and around Olympus and by (early) Noachian crust that at one time was more the Tharsis Montes. magnetized than at present. By analogy with Earth, It has been proposed that the absence of magnetic much of the earliest Tharsis magmatism is likely to fields at the shield [34] and over the northern have been intrusive [64]; it is thus possible that the portion of the Tharsis rise [63] indicate that Tharsis magnetized basement could be uplifted, pre-Tharsis formed after the cessation of a martian dynamo. crust, early Tharsis intrusives, or both. It is not However, these studies do not address observations currently possible to distinguish between these sce- (1)–(4) outlined at the beginning of this discussion. narios: doing so is important since the latter places We suggest that buried crust formed contemporane- constraints on the relative timing of the earliest ously with the existence of a global magnetic field, formation of Tharsis and cessation of the dynamo. and subsequently partially thermally demagnetized by The timing of the Hellas impact relative to the pervasive intrusion, explains the regional-scale mag- construction of Tharsis may be crucial in under- netic field observations over a large part of the Tharsis standing the source of the Tharsis magnetized crust. rise. While this buried crust may be magnetized pre- Phillips et al. [17] suggest that the Hellas impact Tharsis basement, similarly, e.g., to the southern preceded Tharsis formation, as the basin dips into the hemisphere crust of the Terra Cimmeria region, the trough formed by and circumferential to the Tharsis possibility that its crust is formed during the earliest load. By extension, dynamo cessation prior to Hellas phases of Tharsis construction cannot be excluded. formation [29,35,37] would require older magnetized We return to this in the summary. crustal units to underlie much of the Tharsis province, as hypothesized above. In one end-member scenario, no magnetization was acquired in Tharsis as 5. Summary: evolution of the Tharsis Province they cooled through the Curie temperature, since the dynamo had ceased to exist by that time. Thus, much We conclude by offering the following scenario for of Tharsis must be underlain with ancient crust whose the formation and evolution of Tharsis and then remanent magnetization, particularly in deeper crustal incorporating the constraints suggested by the mag- portions, was decreased regionally by high heat flow netic field data. In the early Noachian thermal, and into the crust associated with partial melting in the possibly compositional, buoyancy produced uplift, upper mantle. A pre-Tharsis crust having the charac- partial melting and deep intrusion. All previously teristics (magnetization, thickness, mineralogy) of proposed hypotheses for Tharsis (e.g., [4,7,13]) southern hemisphere crust is supported by Wenzel et require early, voluminous melt generation and uplift. al. [23] who suggest Tharsis overlies a long-lived The oldest exposed Noachian units, such as those at upwelling, focused under the southern highlands, near Claritas Fossae, may represent pre-Tharsis crust [7], the dichotomy boundary. Original Tharsis crust was or the earliest Tharsis volcanics [4]. Deformation at thickened and uplifted by magmatic underplating, as Claritas Fossae produced the observed concentric early magmas were dense, reflecting the high extensional fracturing [7–9], consistent with early content of the mantle [7,65–67]. Magnetization was Noachian uplift. preserved in the upper crust, as discussed earlier. By the mid-late Noachian, Tharsis topography was Magmas became less dense with time as source supported by a combination of membrane and regions were depleted in iron and as upper crustal bending stresses from loading, and crustal thickening intrusion became more prevalent, particularly along [4,16,17]. This transition is consistent with observed zones of tectonic weakness (e.g., Tharsis Montes and circumferential ridges and radial extensional faulting northeastward into Tempe Fossae). This magmatism at Tharsis, the present-day gravity and topography, and its attendant lateral heating of the country rock orientations and regional-scale varia- locally removed much of the remaining upper crustal tions in magnetic anomalies. magnetization, and this transition from regional-scale 252 C.L. Johnson, R.J. Phillips / Earth and Planetary Science Letters 230 (2005) 241–254 deep melting to more localized shallow melt intrusion source and evolution of the magnetic field at Tharsis predicts more extensive demagnetization at younger is an important one, for example, a syn-Tharsis volcanic centers, and of course the production of post- magnetic field could protect Tharsis-generated vola- dynamo unmagnetized crust. Additionally, if hydro- tiles [17] from atmospheric escape, thus influencing thermal demagnetization has been an important climate evolution during Noachian times (e.g., [59]). mechanism for removing crustal magnetization sig- Furthermore, there are implications for the compo- on Mars, then localized effects in the vicinity sition of Tharsis magmas, how they evolved with of volcanoes can be expected. In a system in which time, and the constitution of the upper mantle thermal effects only are considered, the radial extent beneath Tharsis. Thus, we conclude that magnetic of unmagnetized crust around the Tharsis volcanoes field observations at Tharsis provide important could provide a constraint on the duration of the constraints on the crustal evolution and thermal volcanic conduit (assuming a background crustal history of this region of Mars. thermal profile and conduit size). In practice, hydro- thermal effects will complicate this through chemical alteration (perhaps with associated demagnetization Acknowledgements [39]), and by increasing cooling close to the , reducing the radial extent of thermal demagnetization. We thank Matt Golombek and Mark Jellinek for An alternative end-member hypothesis is that discussions, and Sue Smrekar, Walter Kiefer and an crustal magmatism resides exclusively in early Tharsis anonymous reviewer for reviews, which improved the intrusives, which were emplaced while the dynamo manuscript. We acknowledge Judy Gaukel and was still active. In this scenario, magmatism com- Bridget for proofreading the final version of pletely erased preexisting crustal magnetization, and the manuscript. This work was supported under what is observed today is the residual of remanent NASA and Geophysics grants fields acquired as Noachian Tharsis magmas cooled NAG5-11563 (C.L. Johnson) and NAG-5-13243 below the Curie temperature and then was subse- (R.J. Phillips), and Mars Data Analysis grant quently partially erased by younger episodes of NAG5-11202 (R.J. Phillips). intrusion and volcanism. However, simple dimen- sional analysis and estimated Noachian heat flux [53] suggest that early magmas placed deep within the References crust, or underplating the crust, would likely not have cooled below the Curie temperature during the [1] F.G. Lemoine, D.E. Smith, D.D. Rowlands, M.T. Zuber, G.A. dynamo’s existence. Thus, we would argue that for Neumann, D.S. Chinn, D.E. Pavlis, An improved solution of this model to work effectively, magmas would have the gravity field of Mars (GMM-2B) from Mars Global had to penetrate the upper crust early and often. Pre- surveyor, J. Geophys. Res. 106 (2001) 23359–23377. Tharsis crust would have been effectively assimilated. [2] J.B. Plescia, R.S. Saunders, Tectonic history of the Tharsis region, Mars, J. Geophys. 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