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Sixth International Conference on (2003) 3065.pdf

THE REGION OF MARS: NEW INSIGHTS FROM MAGNETIC FIELD OBSERVATIONS. C. L. Johnson1 and R. J. Phillips2, 1Institute for and Planetary Physics, Scripps Institution of Oceanogra- phy, 9500 Gilman Drive, La Jolla, CA 92093-0225 ([email protected]), 2Dept. of Earth and Planetary Sci- ences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, MO 63130 (phil- [email protected])

Introduction: The Tharsis volcanic province a revised scenario for the evolution of Tharsis pre- dominates the western hemisphere of Mars. The re- sented. gional topography comprises a long-wavelength rise of Previous Models for Tharsis Formation: A vari- several kilometers elevation. Superposed on this are ety of models for the formation of Tharsis have been the , and Alba Patera. proposed, and predicted gravity, topography and tec- To the southeast, the Tharsis rise includes Solis, Syria tonic stresses compared with available observations. and Sinai Planae, bounded by , Clari- These models include dynamic support of topography tas Fossae, and the Coprates rise. The region also by a large mantle plume [11,12]; regional uplift due to dominates the gravity field of the western hemisphere, underplating of crustal material derived from the with typical free air anomalies of several hundred mil- northern hemisphere [13]; uplift due to solely mantle ligals, and a peak free air anomaly greater than 1000 anomalies - thermal and/or compositional [14], and milligals over Olympus Mons. including crustal thickening by intrusion [6]; flexural Tectonic deformation is pervasive throughout the loading due to volcanic construction [2, 15, 16]. Tharsis region and provides constraints on the history There are several difficulties associated with plume of uplift and/or loading and/or volcanic construction. models. Single-plume structures are difficult to There have been several major episodes of radial frac- achieve in numerical convection models, and the re- turing from the onwards [1,2,3,4]. Concen- quired maintenance of persistent plume for the 4 Ga tric contractional deformation of mid-Noachian units history of Tharsis [15] is problematic. Furthermore, of the Coprates Rise and Tharsis ridge belt has the lack of sensitivity in geoid kernels at appropriate been reported [5]. Additional information on the earli- mantle depths [17], means that less than ten per cent of est history of Tharsis is provided by observations of the present-day geoid can be attributed to an upper concentric extensional fractures in the oldest (early mantle plume. Support of the Tharsis rise solely by Noachian) units of [6,7,8]. Later con- mantle thermal and compositional anomalies are not centric contractional deformation – the favored since it requires the maintenance of large lat- ridged plains (type locale: Lunae Planum) – is of a eral variations in density over billions of . Pre- fundamentally different wavelgnth and amplitude than sent-day gravity and topography is consistent with the above-mentioned contractional deformation in the flexural loading and crustal thickening at Tharsis South Tharsis Ridge belt and the Coprates Rise. [15,16, 18]. Estimates for lithospheric thickness in the Here we focus on new constraints for models for region are variable and range from 70 - 150 km the origin and evolution of Tharsis, provided by Mars [19,20], nonetheless they are consistent with current Global Surveyor (MGS) magnetic field data. Magnetic flexural support of much of the Tharsis topography. field observations are compared with the geology and Magnetic Field: Observations. MGS magnetic topography of the region. Given that it is unlikely that field data provide critical new constraints on the his- a global magnetic field persisted beyond the late Noa- tory of crustal evolution in the Tharsis region. Here chian [10], we investigate end-member scenarios for we discuss the altitude-normalized (200 km altitude)

thermal demagnetization of pre-existing, or early- radial magnetic field anomalies (Br) of Purucker et al. Tharsis, magnetized Noachian . Specifically, we [21]. Investigations of other magnetic field data sets test whether observed distributions of magnetic field and global magnetic field models (refs) are underway. amplitude over surface units of , Hesperian Figure 1 shows a simplified version of the geological

and Noachian , are consistent with reasonable map of the western hemisphere of Mars [22] and of Br, bounds on the extent of crustal thermal demagnetiza- each draped over MGS-derived topography [23]. tion during the construction of Tharsis. Implications Anomalies of over 50 nT amplitude are observed for the relative timing of the cessation of a Mars dy- above the 8-km-high early Noachian com- namo and the earliest history of Tharsis are discussed. plex at Claritis fossae (unit Nb, Scott and Tanaka [21]). The magnetic field observations are integrated with Similar magnitude anomalies are seen above the Nf constraints from , gravity and topography and units of Nectaris Fossae [21]. In contrast, no magnetic Sixth International Conference on Mars (2003) 3065.pdf

at the limit of accuracy of the magnetometer. The dis- tributions show significant anomalies over units of all ages. The peak anomaly, mean anomaly and variance of the distributions decrease with decreasing surface

unit age. Values of Br over Noachian surface units can be large, but do not approach the several hundred nano-Tesla anomalies of the region. Kolmogorov-Smirnov tests [24] indicate that the Ama- zonian, Hesperian and Noachian cumulative distribu- tion functions are statistically significantly different.

Figure 1. Drape of geological map (upper figure) and

Br (lower figure) over MOLA topography for the re- gion bounded by –180°E, 0°E, 45°S, 45°N. Geologi- cal map is a simplified version of that in [22]. Noa- chian units are denoted by brown colors, Hesperian units by purple, blue and colors, and Amazonian units by red, orange and yellow and white colors. The

color bar gives the scale for Br in nT [21].

field anomalies are associated with the Noachian units (Nf) of and . Negligi- ble anomalies are observed over Sinai Planum, , Solis Planum and western Valles Marineris. Anomalies at the eastern edge of the Terra Cimmeria region are associated with deformed Noachian terrain south-west (Nplr, Npl1) of . Signifi- cant, though lower amplitude, anomalies are observed at eastern Daedalia Planum which rises to elevations of about 4 km above the planetary mean. The surface units here are Amazonian in age. Finally, magnetic anomalies are observed over a small area on the low- ermost western flanks of Olympus Mons. The surface units here are also Amazonian. Inspection of Figure 1 suggests, on average, de- Figure 2: Histograms (upper) and empirical cumula- creasing magnetic field anomaly amplitude with de- tive distribution functions for the absolute value of Br creasing surface unit age. Figure 2 shows magnetic over Amazonian (green), Hesperian (blue) and Noa- anomaly distributions over units of Amazonian, Hespe- chian (red) surface units. Values of Br less than 5 nT rian and Noachian age. The region examined extends are excluded (see text). There are 1423, 2089 and from 45°S to 45°N and from 180°W to 0°E. Similar 1619 observations of Br over Noachian, Hesperian, patterns of results were obtained when various smaller Amazonian units. Noachian, Hesperian and Amazo- areas within Tharsis were analyzed. Magnetic anom- nian distributions have mean values of 37.5 nT, 24.6 aly amplitudes of less than 5nT were excluded as being nT, and 20.7 nT respectively. Sixth International Conference on Mars (2003) 3065.pdf

Implications Some of the oldest and highest ex- sphere at ~ 4 Ga. We assume the base of the magnetic posed Noachian terrain in the Tharsis region exhibits source region (crust) is initially at the tempera- significant magnetic anomalies, indicating long-lived ture. Persistent intrusions and underplating of crustal crustal magnetizations. This provides important con- material are permitted, such that the temperature at the straints on the thermal regime of the magnetization base of the crust is held close to its melting tempera- source region. The retention of significant magnetiza- ture. This provides a crude upper limit on the vertical tion at Claritas Fossae means that models for up- extent of demagnetization of the crust due to intru- lift/construction in that region must allow a significant sions. In this case, the lower 50% of the crust is raised depth extent of the magnetization source region to re- above the Curie temperature. In practice this model below the appropriate Curie temperature. Uplift would correspond to a requirement of continuous in- (inferred structurally [6,7,8]) may be the result solely trusions over a 100 Myr period. of crustal thickening by magmatic intrusion, irrespec- From these preliminary calculations it is clear that tive of the source of early mantle buoyancy in the re- it is possible retain significant magnetization signa- gion. Crust below the young Amazonian units of tures during the construction of Tharsis. We will in- Daedalia Planum is magnetized, but less strongly so, vestigate trade-offs between magnetization distribu- suggesting that burial of older basement units by vol- tion, volcanic construction and the resulting crustal canics has reduced, but not removed, the remnant thermal anomalies. magnetism. The basement below the Tharsis Montes Evolution of Tharsis: Integration of the magnetic is not magnetized. The fracture associated with field observations with constraints from gravity and Tempe Fossae has been proposed to underly the Thar- topography are consistent with the following scenario sis Montes. Since Tempe Fossae exhibits no magnetic for the formation and evolution of Tharsis. In the early signature, it is possible that the crust beneath the Thar- Noachian thermal (and possibly compositional) buoy- sis Mons was never magnetized. Two scenarios con- ancy produced uplift, partial melting and deep intru- sistent with the presence of high-elevation magnetic sion. This is consistent with exposed oldest Noachian anomalies are: (1) Anomalies in these regions reflect units being pre-existing crust, not volcanic construct. pre-Tharsis Noachian basement, that has been uplifted Deformation of these units produced the observed ex- during the construction of Tharsis, (2) Crustal mag- tensional fracture [7,8,9]. Whether the magnetized netization was acquired during the early stages of for- Noachian crust is primarily pre-existing basement or mation of Tharsis, as intruded and extruded crust early Tharsis construct is unknown. Further informa- cooled through the Curie temperature of the magnetic tion regarding the relative timing of the waning and carrier. Scenario 1 would not require the presence of a cessation of a dynamo and the earliest phase of global magnetic field during the formation of Tharsis, Tharsis construction is needed. By the mid-late Noa- scenario 2 requires a global magnetic field at least chian Tharsis topography was supported by a combi- during the early stages of Tharsis construction. nation of membrane and bending stresses from load- Effect of Thermal Demagnetization: We have ing, and crustal thickening [2, 16, 18]. This transition investigated two 1-D end-member scenarios for ther- explains the circumferential ridges and radial exten- mal demagnetization of a magnetic source layer. We sional faulting at Tharsis, the present-day gravity and assume that the crust in the Tharsis region prior to the topography, orientations and the reduc- formation of Tharsis was strongly magnetic, and had a tion in magnetic field anomalies over younger surface thickness of 40 – 50 km [22, 25]. Furthermore we as- units. sume uniform magnetization of the crust, and a Curie References: [1] Plescia and Saunders (1979), LPS temperature of 580°C (typical of single domain mag- X, 986. [2] Solomon, S. C. and J. W. Head (1982), netite). Emplacement of surface flows heat the JGR, 87, 9755. [3] Scott and Tanaka (1980) Proc. underlying crust, and the maximum depth to the Curie LPSC, 11th 2403. [4] Anderson, R. L. et al. (2001), isotherm scales linearly with the flow thickness [26]. JGR, 106, 20,563. [5] Schultz, and Tanaka (1994), For 1-km thick flows, only the upper few hundred JGR, 99, 8371 [6] Phillips, R. J. et al. (1990) JGR, 95, meters of the underlying crust are raised above the 5089. [7] Dohm, J. M. et al. (1997), LPS XXVIII, Ab- Curie temperature. Thus volcanic extrusions after the stract # 1642. [8] Johnson, C. L. and R. J. Phillips cessation of a dynamo will only have a small demag- (2003), LPS XXXIV, Abstract # 1360. [9] Schultz and netization effect on the underlying crust, unless the Tanaka (1994), JGR, 99,8371. [10] Mag field cessa- magnetic carriers are strongly concentrated upwards. tion ref. [11] Hartman, W. K. (1973), JGR, 78, 4096 In the second calculation we use the results of the [12] Carr, M. H. (1974), JGR, 79, 20,563. [13] Wise, nominal thermal model of Hauck and Phillips [25] to D. U. et al., (1979) Icarus, 38, 456. [14] Sleep, N. H. estimate the temperature gradient in the martian litho- and R. J. Phillips (1979) GRL, 6, 803. [15] Phillips, R. Sixth International Conference on Mars (2003) 3065.pdf

J. et al. (2001) Science, 291, 2587. [16] Banerdt and Golombek, (2000), Proc. LPSC 31st, Abstract # 2038. [17] Zhong, S. (2002) JGR, 107, 2001JE001589. [18] Zuber, M. T, (2001), Nature, 412, 220. [19] McKenzie, D. P. et al. (2002), EPSL, 195, 1 [20] McGovern, P. J. et al. (2002), JGR, 2002JE001854 [21] Purucker, M. et al. (2000), GRL, 27, 2449. [22] Scott, D. H. and K. L. Tanaka (1986). [23] , D. E. et al. (2001), JGR, 106, 23,689. [24] Press, W. H. et al (1989, Cambridge University Press [25] Hauck, S. A. and R. J. Phillips (2002), JGR 107, 2001JE001801. [16] Turcotte, D. and G. Schubert, (2002), Geody- namics.