JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91. NO. 88. PAGES 8113-8125. JULY 10. 1986 Orientation, Relative Age, and Extent of the Tharsis Plateau Ridge System THOMAS R. WATTERS AND TED A. MAXWELL Center for Earth and Planetary Studies. National Air and Space Museum. Smithsonian Institution, Washington, D. C. The Tharsis ridge system is roughly circumferential to the regional topographic high of northern Syria Planum and the major Tharsis volcanoes. However, many of the ridges have orientations that deviate from the regional trends. Normals to vector means of ridge orientations, calculated in 10° boxes of latitude and longitude, show that the Tharsis ridges are not purely circumferential. Intersections of vector mean orientations plotted as great circles on a slereographic net show that the ridges are not concentric to a single point but to three broad zones. These data indicate either that the Tharsis ridge system did not form in response to a single regional compressional event with a single stress center or that the majority of the ridge system did form in a single event but with locally controlled deflections in the generally radially oriented stress field. Superposition relationships and relative ages suggest that the compressional stresses that produced the ridges occurred after the emplacement of the ridged plains volcanic units (early in the Hesperian period) but did not extend beyond the time of emplacement of the Syria Planum Formation units or the basal units of the Tharsis Montes Formation (during the latter half of the Hesperian period). Comparison of topographic data with the locations of ridges demonstrates a good correlation between the topographic edge of the Tharsis rise and outer extent of ridge occurrence. Compressional deformation related to Tharsis is present out to about 5100 km from the regional topographic center located near northern Syria Planum. The innermost extent cannot be determined because the ridged plains units have been buried by more recent volcanic units, although ridge formation appears to have extended farther inward than is presently observed. Models involving a combination of isoslatic stresses and stresses resulting from flexural loading appear to explain best the observed lensional features, topography, gravity, and. to a first approximation, the locations and orientations of the compressional ridges. INTRODUCTION lions of the Tharsis Plateatt ridge system and the symmetry of the inferred compressive stress field. (2) the relative ages The Tharsis Plateau of Mars is one of the most extensive volcanic and tectonic regions known on the terrestrial plan- of the compressional events that generated the ridges, and (3) the extent of Tharsis-related compressional deformation. ets. The plateau is characterized by a broad topographic rise In addition, models for the origin of stresses in the Tharsis and dominated by the large shield volcanoes, Olympus, region are investigated in relation to the formation of the Arsia, Pavonis, and Ascraeus mons. The major structural ridges. features of the Tharsis Plateau are a radial fracture system extending over approximately 25% of the surface of the planet [Wise el al.. 1979] and a circumfercntially oriented RECENT STUDIES ridge system. Tharsis ridges range from 2 to 6 km wide and In a comprehensive tectonic study of the Tharsis region, extend from 50 to 150 km long. These ridges occur within, Wise el al. [1979] present a model of an early topographic and in some areas beyond, the radial fracture zone on rise, followed by extensive, long-lived volcanism. Analysis relatively smooth volcanic plains units that have flooded of stereographic plots of fractures and ridges mapped pri- older and more densely faulted cratered terrain. The ridged marily from Mariner 9 and early Viking orbiter images plains units have been faulted but to a much lesser degree confirmed a predominantly radial orientation of the faults than the older cratered terrain. These ridges may be (I) folds and a circumferential arrangement of ridges. Based on crater formed by regional or local compressional stress [Walters ages, they suggested that the ridges were approximately and Maxwell, 1983, 1985<v], (2) the surface manifestation of coeval with the main radial fault system. Deviations from reverse or thrust faults [Plescia and Golombek, 19851, or (3) Tharsis-related orientations were attributed to a NE trending folds with thrust faulting developing as a result of fold cruslal anisotropy related to the boundary between the geometry [Walters and Maxwell, 19856]. highlands and the lowlands, parallel to the line of the Tharsis The Tharsis Plateau is characterized by a large positive volcanoes. free air gravity anomaly [Phillips el al., 1973; Phillips and In studies of the ridged plains units of Coprates and Lunae Sounders, 1975; Balmino el al., 1982]. Applying an isostatic Planum, Sounders and Gregory [1980], Sounders el al. compensation mechanism for the gravity data (Bouguer [1981], and Walters and Maxwell [ 1985c/J suggest that the gravity minus the gravity due to the root) results in a positive regularity of the ridge spacing was controlled by a dominant residual anomaly over the plateau [Phillips el al., 1973; wavelength of folding. An alternative control may be preex- Phillips and Sounders, 1975]. isting basement faults, although ridge orientations are not The purpose of this study is to investigate (I) the orienta- similar to those of the surrounding fault systems [Maxwell, 1982]. In a study of the Tharsis fault system, Plescia and Copyrighl 1986 by the American Geophysical Union. Sounders [1982] present a model involving several discrete Paper number 5B5746. episodes of uplift and faulting. According to their model the 0148-0227/86/005 B-5746S05.00 earliest tectonic activity, located in the Thaumasia region, 8113 8114 WATTERS AND MAXWELL: THARSIS PLATEAU RIDGE SYSTEM Fig. I. Ridges in the western hemisphere of Mars. Ridges form a small circle centered roughly on Pavonis Mons. The largest number of ridges occur on the ridged plains east of the major Tharsis volcanoes in the Coprales and Lunae Planum regions. This projection is centered at 0°N, II0°W and represents 1853 digitized segments (Apt represents volcanic plains of Tharsis Montes region [Sco/i and Can; 1978]). was followed by faulting centered on northern Syria Planum anism dominated in the tectonic evolution of Tharsis. Rela- and then by two more episodes of faulting, both centered tively few geophysical studies have addressed the origin of near Pavonis Mons. They suggest that the ridges in the the Tharsis ridge system or its significance to the tectonic Tharsis region formed late in the tectonic history of the history of the region. province and that the ridges represent a viscoelastic re- sponse of the lithosphere to the Tharsis load. RESULTS Watters and Maxwell [1983] present the results of obser- vations of the ridge-fault crosscutting relations in the Tharsis Orientations of the Ridge System region and suggest a classification system involving three The ridge system of Tharsis is approximately circumfer- distinct types of ridge-fault intersections: ridges crosscut by ential to the regional topographic high in northern Syria faults, superposed by faults, and terminated at one end by Planum and the major Tharsis volcanoes (Figure 1). These faults. Relative age relationships based on these intersection ridges are interpreted as compressional features that formed types indicate that ridges were formed both before and after in response to a radially oriented stress field with a center extensional faulting. Using ridge-fault crosscutting relations near the Pavonis Mons volcano, assuming that the maximum and the angle of intersection between ridges and Tharsis principal compressive stress cr, was perpendicular to the radial faults, Walters and Maxwell [1983] conclude that the long axes of the ridges. major ridge-forming events in the Tharsis region were Our data permit a detailed analysis of the orientations of roughly coincident with, and in many cases prior to, the the Tharsis ridges. Normals to vector means of ridge orien- extensional events that produced the faulting of the Tempe tations were calculated using the over 1850 digitized ridge Terra, Coprates, and Memnonia regions and the rifting of segments sampled in 10° boxes of latitude and longitude Valles Marineris. (Figure 2). The ridges were digitized from U.S. Geological Geophysical studies of the tectonic evolution of the Survey (USGS) 1:2,000,000 Mercator and Lambert projec- Tharsis region have focused on the relative role of stresses tion controlled orthophoto maps. Due to resolution limits, generated from isostatically compensated uplift versus fiex- individual segments of less than 5 km length were merged ural load, using the Tharsis fault system as the major with longer ridge segments or omitted. Orientations of constraint [Solomon and Head, 1982; Willemann and individual ridge segments were determined and weighted by Turcotte, 1982; Banerdt et ai, 1982; Sleep and Phillips, length, and mean orientations or vector means were then 1985], This has resulted in a controversy over which mech- computed [see Reiche, 1938]. The magnitude of each result- WAITERS AND MAXWELL: THARSIS PLATEAU RIDGE SYSTEM 51)5 1 000 km VECTOR SCALE Fig. 2. Normals to vector means using 1853 digitized ridge segments sampled in 10° boxes. Orientations of individual ridge segments were weighted by lengths to determine vector means. Line lengths shown are weighted by vector magnitude, with double width lines representing 4x the magnitude of single thickness lines. Normals to vector means with magnitudes less than 100 km are not shown. This projection is centered at 0°N, I10°W ant vector mean was weighted by the total length of the ridge The vector mean analysis of the ridge system also indi- segments within the given 10° box and normals to the vector cates the relative magnitude of the compressive strain since means were then determined (Figure 2). Normals to vector the resultant vector is length weighted by the total number of means with magnitudes less than 100 km were considered to ridge segments within the sampled areas.