STYLES OF FAULTING AND TECTONICS OF THE THARSIS REGION, J.B. Plescia, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 9 1 103/Department Geol og i ca 1 Sciences, Un ivers i ty of Southern Cal i fornia, Los Angeles, CA 90007 and R.S. Saunders, Jet Propulsion Laboratory, Cal iforn ia lnstitute of Technology, Pasadena, CA 91 103 For understanding the geologic and tectonic history of Mars, one of the most important regions is Tharsis, the site of most of the major volcanic edifices as well as a region of intense faulting. The two most common structural features of the region are graben and irregular ridges resembling those on the lunar maria. The graben, as well as most of the other faults and 1 ineaments, are interpreted to be the expression of tensional forces. The ridges have been interpreted to be compressional in origin as were similar features on the Moon (1). Additional features observed are unpaired normal faults, lineaments, a possible thrust fault, and chains of coalescing pits that form linear depressions. Series of graben occur in groups and singly throughout Tharsis, with the groups being more common. Graben morphology varies around Tharsis. Tantalus and Alba Fossae have long straight walled graben, while southern Syria has graben that are irregular in width and change strike over short distances. The spacing is also variable. Tantalus Fossae, along the eastern flank of Alba, decreases in width and increases in spacing of graben away from the shield. The Sirenum-Memnonia Fossae region exhibits a pattern of en echelon graben. The en echelon graben are typically only a few krn long, while the zones extend for hundreds of km. A final type of feature is seen in the Tantalus Fossae and Memnonia-Si renum Fossae region. Series of pits coalesce down strike into a feature that resembles a graben. This type of feature is particularly common on volcanic shields, and appears to represent some of the most recent Tharsis activity. A series of long scarps, up to 1800 km in length, represent the recent activity of the Clari tas-Thaumasia Fossae region. The strike of these scarps is approximately SE, but changes locally along strike and as branches splay out, or join the main fracture. The two sets outline a large graben-1 ike structure approximately 75-100 km wide. This feature may be analogous to a proto- Valles Marineris, which either is just beginning to form, or which was aborted in its early stages of development. The only interpreted compressional features that have been recognized are the irregular ridges which occur on the plains type material. Surfaces in the Sol is-Sinai-Lunae Planum regions, as well as western Arnazonis and individual large basins of Sirenum region have athese linear ridges. The ridges seem to be general ly circumferential to the central Tharsis reg ion. The Tharsis region on a global scale is the center of a series of radiating fractures and circumferential ridges (Carr (2)). The fracture systems, as well as local volcanic morphology have a preferred NE/SW orientation. This preferred orientation may reflect some deep-seated crustal weakness. We have prepared a series of stereo plot of the major faults of the region. Figure 1 is an abstracted version of the orig- inal map on a stereo plot centered at 120°W of the equator. It is our con- tention that fractures can be separated into two individual centers of differing age. Figures 2 and 3 illustrate the faults related to each of the

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Plescia, J.B., et. al. two centers, again plotted in stereo. One center, the older of the two, is located in northern Syria Planum, just south of Noctis Labyrinthus. The sec- ond, and younger, center is located approximately at the position of Pavonis Mons . The fracture pattern centered on Syria includes the fractures associated with the fol lowing areas: Cerauni us Fossae, the NNE/SSW fractures SE of Uranius Patera, the fractures north of Noctis Labyri thus, and several isolated windows throughout Tharsis. The fracture pattern, centered on Pavonis, is more 1 imited in extent, and is principal ly exposed as the fractures of Tempe-Mareoti s Fossae and the Memnonia-Sirenum Fossae regions. Additionally, fractures are exposed in the Sol is Planum region and as superimpositions on older fractured terrain of the Clari tas-Thaumas ia Fossae region and other areas. From geologic and crater counting data the fol lowing general i zed tectonic history is proposed. An early fracture system is centered in southern Thaumasia - perhaps related to an up1 ift r re^ (3)). This system is poorly represented, save in a few windows in the Solis Planum region and as struc- tural control for the Valles Marineris. The center of activity apparently then moved northward to the center in northern Syria. This stage of fracturing apparently began just after the em- placement of Lunae Planum and equivalent units, as these age units are in places cut by Syria faults. This episode of faulting is also contemporaneous with the early stage of volcanic construction in northeast Tharsis. Faulted surfaces of this age are be1 ieved to under1 ie much of the Tharsis region, and. are now covered by later plains units. Only those topographically high re- gions are now exposed as windows in the plains units. The faulting associated with this center appears to have been constrained in a generally north/south direction. Surfaces that predate this episode of faulting, the plains south of Eos and western Coprates Canyons, are not extensively cut by faults, and only a few structures of the Memnonia- Sirenum region to the west can be correlated with this system. Following this episode of faulting the plains units of Syria-Sinai-Solis Planum were formed. Additional units in the Memnonia Sirenum region were also formed at this time. It is unknown if the plains forming volcanism was con- tinuous with the faulting or if it ceased during this time, and resumed post faulting. A new center of faulting was then established in the region of Pavonis Mons, which at this time did not exist. The fractures of this system may be related to an episode of uplift preceeding the Tharsis volcanic plains and volcanoes (Hartmann (4)). The fracture patterns are also consistent with a failure model of Phil1 ips et al. (5). This system includes the fractures of Mareotis-Tempe Fossae and the fracture of Memnonia-Si renum Fossae. The fracturing principally is restricted to a NE/SW direction, and did not affect the regions to the SE or NW. This general strike of these fractures follows the present alinement of the Tharsis Montes volcanoes. The Tharsis plains were then emplaced over this faulted surface. The Tharsis volcanic plains flood several structural features developed during the previous faulting episode. Following the emplacement of the distal Tharsis cover, the shields began to develop. The development of the Tharsis Montes volcanics appears to have been northeastward. Arsia Mons is the oldest, then Pavonis Mons, and final ly Ascraeus Mons. Olympus Mons is the youngest of the Thars i s volcanic constructs. Following the development of the volcanoes, and apparently in response to 1 i thospheric loading by the volcanoes, the fracture system related to the

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Plescia, J.B., et. al.

Pavonis center was reactivated. These fractures cut the Tharsis cover, and and in some cases extended to near the shields. The fractures of this system are small graben, undifferentiated lineaments, and a series of pits that coalesce to form graben-like structures. This system is poorly developed and represents only a few of the Tharsis fractures. Another ser i es of fractures exposed around Thars i s but apparent 1y unre- lated to the regional structural aspects is the more local faults related to loading of individual volcanoes. Several of the large shields illustrate a pattern of concentric graben which cut the distal ends of the shields and the immediately surrounding region. The fractures around Alba Patera were first attributed by Wise (6) to be the result of loading. From crater counting data, a relationship between age and intensity of faulting is appar- ent. The oldest shield, Alba, is the most intensely fractured area, while youngest, Olympus Mons, has no fractures at all. The Tharsis Montes volcanoes each have fractures and the intensity of fracturing increases in age. This age vs. intensity relationship has been interpreted to be related to an in- crease in strength of the lithosphere with time due to thickening. REFERENCES: 1 . Howard, K. and Muehl berger, W., 1973, ApoT lo 17 Prel irninary Sci. Report; 2. Carr, M., 1973, JGR, 78, 4047-4062; 3. Frey, H., 1977, Goddard Space Fl i ght Center Report ~-922-77-2n; 4. Hartmann, W., 1973, lcarus -19, 550-575; 5. Phillips, R., Saunders, R.S. and Banerdt, B., 1979, JGR (in ~ress);6. Wise, D., 1976, Geol. Romana -15, 430-433.

FIGURE 2 FIGURE 3

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