Lunar and Planetary Science XXXIII (2002) 1358.pdf

NOACHIAN TECTONICS OF AND THE B.M. Webb, and J.W. Head, De- partment of Geological Sciences, Brown University, Providence, RI. [email protected].

Introduction. Syria Planum (Fig. 1) has previously been trends visible in 9 and Viking Orbiter photographs. identified as a volcano-tectonic center partially surrounded by He observed strike-slip faulting and an en-echelon fault-zone a raised annulus. Orbiter Laser Altimeter (MOLA) data in northern Claritas Fossae. He also noted that "Some com- [1] show a cap of over 100 low volcanic shields over the east- pressive movements observed in the of Claritas Fossae ern half of the summit, and a continuous slope of Upper Hes- could be due to extensive movements occurring in the Valles perian lava plains from Syria Planum out to underlying ridged Marineris area." plains [2] which are as old as in age. These observa- Dohm and Tanaka [7] concluded that fault activity in the tions suggest that Syria Planum was an active volcanic center Thaumasia region peaked in the Noachian. They suggest that from the Noachian until Late . The annulus could "deep crustal intrusion and a thickened, buoyant crust may represent the surface of a Noachian Syria Planum volcanic have caused the uplift of the plateau during the Noachian and edifice which became gravitationally unstable. Southeastward Early Hesperian…[this] result[ed] in outward verging fold- lateral movement of the Thaumasia plateau starting in the and-thrust plateau margins." Noachian and tapering off in the Early Hesperian likely caused Recently, Anguita et al. [9] proposed that Thaumasia pla- extension in Syria Planum, creating the annulus, and compres- teau was an independent lithospheric block that underwent sion in the south ridge belt. This collapse of the Syria buckling and thrust faulting in the Late Noachian or Early Planum edifice is similar to gravitational collapse common in Hesperian. In their analysis, Hesperian lithosphere terrestrial volcanic complexes [3]. may have been similar to the current terrestrial oceanic litho- We propose that the strain from Syria Planum was trans- sphere, based on buckling wavelengths. They consider Valles ferred along proto-, forming a sinistral tran- Marineris to be a transtensive, dextral shear zone. stensional zone which provided tectonic control for later valles Observations formation. At the east end of Valles Marineris, the Coprates Syria Planum. Syria Planum is at the peak of the Tharsis Rise (Fig. 1) is interpreted as a lithospheric buckle with a Rise and is the location of the thickest crust on Mars (92 km) thrust fault along the eastern edge [4]. We also interpret the [10]. To the north and west it is surrounded by an elevated, southern edge of the Thaumasia Highlands as the surface ex- extensional annulus of NHf material [8]. It is also the tectonic posure of a thrust fault [5]. The compressional structures of center of a large radial graben set [11]. The annulus itself con- the Coprates Rise appear to topographically extend into the nects topographically to both Valles Marineris in the east and Thaumasia Highlands. In this hypothesis, Claritas Fossae Claritas Fossae to the south. While some of the circumferen- (Fig. 1) represents a dextral transpressional zone. tial fractures of the annulus may be due to subsidence after Previous Work. Masson [6] first suggested tectonic depletion of subsurface reservoirs [11] or the loss of thermal movement of Syria Planum to the southeast based on tectonic

Figure 1: Tectonic sketch map with MOLA shaded relief map of the Thaumasia and Syria Planum regions. Movement of the Thaumasia plateau to the southeast beginning in the late Noachian caused uplift along the Thaumasia Highlands and shearing along Valles Marineris and Claritas Fossae. Labeled features are: SP-Syria Planum; VM-Valles Marineris, TH-Thaumasia Highlands, CR-Coprates Rise, CF-Claritas Fossae, AM- . 40°-130° longitude and 0°- -45° latitude, spherical projection centered on (14°S, 105°W) center of radial graben set from (11). Lunar and Planetary Science XXXIII (2002) 1358.pdf

NOACHIAN TECTONICS OF SYRIA PLANUM: B. M. Webb and J. W. Head

support, the annulus is not symmetrical. In this model, south- requires three elements: a sufficiently massive volcanic center, eastward movement of the Thaumasia plateau caused exten- adequate heat (magma intrusion) input, and a weak basal layer sion in Syria Planum and resulted in the circumferential exten- [18]. The first two are qualitatively satisfied by the observed sional fractures. The absence of extensional fractures in the topography and volcanic nature of the region. On the scale of southeast may be due to volcanic resurfacing of a low part of the Thaumasia plateau, a weak basal layer would have to be a the annulus, but is more likely due to the absence of exten- large scale structure, probably a weak crustal layer. On the sional strain in that region. basis of gravity data, it was suggested by [16] that there ex- Thaumasia Highlands. The Thaumasia Highlands form isted a thick, weak lithosphere in the southern hemisphere. A the southeastern edge of the plateau and are part of the south method for the formation of a ductile layer in areas of thicker Tharsis ridge belt as defined by [4]. The lobate scarp which martian crust has been proposed by [19]. They proposed that forms part of the southern edge of the highlands was first sug- areas of thinner crust (north of the dichotomy boundary) are gested as a possible thrust fault by [5]. The highlands have supported by the strong upper mantle, while in regions of been mapped mostly as Noachian basement and Hesperian and thicker crust (south of the dichotomy boundary) crustal mate- Noachian fractured units [8]. The Highlands resemble terres- rial is weaker than mantle material at the same depth. This trial fold-and-thrust belts [7], and also contain four to six vol- produces the weak/ductile layer required for gravity-controlled canic edifices [8,7]. Uplift of the plateau took place during the movement of the Thaumasia plateau. Late Noachian [7] which is consistent with deformation of the A difference in crustal thickness such as that represented annulus around Syria Planum. by the dichotomy boundary could account for the southeastern Claritas Fossae. To the west of the Thaumasia Highlands direction of gravity sliding. The location of the dichotomy lies Claritas Fossae, which connects topographically to Syria boundary is not well constrained in the region of Syria Planum Planum in the north. Like the highlands, it has been mapped as due to the younger lava plains, but the possible trend sug- Noachian fractured and Noachian basement units [8]. Ander- gested by [10] would place it near or through the northwest son, et al. [12] identified Claritas Fossae as the most active side of Syria Planum. The northwestern flank may also have Noachian tectonic center in Mars' western hemisphere. At the been buttressed by the growth of a proto-Arsia Mons edifice. northern end of Claritas Fossae a complex zone of graben Conclusions. On the basis of our observations, we believe indicates shearing [6]. This would be expected for movement that the following model provides a solid explanation for ob- of the Thaumasia plateau to the southeast. As transtensional servations of disparate, major tectonic features of the Tharsis movement progressed along Valles Marineris, Claritas Fossae rise: Gravity controlled movement of the Thaumasia plateau experienced transpression, which produced thickening and to the southeast began in the Noachian and tapered off in the uplift. However, there are no obvious compressional tectonic Early Hesperian. This movement was facilitated by a ductile features except in southern and western Claritas. There is no lower crustal layer which developed south of the dichotomy evidence of volcanic construction along Claritas Fossae, al- boundary [16,19]. The annulus around Syria Planum was though it is possible that subsequent cratering has obliterated initially formed by extensional tectonic modification of the evidence of ancient flows. Noachian Syria Planum edifice and may later have undergone Valles Marineris. Valles Marineris, which forms the subsidence after depletion of subsurface reservoirs [11] or the northern boundary of the Thaumasia plateau, opened in the loss of thermal support. The tectonic control for Valles Early Hesperian [13]. There is strong tectonic control on the Marineris [14] was produced by a sinistral transtensional shear alignment of the canyons [14]. In this model, movement of the zone which transferred strain from Syria Planum to the Thaumasia plateau produced the tectonic features which con- Coprates Rise. Along the western edge of the Thaumasia pla- trolled later Valles formation, possibly as described by [14]. teau, Claritas Fossae represents a complex, dextral transpres- While Valles Marineris does not fit traditional rift models sional shear zone. Finally the compressional strain was taken [15,16], observations are consistent with a sinistral trans- up by the Thaumasia Highlands which are a compressional tensional shear zone subsequently modified by significant fold-and-thrust belt [7], with a volcanic component [8,7]. erosion. To connect western extensional and eastern compres- References: 1) D. , et al., Science, 284, 1495, 1999. 2) B. sional environments, strain would have been transferred down Webb, et al., LPSC 32, 1145, 2001. 3) B. Webb, and J. Head, LPSC Valles Marineris yielding a sinistral sense of motion. An ex- 33, 2002. 4) R. Schultz and K Tanaka, JGR, 99, 8371, 1994. 5) J. tensional component of movement is consistent with compres- Plescia and R. Saunders, JGR, 87, 9775, 1982. 6) P. Masson, Moon and Pl., 22, 211, 1980. 7) J. Dohm and K. Tanaka, PSS, 47, 411, 1999. sion in the Thaumasia Highlands and Claritas Fossae. 8) D. Scott and K. Tanaka, USGS I-1802A, 1986. 9) F. Anguita, et al., This tectonic situation is similar to terrestrial zones of tec- JGR, 106, 7577, 2001. 10) M. Zuber, et al., Science, 287, 1788, 2000. tonic escape such as the North Anatolian Fault Zone, except 11) K. Tanaka and P . Davis, JGR, 93, 14893, 1988. 12) R. Anderson, that movement of the Thaumasia plateau is driven by gravity et al., JGR, 106, 20563, 2001. 13) B. Lucchitta, et al., in Mars, U. Az, instead of plate tectonic processes. If this is a robust analogue 453, 1982. 14) J. Spencer and F. Fanale, JGR, 95, 14301, 1990. 15) F. then Valles Marineris tectonics may resemble a negative Anderson and R. Grimm, LPSC XXX, 1954, 1990. 16) D. Smith, et al., flower structure (a strike-slip master fault at depth with normal Science, 286, 94, 1999. 17) A. Aksu, et al., GSA Today, June, 3, 2000. faults connecting to it from both sides) [17]. 18) A. Borgia, JGR, 99, 17791, 1994. 19) G. Montési and M. Zuber, Mechanics of gravity-driven Noachian tectonics. Ter- LPSC XXXI, 1879, 2001. restrial analogues show that gravity-controlled edifice collapse