THE CRUSTAL DICHOTOMY OF MARS

The crustal dichotomy of Mars describes the topographic division betwe en the young plains in the northern hemisphere and the old terrain in the southern hemisphere. The highland-lowland boundary separates the y ounger plains from the older, high-standing terrain and consists of th ree geologically-distinct regions: the Tharsis Province, the chaotic t errain, and the fretted terrain (which includes gradational boundary t ypes)-all are characterised by tensional tectonics. This paper present s new geological evidence that shows the topographic division at the f retted terrain formed in the late Noachian-early Hesperian time period : the same time period in which the Tharsis Province and chaotic terra in formed, and fracturing of a southern-hemisphere-type surface beneat h the northern plains occurred. These are inherent features of the cru stal dichotomy, indicating it must have also formed during the late No achian-early Hesperian time period. An analogy is made between the nor thern lowlands and sedimentary basins on Earth: both are basin like an d are surrounded by provinces that have been subjected to pronounced t ensional tectonics. This paper uses the White and McKenzie model (1989 a) to propose that a lithospheric-stretching event on Mars, in the lat e Noachian-early Hesperian time period, produced the crustal dichotomy ; the Tharsis Province formed by uplift (over a sub-surface hotspot) a nd gave rise to lithospheric stretching, and the northern lowlands for med by subsidence (over normal asthenospheric temperatures). Detachmen t faults, operating from the Tharsis Province and around northern lowl ands, allowed structural equilibrium and large lithospheric extensions to be attained during this period: they also defined the geometry of the lowlands. The proposal is supported with calculations used to esti mate the amount of subsidence that can be achieved in this way.