Mars: Stratigraphy and gravimetry of Olympus Mons and its aureole

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Abstract
Olympus Mons and its surroundings have been investigated using impact crater statistics. Crater retention ages of all major geological units show that their emplacement occurred during three time intervals. The underlying fractured plains are part of the northern plains in general (N(1) > 4 × 10−3 (km−2)). This time is followed by the time interval represented by crater retention ages N(1) = 8 ×10−4 to 3 × 10−3 (km−2). Some lava flows in the fractured plains, the smooth plains, and the aureole materials were emplaced during this time. The aureole probably developed by some sort of collapse of the outer parts of an ancient volcano (‘Proto‐Olympus Mons’) underlying the present shield. Its remnants may show in the scarp and small parts of the shield not covered by younger lava flows. The third interval covers crater retention ages N(1) = 5 × 10−5 to 3 × 10−4 (km−2). This volcanic activity followed a period of dormancy. A series of lava flows forming caldera and present shield surface and flooding a depression around the shield are distinguishable. The regional stratigraphy is discussed as part of the global geological history. The question of the genesis of the aureole material is addressed by gravity models. The topographic mass of Olympus Mons contributes about 80% to the aureole gravity high northwest of the mountain. The gravity high of the entire aureole can be explained by a density model consisting of the topographic masses of Olympus Mons and the aureole materials. It does not require additional high‐density materials underneath the aureole. Thus aureole materials of an allochthonous origin by collapse are in agreement with the gravity data. However, the gravity data do not preclude an autochthonous and volcanic origin of the aureole materials. In the latter case, possible subsurface intrusions should have no density contrasts with the surroundings.