Demagnetization of crust by magmatic intrusion near the Arsia Mons volcano: Magnetic and thermal implications for the development of the Tharsis province, Mars

摘要

A sharp crustal magnetic field contrast of almost two orders of magnitude at 185 km altitude, as determined by electron reflection (ER) magnetometry, exists between the nonmagnetic bulk of the Tharsis province and its relatively strongly magnetized southwestern region. The 3 nT magnetic field contour passes west of Ulysses Patera, south of Arsia Mons, through Thaumasia Planum and appears largely unmodified by impact craters, suggesting a post-Noachian origin. This sharp magnetic boundary is most easily explained by thermal demagnetization caused by pervasive magmatic intrusion throughout the upper crust on its nonmagnetic side. Using a best guess range of assumptions, we model these intrusions and their demagnetizing effects on preexisting crustal magnetization distributions and fit the resulting model magnetic field magnitudes to ER magnetic profiles across the boundary. Within the framework of our assumptions, we find that the magmatic boundary may not be as sharp as its magnetic counterpart, extending over 0-600 km, and that a minimum of ～10-35 km average accumulated thickness of intrusions are required to completely demagnetize the crust on the northeast side of the boundary. The best-fit modeled intrusions extend horizontally between ～120 km and 220 km beyond the magnetic boundary to the southwest for most of its length, with the inferred intrusion thickness and penetration distance being larger for minerals with higher magnetic blocking temperatures (magnetite vs. pyrrhotite). Such thicknesses of intrusion are consistent with magma production rates similar to those at Hawaii, if we allow accumulation over 0.1 to 1 Ga. If the volume of intrusion is representative of most of Tharsis, these thicknesses imply average intrusive-extrusive ratios higher than previously estimated, and in closer agreement with previous magma production estimates based on heat flow. Mapped fields of late Amazonian small volcanic vents, with diverse morphologies and a wide spatial distribution, may represent the latest stages of volcanism and record some of the polybaric processes that likely have occurred as magma intruded multiple levels of the crust. Also, intrusions are inferred to extend beneath most of the length of the upper southwest rift apron of Arsia Mons, implying that localized extrusion may be responsible, along with volcanism from the rift zone, for the apron's plateau shape. Lastly, within our model, the maximum pre-intrusion lateral magnetization coherence scale in this region is found to be less than ～200 km.