MAGNETIC SUSCEPTIBILITY OF FAULT ZONES: ARE COSEISMIC FLUIDS UBIQUITOUSLY FERROUS?

Ubiquity of fault zone fluid during rupture is attested by closely post-seismic crystallization of hydrated zeolites and hydrous clays in brittle fault zones. At deeper brittle and upper ductile strain horizons, co-seismic water enters a post-seismic alteration reservoir of metasomatic minerals; especially epidotes, micas, chlorites, kaolinites, and serpentinite. Accordingly, zeolite, clay, and kaolinite petrology occupies attention in studies of seismogenic continental faults. However these products imply a set of reactants – iron-bearing minerals – which also reveal interesting systematics.

The source of primary water in basement fault zones must be the common rock-forming minerals, biotite and amphiboles. These mafic minerals also are iron-bearing, while fault zone zeolites, kaolinite, and the most common clays are iron-barren. Since iron is among the most soluble and abundant crustal cations, its flux in fault zones also should be ubiquitous. If the high iron activity implied by common fault zone zeolites does not ultimately enter a metasomatic or minor clay phase, it will produce iron oxide or sulfide minerals, or else be advected through the fault zone at a given observation level.

I will describe several well-exposed continental fault zones in crystalline terrane which are characterized by highly structured iron depletion or enrichment profiles, as mapped by magnetic susceptibility transects in context of petrologic and structural details. These plate boundary and hinterland faults include the San Andreas, San Gabriel (ancestral San Andreas), Laerdal-Gjende, Highland Boundary, Leannan, and Alpine Faults.

It seems possible that a general structural-stratigraphic behaviour of fault zone ferrous fluid exists, though continued testing is required. Deep, ductile regimes may act most nearly as closed systems, exchanging iron between silicate phases. Shallower, ductile regimes preferentially crystallize iron sulfides. Relatively deep, brittle faults tend to advect ferrous fluid to other structural levels. Shallow, brittle faults generally crystallize magnetite. Fault zone hematite is probably generally a post-seismic alteration phase.