THE ORIGIN OF MAGNETIZATION AND GEOCHEMICAL ALTERATION IN A FAULT ZONE, KILVE, ENGLAND

The origin of an apparently syndeformational chemical remanent magneti zation (CRM) and geochemical alteration in a fault zone in the Bristol Channel Basin, southwest England, was investigated. Deformation in th e fault zone occurs in Jurassic aged, organic-rich limestones and cons ists of numerous normal and oblique-slip faults and associated folds. Migration of basinal, radiogenic fluids is indicated by elevated Sr-87 /Sr-86 values for calcite veins that occur throughout the fault zone. Some of the calcite veins contain hydrocarbons sourced from deeper str ata. Elevated Sr-87/Sr-86 values in the host Jurassic limestones indic ate that they were also extensively altered by radiogenic fluids that migrated through microfractures in addition to major fault and fractur e planes. Folded and tilted host limestones contain a pervasive second ary CRM residing in magnetite that was acquired during deformation in the Tertiary. The association between this pervasive CRM and the perva sive geochemical alteration is consistent with a genetic connection be tween the orogenic fluids and the CRM although the timing of CRM acqui sition (Tertiary) is not consistent with structural interpretations fo r the timing of most veining. An alternative remagnetization mechanism which is not triggered by externally derived fluids, such as diagenes is of hydrocarbons, might account for the CRM. Hydrocarbon-bearing vei ns also contain a CRM that resides in magnetite, although the time for remanence acquisition is not well constrained by field tests. (C) 199 8 John Wiley & Sons, Ltd.