Reworking characteristics of quaternary pyroclastics, Thera (Greece), determined using magnetic properties

Abstract

The magnetic properties of unwelded, poorly consolidated Quaternary tephra from Thera, Greece, are examined for the evidence on their emplacement, particularly the extent to which individual clasts have been reworked after initial deposition. Most samples have stable remanences with single component magnetization acquired between 350–450°C (some from 540°C) and 100/150°C, indicating that each of these samples was in a fixed position, relative to the ambient geomagnetic field, when they originally cooled over these temperature ranges. The between-sample directions within each tephra layer are either: (a) mutually consistent, i.e. within measurement and collection error; (b) grouped but more scattered than expected; or (c) randomly scattered. These groupings are interpreted in terms of hot emplacement followed by respectively: (a) no re-working; (b) cold, internal settlement of individual clasts; and (c) extensive cold, reworking. In each case, the percentage of reworked material within any given layer can be estimated from the scatter of the directions of remanence of individual clasts. Such estimates are consistent with some, but not all, volcanological appraisals. This magnetic method provides a simple, inexpensive method for establishing the degree of cold reworking in such unwelded tephra.