| Popis: |
The 3.48 Ga Dresser Formation, exposed in the North Pole Dome of the Pilbara Craton, Western Australia, previously considered as a quiet, shallow-water marine environment. According to recent studies the Dresser Formation was deposited in an active volcanic caldera floored by an extensive network of hydrothermal chert-barite veins. Although these veins have been recognised as filling fractures, no geometrical data, history of vein emplacement, or kinematic reconstruction of vein offsets have previously been gathered. Was there a regional stress field, or the veins are related directly to magma supply and discharge processes within an evolving caldera system? Is there more than one vein system and, if so, how do these relate to the developing caldera? Over 191 veins were mapped in the field, their geometry and their age relationship to nearby veins observed to constrain the history of veining and fault-related offsets. Nine structural sub-blocks bounded by faults were identified, within which Dresser Formation bedding strike and dips were recorded. Measured veins were plotted using the GEOrient computer program then back-rotated to their original orientation by un-tilting the bedding of each sub-block to the horizontal. Rose diagrams of back rotated veins show that: 1) There are four main vein systems, the last of which were emplaced within listric growth faults; 2) The vast majority of veins were vertical prior to regional tilting; 3) Conjugate vein networks (NE-SW and NW-SE) are concentrated in two sub-blocks in the middle and northern parts of the area; 4) Veins in other sub-blocks generally are characterised by one dominant direction of veins, corresponding to ring faults. The results are consistent with fracturing under a stress regime with a vertical ?1. The geometry of a conjugate core zone flanked laterally by unidirectional veins is consistent with sandbox models of overburden fracturing during emplacement and discharge of magma chambers in a volcanic caldera environment. The two loci of conjugate veins represent possibly separate, but linked calderas. Areas of single vein orientations reflect marginal areas, dominated by ring faults. The veins in late listric faults represent the final period of caldera collapse, which extended up into overlying strata. |
