HOWARD'S PASS PB-ZN SEDEX MODEL NEEDS REVISITING: NEW INTERPRETATION OF DEFORMATION HISTORY
We propose that the existing distribution of rock types is primarily controlled by thrust imbrication, not simply by folding. Northeast-verging thrusts are proposed to root into a detachment zone observed at Howard’s Pass, herein termed the Howard’s Pass décollement. The Howard’s Pass décollement displays significant ductile strain. Above the décollement, a series of imbricated thrust faults disrupt the clastic-dominated lead-zinc ore and stratigraphic succession. This zone of complex deformation is capped by a thrust that is the roof of a complexly formed duplex, and above which less shortening has been accommodated. We suggest that the pressure solution cleavage, and associated folds and transposition, formed 250-300 Ma after deposition of sediments.
The best structural interpretation for the Howard’s Pass district is that layer-parallel shortening during Jura-Cretaceous orogenesis resulted in a regional-scale duplex structure. The bulk of the deformation is Jura-Cretaceous, not the Silurian in age.
Recent studies on ‘SEDEX’ deposits of the Selwyn Basin and world-wide suggest that, in some cases, there is little evidence for exhalation and that the bulk of the Pb-Zn sulphide minerals precipitated below the seafloor. These studies, together with the structural re-interpretation of Howard’s Pass demanded by our work, highlight the need to re-examine the currently accepted models for all ‘SEDEX’ in the Selwyn Basin.