East-directed overthrusting in the Melbourne Zone, Lachlan Fold Belt
Title:
East-directed overthrusting in the Melbourne Zone, Lachlan Fold Belt
Author:
Murphy, N. C. Gray, D. R.
Appeared in:
Australian journal of earth sciences
Paging:
Volume 39 (1992) nr. 1 pages 37-53
Year:
1992-02
Contents:
Inclined northeast-vergent asymmetric regional F1 folds cut by thrust faults characterize a fold-to-fault transition into the Mt Wellington Fault Zone on the eastern margin of the Melbourne Zone of central Victoria. In the most southerly exposed part of the Mt Wellington Fault Zone at Lake Glenmaggie, polydeformed interbedded sandstone and mudstone of Silurian(?) age are unconformably overlain by weakly deformed Late Devonian conglomerate and volcanic rocks. Three deformations pre-date the Late Devonian succession, which has itself been warped into a series of open folds. Thrusting (D1) along southwest dipping reverse faults and thrusts produced imbrication and juxtaposition of different lithological groups. Fault slices of strongly cleaved, interbedded sandstone and mudstone abut against a weakly deformed mudstone-siltstone sequence, thereby suggesting significant displacement on some of these faults. Thrust sheets consisting of large panels of overturned homoclinally dipping strata represent the faulted out lower limbs of inclined to overturned regional F1 folds. Northwest-trending polydeformed high strain zones (slides), varying from 50 to 800 m in width, cut across the regional F1 folds. These slides are dominated by strong D2 deformation, typified by refolded F1 folds and subvertical northwest-trending crenulation cleavage (S2). The D3 deformation is more regionally extensive and characterized by a subvertical north-trending crenulation cleavage (S3) which locally overprints the D1 and D2 structures within the high strain zones. In slide zones where D2^D3<30°, D3 structures reorient the D2 structures rather than producing F3 fold interference structures. The marked increase in intensity of deformation from west to east across the Melbourne Zone, coupled with a development of fold asymmetry, is typical of shear strain gradients from shallow to deeper levels within thrust belts. Both the direction of movement on faults and the sense of asymmetry of the F1 folds indicate tectonic transport from the southwest to the northeast. This suggests that the Melbourne Zone is allochthonous and has been involved in eastward-transport along a major mid-crustal detachment whose surface expression is considered to be the Mt Wellington Fault Zone.
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