SHALE TECTONICS AND DEFORMATION ASSOCIATED WITH ACTIVE DIAPIRISM - THE JERUDONG ANTICLINE, BRUNEI-DARUSSALAM

The Jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock. The N-S-trendi ng anticline overlies a high-angled basement fault zone that was episo dically active. moving predominantly sinistrally in response to transp ressive stresses. The earliest stage of deformation in the study area was mid-Miocene E-W to NE-SW-trending growth faulting and shale-diapir growth. Early normal faults exerted an important influence on the lar ge- and small-scale bedding geometries and facies changes of syn-tecto nic shallow-marine, shoreface and tidal strata. In particular there ar e laterally rapid Facies changes and differential rotation of strata a cross growth faults. Diapir activity is indicated by the presence of s hale dykes which commonly intrude normal fault planes. and concordant shale intrusions. Shale intrusions associated minor thrust faults and normal faults were rotated during a late Miocene-early Pliocene phase of folding related to transpression. Few shale dykes were intruded dur ing this phase, probably because the horizontal principal stresses wer e relatively large compared with the pore-fluid pressure. At the end o f the transpressive phase, shale dykes were intruded into steeply dipp ing beds as a result of stress relaxation. Continued uplift and erosio n elevated overpressured horizons to a point where hydraulic fracturin g reached the surface and Holocene-age mud volcanoes were developed.