Abstract

The Jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock. The N–S-trending anticline overlies a high-angled basement fault zone that was episodically active, moving predominantly sinistrally in response to transpressive 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 large- and small-scale bedding geometries and facies changes of syn-tectonic shallow-marine, shoreface and tidal strata. In particular there are laterally rapid facies changes and differential rotation of strata across growth faults. Diapir activity is indicated by the presence of shale 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 during this phase, probably because the horizontal principal stresses were relatively large compared with the pore-fluid pressure. At the end of the transpressive phase, shale dykes were intruded into steeply dipping beds as a result of stress relaxation. Continued uplift and erosion elevated overpressured horizons to a point where hydraulic fracturing reached the surface and Holocene-age mud volcanoes were developed.

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