James S. Jackson¹

(1) Consultant, Portland, OR

ABSTRACT: Modification of Petroleum Systems by Movements on Transform Faults

Transform faults on passive margins occur in two settings: between offsets on the mid-ocean ridges and at the transform plate boundary between continental and ocean crust. Lateral movement occurs on the transform faults between the mid-ocean ridges. Vertical movement occurs at transform plate boundaries where thermal subsidence effects the ocean plate. Less well known is the range of movements that occur on transform faults out board of the mid-ocean ridges when there is a change in sea floor spreading geometry.

Vertical movements at transform plate boundaries result in a narrow zone of steeply dipping faults bounding relatively small fault blocks. Rapid subsidence of the extended continental crust and ocean crust across the boundary leads to maturation of potential source rock facies, with subsequent hydrocarbon migration following predominantly vertical pathways.

Lateral movements of transform faults extending outboard from the mid-ocean ridges during changes in sea-floor spreading geometry can result in the inversion of extensional fault systems and associated pre-rift and syn-rift sediments. Folds may also form above the transform fault extensions within the post-rift sediments. The regional vertical subsidence may occur between transform fault extensions. All of these movements effect patterns of sedimentation, source rock maturation, and hydrocarbon migration.

Examples from basins within the Atlantic and Indian oceans illustrate the impact of transform fault movement on petroleum systems.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado