The Evolution and Interaction of Normal Faults in Multi-
Phase
Rifting: A Numerical Modelling Approach
Abstract
Continental rifts commonly undergo multiple phases of rifting, with variations in both the rate and orientation of extension through time. Physical analogue experiments have demonstrated that the fault network developed during the initial rift phase
influences subsequent fault populations. However, the full 3D geometry, evolution and interaction of fault networks are difficult to constrain from such models. A 3D discrete element model is employed to compare the evolution of normal fault networks in multi-
phase
rift environments with networks developed during a single rift
phase
. Faults are defined as an accumulation of broken bonds in the brittle layer, and their location, throw and interaction are recorded through time. Thus incremental fault displacement and geometry and the 4D evolution of the fault network can be examined. We investigate how the maturity of an initial normal fault network impacts fault network evolution and geometry during a second rift
phase
. We examine how strongly the presence of
Phase
I structures controls the initiation and localization of subsequent structures by setting secondary extension directions at 30, 45 and 60 degrees to the initial
phase
, and varying the length of
Phase
I relative to
Phase
II. Extension in the initial rift
phase
results in conjugate fault sets that nucleate and organize themselves by segment growth, interaction and linkage into co-linear fault zones. Increased extension leads to a preferred dip polarity and crustal-scale half grabens. The degree of development of this first-
phase
fault network strongly influences the second
phase
fault geometry and evolution. A small amount of
Phase
I extension promotes fault orientations in
Phase
II that are initially controlled by the orientation of
Phase
I before
Phase
II dominates. An intermediate level of
Phase
I extension results in complex
Phase
II fault geometries where reactivation of
Phase
I faults is common and new faults form to accommodate displacement on earlier faults. Sigmoidal planform fault geometries develop, with complex, zig-zag and rhomboidal fault patterns. A mature initial fault network results in
Phase
II being dominated by, and deformation localized onto,
Phase
I faults. Domains dominated by new
Phase
II faults occur where the density of
Phase
I faults is low. In all models, fault geometry shows clear variation with depth - faults become less segmented and are better represented by the
Phase
I orientation at deeper structural levels.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015