Abstract - Knowledge of the structure and content of fault zones is a pre-requisite for the
construction of fault zone models and for assessments of fluid capacity and flow
behaviour within faulted sequences. In this paper, we summarise field observations of
the Maghlaq Fault Zone in Malta which is characterised by localised complexities in
fault zone architecture arising from the linkage of an array of fault segments.
The Maghlaq Fault is a >200m displacement, left-stepping, normal fault
outcropping in coastal exposures of southwest Malta. The fault displaces Oligo-
Miocene, pre- to syn-faulting carbonate sediments, that exhibit a range of deformation
styles reflecting their state of lithification during deformation. We focus on the fault
zone geometry and fault rocks of the footwall sequence of pre-faulting, and generally,
massive limestones that were lithified prior to faulting. Though the footwall fault
surface is usually relatively planar and coated with a mm-cm thick zone of finegrained
breccia and cataclasite, heterogeneities in fault zone structure and content are
common and can be related to specific linkage-related structures.
Breached relay zones - Deformation within breached relay zones varies with
the nature of the faulted sequence and tends to be more complex within massive rather
than well-bedded carbonate sequences, because of the space problems associated
with accommodating relay-related bed rotations. Ramp rotations predominantly
accommodated by flexural slip within well-bedded sequences are instead
accommodated by antithetic faults, synthetic faults and bed-parallel slip surfaces that
combine to form a linked slip system within massive limestones. This ramp
deformation gives rise to fault densities that are one to two orders of magnitude
greater than background values.
Branch-lines - Progressive rotation of a relay ramp eventually leads to
segment linkage and the formation of a branch-line, the plunge of which depends on
the relative orientations of the intersecting faults, but is usually sub-vertical.
Continued interaction between the intersecting faults, combined with the often very
high strains localised within the intervening rock volume, results in the formation of
anomalous thicknesses of coarse fault breccia.
Bends –Increasing displacements on faults that link along branch-lines give
rise to an angular fault bend on the footwall fault. Where the axis of this bend is
oblique to the fault slip vector the local slip direction swings into parallelism with the
axis. In some circumstances this scenario is kinematically unfavourable and the fault
bends are modified by abrasion of the footwall to form a cylindrical fault geometry, in
which a constant slip direction can be maintained around the bend. Bends are
therefore characterised by anomalously thick fault zones which are occupied by
decametre-scale lenses of deformed footwall limestone.
The Maghlaq Fault Zone is characterised by increased brecciation and
fracturing associated with the linkage of fault segments, along zones that tend to be
elongate in the fault slip direction. These findings suggest that the evolution of
segmented fault arrays within carbonate rocks, will tend to provide linear zones of
enhanced vertical permeability at pre-existing segment boundaries.
Abstract of talk given to:
Irisag Geological Research Meeting, Galway, March 2004