Abstract -
Relay ramps between segments of normal fault arrays have a wide range of aspect ratios (ramp length : width). A recently compiled dataset of relay ramps and breached relay ramps (N = 600) from outcrop and 3D seismic datasets has an average ramp aspect ratio of ~3.8. The collected data demonstrate a broad relationship between aspect ratio and strength of the offset lithologies with lower aspect ratios more often associated with weaker rocks. Relay ramps in weaker rocks occasionally have negative aspect ratios i.e. the segments are underlapping. Relay ramps with typical aspect ratios (1 - 5) are characterised by bed dip directions at a low angle to fault strike. Where fault segments underlap bed dip directions approach those of the fault dip direction. In this case the relay ramp is a monocline that links the tips of the underlapping faults.
When overlapping fault segments become linked, the tip region of one or both segments is preserved as a by-passed splay(s) that can often be used to infer the location of the earlier segment boundary. When two underlapping fault segments separated by a monocline become linked, the monocline will be preserved as a zone of drag associated with the through-going fault. In this case the tip regions of the initial segments may not be preserved and the previous existence of a segment boundary will be recorded as an irregularity in the fault surface with no associated splays. Where multiple underlapping fault segments become linked together the fault trace will have a corrugated geometry with associated localised normal drag. Depending on how the through going fault propagates the drag may be confined to the footwall or hanging wall or may occur on both sides of the fault. According to this model we expect a continuous spectrum of fault zone structure to form as a result of segment linkage in rocks of different strengths, from fault zones characterised by high aspect ratio breached relay zones in strong rocks, through to corrugated fault surfaces with associated normal drag in weak rocks.
Within the proposed spectrum of fault zone structure, in our experience, fault zones with high aspect ratio relay ramps without extensive normal drag are common but areas that demonstrate the other end member i.e. with pronounced drag, are relatively rare. Lignite mines in the north of Greece provide an example of extensive drag associated with normal faults; here there is a clear association between the occurrence of drag and the boundaries between segments of fault arrays. An example of a case in which normal drag is widely developed is provided by normal faults offsetting unlithified or weakly lithified clastic sequences in the Rio Grande Rift. Here fault zone structure is relatively simple with typically one, often corrugated, fault surface flanked by pronounced drag on the footwall, hanging wall or both sides of the fault . Mapping of 3D seismic reflection datasets in areas with pronounced drag indicates that discontinuous (fault) and continuous (drag) deformations can be combined to provide the smoothly varying total displacement associated with individual faults. This complementary relationship between fault offset and drag reflects the fact that that they are components of a single displacement field with drag occurring where fault offsets on their own may suggest a displacement low i.e. at some segment boundaries.
Several elements of the model presented here have been described previously. Monoclines at the tips of dip-slip faults, particularly in weaker units, are well known, and drag between normal fault segments in cross-section has often been described. However, as far as we are aware,
the suggestion that normal drag and relay ramps can be equivalent structures but with different expressions depending on the offset sequence (or depth of burial) has not previously been made.
Abstract of talk given to:
Geometry and Growth of Normal Faults, Geological Society of London, June 2014.