Boersma, N.D., Kattenhorn, S.A. (2006)
Normal fault-related surface monoclines: 3-D developmental controls and implications for fault evolution
Eos, Transactions of the American Geophysical Union 87(52), Fall Meeting Supplement, Abstract T43A-1622.
Fault-related folds are usually described in sedimentary sequences but surface folds are also associated with normal faults in volcanic rocks erupted at divergent plate boundaries or in continental rift zones. At the active plate boundary in SW Iceland, normal fault traces in basalt lava flows are commonly marked at the surface by laterally continuous, narrow monoclinal flexures that formed above the fault tip as it propagated towards the surface. Although they tend to increase in height and width towards fault centers, they can be asymmetric about the fault center or variable due to relict fault segmentation. Some small monoclines formed by delamination of a surface lava flow where it covered an active fault; they commonly collapse into the resultant underlying cavities, creating laterally discontinuous monoclines. This complexity in monocline geometry raises the question: is it possible to generate a predictive growth model for fault-related surface monoclines? We combine field measurements of fold geometries as a function of distance along fault strike with numerical models to characterize monocline development and its relationship to the 3-D evolution of the underlying fault.
Monoclines flank the hanging wall side of surface-breaking normal faults; however, monocline growth precedes surface rupture. Monoclines thus act as a proxy for the growth evolution of the underlying fault. After being breached along the upper hinge, the monocline becomes a passive structure in the hanging wall. At that point, all surface throw is accommodated along the fault itself. Monocline height along fault strike mimics the elliptical shape of the fault throw distribution, showing variability due to fault segmentation and kinematic coherence effects. Hence, monocline height is a function of the amount of fault slip prior to breaching in addition to location along the fault. In contrast, monocline width does not necessarily correlate with monocline height or location along fault strike; it is laterally variable, indicating that width is not controlled by the amount of fault slip prior to breaching.
Numerical models based on linear elastic fracture mechanics reveal multiple controls on monocline width and height: fault dip, shape and aspect ratio; upper tip line depth; the height of a vertical fracture at the upper fault tip where it approaches the surface; and rock elastic properties. The depth of the upper tip line is the primary control on monocline shape along fault strike. Shallower upper tip lines produce narrower monoclines. Although monocline height decreases towards the tips, monocline width in the models is relatively constant along fault strike, especially as fault aspect ratio (height/length) exceeds 2. Also, the width is independent of the amount of fault slip and the elastic properties of the rock for a given fault configuration. Therefore, as a subsurface fault accumulates slip, the monocline gets higher but its width remains constant. Along-strike monocline variability thus provides a means of interpreting the vertical and lateral growth evolution of the subsurface fault prior to breaching, although fault geometry cannot be uniquely determined. We infer that monoclines in Iceland formed over faults with high aspect ratios and which only extend a few km into the subsurface, possibly due to growth above shallow crustal dikes. Differences in monocline width are attributed to variable fault tip line depths, vertical fracture heights, and fault segmentation. We used the variable monocline geometry along the Almannagja fault at Thingvellir to show that the fault grew progressively upwards and towards the NE in the subsurface, producing a final aspect ratio of ~4.3.
External link: AGU database
See the poster! [JPEG format]
Citations:
This abstract has been cited in the following works:
None at present.
