Boersma, N.D. and Kattenhorn, S.A. (2005)
Surface Monocline Development Along Normal Faults in Basalt, Southwest Iceland
Eos, Transactions of the American Geophysical Union, 86.
The subaerial exposure of the Mid-Atlantic Ridge in southwest Iceland is characterized by a high angle of spreading obliquity (30°), resulting in a complex structural fabric along the ridge segment at the Reykjanes Peninsula (RP). Tension fractures, normal, oblique-slip, and strike-slip faults, eruptive fissures, and prominent hyaloclastite ridges demarcate a series of four closely-spaced fissure swarms across the length of the RP. Monoclines flank the hanging wall sides of numerous normal and oblique-slip faults in the fissure swarms. The formation of these surface flexures is likely the result of upward fault propagation from depth, facilitated by the growth of vertical fractures that nucleate at the upper fault tip and then rip through the highly fractured basalt lava pile. Subsurface slip along the buried fault causes a monocline to develop at the surface above the fault tip. As slip continues, the monocline grows in both height and width, accommodating throw at the surface purely by flexure. Bending stresses within the flexure induce tensile fracturing that breaches the upper hinge at the surface. Further movement along the underlying fault increases the dilation of this fracture resulting in a gaping chasm that may widen by collapse along the fracture walls. No throw occurs along this hinge fracture until the fault ultimately breaches the surface via the fracture. At this time, the monocline becomes a passive and detached hanging wall structure along a vertical fault scarp, thus preventing further growth of the flexure.
Detailed field and aerial photo mapping from scanned then digitally orthorectified photos (0.125m/pixel) were used to analyze fracture style, surface monocline geometries, and to assess the spatial variability of monoclinal flexures across the RP and northwards into the Western Volcanic Zone (WVZ). To capture along-strike changes in monocline shape, elevation profiles were constructed at numerous locations along, and orthogonal to, fault traces using a Trimble real-time differential GPS Pathfinder Pro XR data collection system. The observed monoclines are variable in scale, the largest of which exceeds 8 km in length, located on the SE side of the western graben-bounding fault at Thingvellir in the WVZ. Near the center of this fault, a throw of nearly 20 m across the upper hinge fracture combines with an additional 15 m of throw that was accommodated by the monocline itself prior to the fault breaching the surface, for a total of 35 m of throw. As the throw approaches zero towards the tips of the fault, there is an associated decrease in monocline-accommodated throw and monocline width until the monocline eventually disappears at the fault tips. Monocline geometries display the same relationship along individual, 10s of meters long fault segments near Burfell on the RP. An oblique-slip fault here is characterized by left-stepping en echelon segments that each exhibit a discrete monocline. Elsewhere on the RP, examples of monoclines lacking surface rupture (i.e., blind faults) have been mapped in regions of active faulting in the rift zone, suggesting that monocline formation is an active and ongoing process. Some examples of unbreached surface flexures contain underlying cavities several meters across that appear to occur above a vertical fracture linked to the upwardly propagating, underlying fault. The occurrence of these cavities suggests that the formation of monoclines may not necessarily be related to broad-scale warping of the surface (as appears to be true of Thingvellir), but rather a delamination phenomenon involving only the most recent lava flows that covered an active fault.