Spatial and temporal variation in extensional strain in response to a tectono-magmatic cycle: insights from an obliquely spreading ridge segment, SW Iceland

Rifting in continental lithosphere commonly involves an interplay between tectonic and magmatically assisted extension, characterized by spatial and temporal cyclicity. In the case of oblique extension, transtension models can be used to ascertain the instantaneous strain orientation, which can then be compared to field observations of the orientation of deformation features (normal faults, joints, and dikes) and any kinematic indicators to see if theory and observation can be reconciled. Where magmatic events are episodic, a secondary state of stress may be superimposed on a region of the lithosphere during dike intrusion, resulting in variable kinematic behavior along rift zone faults from magmatic to amagmatic periods. Punctuated magma-assisted extension episodes may ultimately result in a transition to transform-like (or highly oblique) motions during later amagmatic periods in order to average out the long-term oblique displacements across the rift zone. In response, the instantaneous extensional strain axis may oscillate as magmatic episodes come and go, greatly affecting fault activity. As similar extensional processes and tectono-magmatic cyclicity exist along oblique spreading centers, they may provide insights into the long-term effects of magmatic influences during continental rifting. The Reykjanes Peninsula of SW Iceland is one potential analog. As the Mid-Atlantic Ridge comes onshore at the Reykjanes Peninsula, it bends towards an 075 trend that is about 30 degrees oblique to the 105 degree NUVEL-1A plate motion direction. The Reykjanes Peninsula ridge segment is composed of normal, oblique-slip, and strike-slip faults, gaping vertical fractures, and eruptive fissures. Much of this deformation is localized within four NE-trending fissure swarms, each associated with its own magma system, and comprised of an assemblage of closely related fractures and normal faults. The last eruption on the peninsula occurred in the 13th century. Despite the long-term plate motion direction, current motions along the rift axis deduced from GPS-derived velocities and seismic data are distinctly transform-like. The presence of large normal fault scarps despite the lack of contemporary normal fault motions provides evidence for a temporal partitioning of strain into transform (strike-slip fault) and extensional (normal fault) components. Most of the left-lateral component of the oblique spreading is taken up by NNE-trending, right-lateral strike-slip faults (bookshelf faulting) that span the full length of the peninsula with no correlation to the locations of fissure swarms. These are the faults that have been historically active. The fissure swarms trend perpendicular to the long-term strain direction associated with plate motion, implying that the broad-scale structural fabric of the lithosphere is directly related to the spreading obliquity. Nonetheless, at the 100s of m scale, fault traces have a zigzag pattern comprised of two orientations (30 degrees different in strike) where they broke through postglacial lava flows that covered the faults, implying two distinct strain directions. At the 10s of m scale, faults and fractures display left-stepping, en echelon segments oriented 10-15 degrees clockwise from the general trend of the array, but this pattern is not consistent in the eruptive fissures, which may thus have been the cause of these fault geometries. The left-stepping geometry suggests a rotation of the stress field that induced right-lateral oblique slip events. Interestingly, opening vectors across these left-stepping fractures often indicate left-lateral motion. The faults thus have a complex, oscillatory kinematic history. As there has been minimal recent slip on normal faults having up to 10 m of throw in 2-12 ka lava flows, there must have been periods of time in the past 12,000 years when normal faulting dominated. Eruptive fissures (dikes) are consistently found in close proximity and subparallel to the fault traces. The field observations, combined with numerical models of dike effects on faults, imply that normal faulting predominates during the extensional strain field characteristic of magmatic periods (with right-lateral oblique slip partially due to perturbations of the stress field by the intruding dikes), whereas strike-slip faulting predominates during amagmatic periods. As normal fault activity decreases, there is also a transition to left-lateral oblique motion due to the rotation of the extensional strain axis as the magmatic influences wane. These oscillations repeat on the time scale of magmatic episodes on the peninsula, perhaps every 1000-2000 years.

Citations:

This abstract has been cited in the following works:


None at present.

BACK