TECTONIC-MAGMATIC INTERACTION AND STRUCTURAL ARCHITECTURE ASSOCIATED WITH OBLIQUE SPREADING, REYKJANES PENINSULA, SW ICELAND

Sinistral-oblique spreading along the Reykjanes Peninsula (RP) rift zone in SW Iceland is indicated by a 30° angle between the 075° plate boundary and the 105° plate motion direction (NUVEL-1A model). The plate boundary is comprised of 5 NE-SW oriented, en echelon ridge segments. Similarly oriented normal faults are pervasive on the RP; however, the largest earthquakes (up to M5.9) have historically occurred along ~N-S oriented, right-lateral strike-slip faults. These contrasting, superposed fault types with disparate orientations are somewhat enigmatic. Published clay models show that simultaneously active, multiple fault orientations are possible in oblique (transtensional) rift zones; however, the structural fabric of the RP shows a mismatch with 30° obliquity predictions. These differences likely reflect the effect of intermittent magmatic episodes, which occur on a ~1000 yr cycle on the RP (the last episode ended in 1241). Despite a 105° spreading direction, pre-2000 GPS data indicated predominantly plate-boundary-parallel motions, implying a portion of the long-term motion (directed toward 135°) was missing. This strain deficiency may result from temporal strain partitioning related to magmatic cyclicity. Oscillating principal extensional strain directions cause different fault types and orientations to be active in magmatic (eruptive) vs. amagmatic periods (i.e., a ratchet-like plate boundary). In the current amagmatic regime, strike-slip faults are active in response to transform-like plate boundary motion. Normal faults will likely become active during the next eruptive period, accommodating the missing spreading component of long-term plate motion, along with dikes. Accordingly, oscillatory motions occur along normal faults observed in the Krisuvik region. Left-stepping fractures in 2-3 kyr lavas indicate a right-lateral shear component on normal faults. However, piercing points in lavas reveal more recent left-lateral shear, indicating temporally variable stresses. This sequence is consistent with predicted oscillatory extension directions moving from the last magmatic period into the current amagmatic period. Such tectonic-volcanic interactions in oblique rift zones could potentially exist on all volcanically active bodies in the solar system.