Micro-seismicity in the vicinity of oceanic-core complexes – median valley centered versus rift boundary fault located activity

Lithospheric accretion along the slow-spreading Mid-Atlantic ridge (MAR) is driven by two contrasting modes: (i) asymmetric accretion involving active detachment faulting and core complex formation and (ii) symmetric magmatic accretion with abyssal hill formation on both conjugated ridge flanks. Oceanic core complexes (OCCs) are lower crustal or upper mantle rocks exhumed at the seafloor by long-lived detachment faults. At some settings they may accommodate extension for 1-2 Myr, causing profound asymmetries of the crustal structure and faster spreading at the OCC with respect to the conjugated rift flank. Characteristic features of OCCs are blocky topography with dome-shaped structures and often spreading-parallel corrugated surfaces. Hydrophone monitoring networks with a detection threshold of magnitude M~3.5 suggest that active detachment faulting is associated with near constant levels of seismic activity in a colder thermal regime compared to magmatic segments and occurs generally at ridge crest discontinuities. I report new results from three micro-seismicity surveys conducted at spreading segments with off-axis evidence for OCCs, including topographic evidence and dredging of serpentinized peridotites and gabbroic rocks. Two networks were located at the Ascension double transform fault. The first segment was a short 20-km wide intervening segment with a deep valley of >4500 m. Immediately to the south of the Ascension fracture zone the median valley changes from a median valley to an axial high south of 8° 30.S. The second network was located at the intersection of the median valley with the transform fault and extended southward towards the magmatic robust zone. The third network was located in the vicinity of the Logatchev hydrothermal vent field roughly 60 km to the south of the 15°20’N transform fault. The Logatchev vents occur at 3000 m water depth on the eastern rift shoulder roughly 8 km off-axis and 1000 m shallow then the median valley. Studies of vent fluids and alteration of rocks from the Logatchev massif indicated that the Logatchev vents are located in a serpentinite hosted setting. The two spreading segments in the vicinity of the Ascension transform were characterized by median valley centre micro-seismicity and may indicate thermal stresses at the plate boundary rather than fault located activity. Thus, both core complexes are perhaps tectonically inactive. In contrast, at the Logatchev spreading segment a very high degree of micro-seismicity with 40 events per day with magnitudes <2.5 was detected. Further, activity was shifted away from the centre of the median valley and occurred at the normal faults flanking the median valley just westward of the Logatchev vent field. Fault plane solutions suggest that the hydrothermal vents are located where a set of normal faults and transfer faults meet, suggesting that faulting governs the occurrence of hydrothermal activity. However, high backscatter in Kongsberg’s EM120 swath mapping data may indicate that faults are rooted in an intrusive body.