Deep brittle-ductile transition, water percolation into the mantle and
reactivation of weakness zone: new insights of the seismicity on St.
Paul Transform System, Equatorial Atlantic
Abstract
Four transform faults and three intra-transform segments located at
Equatorial Atlantic form the Saint Paul Transform System (SPTS), with a
long-offset of 630 km. In the northern transform, the 200 km long and 30
km wide Atoba Ridge is a major topographic feature that reaches the sea
level at the St. Peter and St. Paul Archipelago island (SPSPA, 00º 55
‘0’ ‘N and 29º 20 ”43”W). The islets have an average uplift rate of
approximately 1.5 mm/year. The southern and northern flanks of the Atobá
Ridge are marked by a series of large thrust faults visible in the
bathymetry and clearly imaged through seismics and correspond to an
exceptionally serpentinized mantle. We have determined the hypocentral
location of 62 minor-moderate earthquakes of SPTS, with magnitudes 1.9 ≥
M ≤ 5.3. The earthquakes occurred in 2013 and were recorded by a
seismometer installed in SPSPA and three autonomous hydrophones deployed
during the COLMEIA cruise. The HYPOCENTER software and Seismic Analysis
Code (SAC) were used for data analysis and hypocenter location. The
depth range is from 0.2 to 17.5 km and are concentrated in three
different zones: the East Shear Zone (ESZ), the Atobá Ridge Zone (ARZ)
and the Central Fracture Zone (CFZ). A seismogenic zone with a deep
britle-ductile transition was identified in SPTS, with hypocenters
reaching 18 km beneath the seafloor. We observed that this lithospheric
structure presents relation with the offset age and controls the maximum
hypocentral depths of oceanic transform faults. Besides, the earthquakes
indicated the existence of a broad serpentinization depth reaching 18 km
beneath the ARZ. This was interpreted as the effect of deep water
percolation into the mantle in the SPTS, which caused a fluid-mantelic
rocks interaction and allowed the expansion of faults into the mantle.
Some hypocenters were located in the central fracture zone (CFZ) segment
of SPTS and their depths reached 8.8 km beneath the seafloor. We
interpreted this seismicity as reactivation of a weakness zone existent
in CFZ due to the transpressive load-induced stress originated in Atobá
Ridge.