Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region

Abstract

Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike-slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6-5.9 km, which compares with the observations reported for slow-slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast-slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower-order Mid-Atlantic Ridge segmentation around the equator.

Lithospheric thinning due to hydration and melting at oceanic transform plate boundaries

“…3a). Our results are consistent with crustal structures obtained from other independent seismic studies 8,17 . Then the mantle velocity is estimated by inverting the manually picked Pn arrival times using a 2-D ray-based travel time tomography method (see Methods).…”

“…The presence of thin lithosphere would have a signi cant effect on the geodynamics of the ridgetransform system. It might account for some recent observations of thick magmatic crust at oceanic FZ 17 and TF 8 . The occurrence of volcanisms 31 along oceanic TFs might be caused by the presence of a thin lithosphere.…”

“…The thickness of sediments is estimated using the two-way travel times (TWTTs) of the basement events, assuming the average velocity of sediment is 1.86 km/s (ref. 8,17). The TWTTs of the basement events are manually picked on the migrated image of the seismic re ection data acquired along the pro le.…”

“…The crustal velocity and the depth of the Moho in the study region are constrained by the travel times of crustal refractions (Pg) and Moho re ections (PmP), using a method similar to Gregory et al 8 and Marjanovic et al 17 . Then the crustal velocity and the depth of the Moho are held xed whilst performing tomography using mantle refractions (Pn).…”

“…Ray-based travel time tomography of mantle refractions. In this section, we introduce the tomography using the Pn arrivals, and readers can refer to Gregory et al 8 and Marjanovic et al 17 for the tomography of the Pg and PmP arrivals. The seismic data are ltered by a 4-5-12.5-15 Hz bandpass lter before picking the Pn arrivals.…”