Segment-scale variations in the crustal structure of 150-300 kyr old fast spreading oceanic crust (East Pacific Rise, ) from wide-angle seismic refraction profiles

Abstract

Summary We have simultaneously inverted seismic refraction and wide‐angle Moho reflection traveltimes for the 2‐D crustal thickness and velocity structure of 150–300 kyr old crust along the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones (FZs). Our results show a strong correlation between ridge segmentation and upper‐ and mid‐crustal seismic velocities, with higher velocities near segment centres and lower velocities near segment ends. Low crustal velocities at the Clipperton and Siqueiros FZs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain. A relict overlap basin left on the Pacific Plate by the 9°03′N overlapping spreading centre (OSC) as it propagated southward is associated with a large (∼1 km s−1), negative upper‐ and mid‐crustal velocity anomaly. This anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration, fracturing and shearing arising from rotation of the basin as it was formed. The discordant zone left by this OSC on the Cocos Plate is characterized by moderately low crustal velocities, probably because of crustal fracturing as the OSC propagated into older crust. Higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust, and lower‐intensity tectonic alteration of the crust, than near segment ends. The mean crustal thickness along the EPR between the Siqueiros and Clipperton FZs is 6.7–6.8 km. The thickest crust is found beneath the Lamont seamounts (∼9 km), and in a southward‐pointing, V‐shaped band located just north of the off‐axis trace of the 9°03′N OSC (7.3–7.8 km). The thinnest crust (<6 km) is found proximal to the Clipperton and Siqueiros FZs. The crust associated with the off‐axis trace of the 9°03′N OSC is not anomalously thin, suggesting that magma supply beneath the OSC is similar to that of the northern and southern segments. We see a similar pattern of crustal thickness variation to that determined using multichannel reflection data, including a gradual thickening of the crust from north to south along the northern ridge segment, and the location of the thickest crust just north of the 9°03′N OSC. However, the magnitude of the along‐axis crustal thickness variation we observe along the northern ridge segment between 9°50′N and 9°15′N(∼1.3–1.8 km, excluding the Lamont seamounts) is significantly less than the 2.3 km of variation previously reported, weakening the case for the existence of a low‐density mantle diapir at 9°50′N inferred from gravity data. The band of thick crust located just north of the off‐axis trace of the 9°03′N OSC suggests a close genetic link between this feature and the OSC. Thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward‐propagating 9°03′N OSC over the past 0.5 Myr, and not to along‐axis melt migration away from a mantle diapir as previously proposed.

Crustal Architecture at the Collision Zone Between Rivera and North American Plates at the Jalisco Block: Tsujal Project

“…The OSC could act as a trap where magma stagnates, which would lead to an increase of the thickness in the extrusive layer and the bulk of the porosity of the upper crust (Bazin et al 2001) giving a plausible explanation to the anomalous layer geometry. An analogous pattern has been reported beneath overlap relict basins in the EPR (Canales et al 2003).…”

Geophysical and geochemical constraints on the evolution of oceanic lithosphere from formation to subduction

“…3-6, 3-7). Near the ridge axis, the wakes of ridge axial discontinuities are known to be associated with prominent negative P-wave velocity anomalies throughout the crust that reflect greater fracturing within the inner pseudofault than in the surrounding crust [e.g., Canales et al, 2003;Weekly et al, 2014]. We find upper crustal (layer 2B) velocities within the propagator wakes are only slightly reduced relative to neighboring crust, suggesting more modest fracturing and/or alteration ( Fig.…”

Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings

“…This segment is one of the best studied portions of the world's MOR system [e.g., Canales et al, 2003;Carbotte and Macdonald, 1992;Christeson et al, 1996;Christeson et al, 1994a;Christeson et al, 1997;Escartín et al, 2007;Fornari et al, 1998a;Fornari et al, 1998b;Fornari et al, 2004;Harding et al, 1993;Haymon et al, 1991;Haymon et al, 1993;Soule et al, 2009;Soule et al, 2008;Soule et al, 2005;Soule et al, 2007;Toomey et al, 1990;Toomey et al, 1994;Wright et al, 1995a;Wright et al, 1995b]. The full spreading rate in this area has been approximately 110 mm yr -1 during the past 2 Myr [Carbotte and Macdonald, 1992].…”