Central anomaly magnetization high: constraints on the Volcanic construction and architecture of seismic layer 2A at a fast-spreading mid-ocean ridge, the EPR at 9 degrees 30 '-50 ' N

The central anomaly magnetization high (CAMH) is a zone of high crustal mag netization centered on the axis of the East Pacific Rise (EPR) and many oth er segments of the global mid-ocean ridge (MOR). The CAMH is thought to ref lect the presence of recently emplaced and highly magnetic lavas. Forward m odels show that the complicated character of the near-bottom CAMH can be su ccessfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava mag netization intensity with age. This lava magnetization function is the prod uct of geomagnetic paleofield intensity, which has increased by a factor of 2 over the last 40 kyr, and low-temperature alteration which decreases the remanence of lava with exposure to seawater The success of the forward mod eling justifies the inverse approach: deconvolution of the magnetic data fo r lava distribution and integration of that distribution for magnetic layer thickness. This approach is tested on two near-bottom magnetic profiles AL 2767 and AL2771, collected using Alvin across the EPR axis at 9 degrees 31' N and 9 degrees 50'N. Our analysis of these data produces an estimate of th e relative thickness of the magnetic lava layer which is remarkably consist ent with existing multichannel estimates of layer 2A thickness from lines C DP31 and CDP27. The similarity between magnetic layer and seismic layer 2A at the 9 degrees-10 degrees N segment of the EPR crest provides independent support to the notion that seismic layer 2A in young oceanic crust represe nts the highly magnetic lava layer, and that the velocity gradient at the b ase of layer 2A is related to the increasing number of higher-velocity dike s with depth in the lava-dike transition zone. The near-bottom magnetic ano maly character of the CAMH is a powerful indicator of the emplacement histo ry of upper crust at MORs which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints. (C) 1999 Elsevier Science B.V. All rights reserved.