NEAR-AXIS SEAMOUNTS ON THE FLANKS OF THE EAST PACIFIC RISE, 8-DEGREES-N TO 17-DEGREES-N

With complete SeaBeam and SeaMARCII bathymetry and nearly complete Sea MARCII side scan coverage of the northern East Pacific Rise and its fl anks from 8 degrees to 17 degrees N, we characterize the entire popula tion of seamounts on young seafloor (0 to similar to 2 Ma) formed alon g this fast spreading center. There are 179 seamounts in this area tal ler than 200 m, most of which belong to one of 21 seamount chains. The se chains are oriented between the relative and absolute plate motion directions of the Pacific and Cocos Plates, and most of the larger cha ins are more voluminous near their centers than near their ends. Sever al of the seamount chains contain bends in orientation which are not c oincident with each other. The two largest seamount chains, at 9 degre es 55'N (Lament Seamounts) and 15 degrees 40'N, are aligned with porti ons of the East Pacific Rise crest which are among the shallowest and broadest of this area. Overall, the maximum abundance arid size of sea mounts increase with shallower and broader ridge axes. No near-axis se amounts form on the summit of the axial ridge; nonetheless, within 15 km (similar to 0.3 Ma) of the axis, the abundance of seamounts achieve s its overall average. Observations of the numbers and cumulative volu me of seamount edifices versus distance from the axis are consistent w ith a model where the majority of near-axis seamounts are formed in a narrow zone on crust aged similar to 0.1-0.3 Ma (5-15 km from the axis ), with significant growth in a zone several times wider. Side scan so nar reflectivity and seamount magnetization analyses are largely consi stent with the predictions of this model. Two exceptions to this model , however, are seamount chains at 15 degrees 40'N and 8 degrees 25'N w hich show evidence for activity over a broader region; this may be due to the presence of fracture zones within about 30 km of these chains. A compilation of seamount counts from a wide range of spreading cente rs indicates that seamounts taller than 400 m are more abundant at gre ater spreading rates and with transitions from a rift valley to an axi al ridge. The variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results, although these influences cannot be distinguished in this ana lysis.