Definition of the plate boundary along the East Pacific Rise off Mexico

Abstract

Tectonic and volcanic activity along the East Pacific Rise near Lat. 21°N is generally restricted to a 3–4‐km‐wide area centered over the rise axis. The East Pacific Rise is a medium‐rate (60 mm/yr) spreading center characterized by modest (100–200 m) relief of hills and seapeaks across the crestal region that is typical for such spreading centers. Few tectonic features appear in an axial volcanic zone 600–1, 200 m wide characterized by fresh, glassy pillow basalt and little or no sediment cover. This volcanic terrain is commonly flanked by tectonic zones where older lavas are cut by numerous normal faults bounding horst and graben systems and open fissures; these tectonic zones are commonly of unequal width on each side of the central volcanic zone and locally may be absent on one side. Bottom photographs and visual observations from a manned submersible indicate that most faults and fissures in the tectonic zones are young. Farther than 2 or 3 km from the axial volcanic zone, recent tectonic activity ap...