Origins of Nonvolcanic Seamounts in a Forearc Environment

  1. Barbara H. Keating,
  2. Patricia Fryer,
  3. Rodey Batiza and
  4. George W. Boehlert
  1. Patricia Fryer and
  2. Gerard J. Fryer

Published Online: 18 MAR 2013

DOI: 10.1029/GM043p0061

Seamounts, Islands, and Atolls

Seamounts, Islands, and Atolls

How to Cite

Fryer, P. and Fryer, G. J. (1987) Origins of Nonvolcanic Seamounts in a Forearc Environment, in Seamounts, Islands, and Atolls (eds B. H. Keating, P. Fryer, R. Batiza and G. W. Boehlert), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM043p0061

Author Information

  1. Hawaii Institute of Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii 96822

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1987

ISBN Information

Print ISBN: 9780875900681

Online ISBN: 9781118664209



  • Seamounts;
  • Coral reefs and islands


The outer half of the Mariana forearc, the region between the trench axis and the active volcanic arc, contains numerous large seamounts formed entirely by nonvolcanic processes. These seamounts are up to 30 km in diameter and rise as much as 2 km from the seafloor around them. Within about 50 km of the trench axis most of the seamounts are horst blocks of uplifted forearc material. From 50 to about 120 km from the trench axis the seamounts are either sites of updomed forearc material caused by diapiric intrusion, or sites of extrusion of diapirically emplaced serpentinized ultra manes fiom the lower crust/upper mantle of the underlying forearc. The formation of the diapiric material comprising these seamounts is dependent on the evolution of the thermal structure of the shallow (above 30 km) portion of the overriding plate as a convergence zone develops. Changes in the thermal structure influence the distribution of the stability fields of various regional metamorphic facies within the forearc region. As a convergence zone evolves, the greenschist stability field retreats from the region of the trench axis and is replaced by the stability field of the lawsonite-albite-chlorite facies at shallow levels, and by that of the the blueschist facies at depth. The disappearance of the greenschist facies stability field from the forearc suggests that the serpentinite diapirs are either emplaced early in the history of the forearc or that serpentinite remains metastable within the outer forearc for tens of millions of years. The growth of the chlorite and blueschist stability fields may explain the apparent capacity of forearc regions to accommodate large amounts of fluids driven off the downgoing slab by compaction, desiccation, and dehydration reactions. Although conditions appropriate for the formation of either fault block seamounts or diapirically formed seamounts may exist in any forearc, the occurrence of the seamounts is dependent on the local tectonic environment. In the case of the Mariana forearc the horsts and diapirs are related to fracturing of the forearc and to subduction related vertical tectonic movement. In those convergent margins with simpler tectonic conditions, horst seamounts may be absent and discrete diapiric seamounts may be replaced by regional upwarp or development of a low forearc ridge.