Volcanic plates overriding the ocean crust: structure and dynamics of Hawaiian volcanoes
Authors: Borgia Andrea , Treves Benedetta 
Published: April 2008

We suggest that Hawaiian volcanoes may be viewed as a set of juxtaposed
volcanic plates
that override the ocean crust. These plates are bounded by the rift zones of the volcanoes that act as spreading ridges where
volcanic crust
(the flank of a volcano) is generated, transform faults connecting subparallel non-coaxial sections of the rifts, and obduction zones where the volcanic crust overrides the ocean floor. Accordingly, a phenomenological continuum of volcanic processes exists between small volcanic cones and ocean plates. Hawaiian volcanoes are the link between these two extremes. Like volcanoes they are constructed around a conduit that carries magma through the crust to the surface; similar to ocean plates their flanks spread away from rift zones; unlike ocean plates, however, they are too small to have asthenospheric roots below the flanks. A review of published work on Hawaiian volcanoes shows that they have four major stages of growth: (1) the alkali basalt pre-shield stage; (2) the tholeiitic basalt shield-building stage; (3) the alkali basalt post-shield stage; and (4) the rejuvenated nephelinite stage. Magma mixing of relatively primitive undegassed mantle, ocean lithosphere, and depleted asthenosphere produces alkali basalts and tholeiitic basalts accompanied by subordinate amounts of more evolved lavas belonging to the peralkaline, alkaline, and tholeiitic trends. The magma forms subcrustal plutonic complexes up to 200 km wide and 6 km thick. It also ponds within the volcanic edifices forming basal and summit reservoirs. The ratio of magma intrusion to magma extrusion seems to increase with time approaching 1 at the end of the shield-building stage. The volcanoes have rift zones that are the linear loci of most eruptions. The rifts vertically dissect the whole volcanic edifices and can be subdivided into three zones: (1) a superficial rift zone characterized by lava flows, feeder dykes and open fractures; (2) an intermediate rift zone comprising a gabbroic dyke complex; and (3) a deep rift zone forming an extensive magma reservoir characterized by cumulitic mafic-ultramafic intrusions. This rock suite is comparable to that found in ocean plates and ophiolitic complexes. Magma emplacement in the rift zones is accompanied by flank displacement away from the rifts. The displacement (
c.
0.1 m a
−1
) is forced by both the weight of the flanks and the hydrostatic pressure of magma intruding the rifts. It is thought to occur on thrust faults (decollements) located in the sediment layer between the ocean crust and the overlying flanks of the volcanoes. In the distal section of the flanks these decollements ramp up towards the sea bottom producing anticlinal ridges. We suggest that the metamorphic environment present along the decollement is similar to that of metamorphic soles characterizing the bottom of many ophiolitic complexes. In addition, the giant submarine landslides (up to 200 km long and 1 km thick, with blocks up to 25 km in maximum dimension) originating at Hawaiian volcanoes once accreted in orogenic belts could resemble ophiolitic melange deposits. Finally, we speculate that some ophiolitic complexes of eastern Kamchatka could represent accreted fragments from the subducted extension of the Hawaiian-Emperor Ridge.