Melt Sill Research Articles

Uniform accretion of oceanic crust south of the Garrett transform at 14°15′S on the East Pacific Rise

Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second‐order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along‐strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30′N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27′S ridge axis discontinuity by a modest 250–300 m and appears to be continuous across this feature. Given the small aspect ratio (∼1.0 km by ∼50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27′S offset is likely the result of a mixing boundary which is sustained through an along‐strike impedance in convection. Wide‐angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200–250 to 500–600 m) in thickness within 1–2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110–120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along‐strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off‐axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14°27′S discontinuity are caused by long‐term reductions in magma supply. Second‐order differences in the style of extrusive thickening may be related to structural differences within the low‐velocity zone underlying the rise axis and/or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis. Images of Mono on cross‐axis profiles may be traced to within ∼1.0 km of the melt sill edge; this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber.

Seismic Structure of the Southern East Pacific Rise

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges.

Accumulations of melt at the base of young oceanic crust

As hot, solid rock ascends through the mantle, the pressure around it decreases while its temperature decreases approximately adiabatically. During the ascent, the melting temperature decreases faster than the adiabat temperature, and if the initial temperature of the rock is high enough, the rock will begin to melt at the depth where the adiabat crosses the melting-temperature curve. Models of upwelling beneath mid-ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge-axis region suggesting that some of the ascending melt must arrive away from the ridge axis. Here I report ocean-bottom seismograph observations of an uncommon and previously unreported marine seismic phase, which provide strong evidence that melt accumulates in sills at the base of young oceanic crust. The phase results from the conversion of compressional (P) waves to shear (S) waves on interaction with the crust-mantle boundary (the Moho). This phase is rare and very difficult to model, but when visible, it is strikingly prominent and usually appears at unexpectedly great source-to-receiver ranges. The observations imply the presence of bodies of extremely low rigidity, presumably melt sills, at the Moho close to the receiver. Melt may be fairly mobile at the base of the crust, allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle. Escape of this melt to the sea floor may be responsible for the formation of off-axis seamounts.