A new isostatic model for the East Pacific Rise crest

Abstract

We have developed a new isostatic model for the East Pacific Rise (EPR) that explains the origin of both the axial topographic high and the rise crest gravity anomaly. Our model involves a thin elastic plate, with a constant thickness crust, that is broken at the ridge crest. We assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly. We have used this model, and two conventional isostatic models, to examine the isostasy of three segments of the EPR: at 9°–l4°N, 6°–11°S and 16°–21°S. Rather than modeling individual profiles we have stacked (summed) the gravity and bathymetry data in each area and computed a single, 200‐km‐long, mean, mirrored profile for each different ridge segment. These stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly. In all three areas the best fitting model parameters are similar and give average compensation depths of about 6–7 km below the seafloor (near the base of the oceanic crust) and an upper limit on the flexural rigidity of the plate that increases from about 1018 N m at the ridge axis to about 3×1019 N m 5 km from the axis and 1020 N m 25 km or more from the rise crest. These results indicate that the EPR crest is compensated at much shallower depths than suggested by previous investigators. Our results are consistent with the presence of a small, low‐density magma chamber in the lower crust, although at least part of the compensating root for the axial high must extend into the upper mantle. We interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished.

Multitaper coherence method for appraising the elastic thickness of the Indonesian active continental margin

“…T e values of 4 km (Cochran, 1979) and 10 km has been estimated in the East Pacific high and Mid-Atlantic Ridge respectively using admittance and mirrored periodogram method. Madsen et al (1984) in the East Pacific Rise ($0.7 km) have observed further, lower T e values. The variations in the method of T e have thus shown varying relationships between elastic thickness and seismogenic thickness.…”

Mechanisms of lithospheric extension at mid-ocean ridges

“…Here the plate spreads with a half-spreading rate of 4.3 cm yr-l. This topographic feature is typical of other segments of the EPR with faster spreading rates (Madsen et al 1984). Gravity anomalies give another important constraint on the thermal structure of mid-ocean ridges.…”

Oceanic crust thickens approaching the Clipperton Fracture Zone

“…Gravity studies of slow-spreading ridge segments predominantly indicate some combination of thinner crust and cooler mantle approaching transform offsets, evidenced by mantle Bouguer anomaly highs over the fracture zones (Kuo and Forsyth, 1988;Blackman and Forsyth, 1989;Lin et al, 1990;Morris and Detrick, 1991). Gravity studies over fast-spreading ridge segments show a more uniform pattern along isochrons, lacking strong evidence for systematic changes approaching fracture zones (Madsen et al, 1984;. Thus the observational evidence does not overwhelmingly support the expectation that oceanic crust thins systematically approaching fracture zones, particularly for crust formed at fastspreading ridges.…”