GRAVITY-ANOMALIES OVER EXTINCT SPREADING CENTERS - A TEST OF GRAVITY MODELS OF ACTIVE-CENTERS

Gravity fields over active mid-ocean ridges display long wavelength free-air gravity highs with superimposed short wavelength anomalies over the spreading axis. These axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30-70 mGal and wavelengths of 40-60 km but decrease in amplitude and wavelength with increasing spreading rate until, at fast spreading centers, the anomalies are positive (10-20 mGal) with wavelengths of about 20-30 km. Similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo-spreading rate dependency, suggesting that a large component of the anomaly is not dynamically maintained. Calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here, even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows. The amplitudes of the residual lows at slow paleo-spreading rates are 25-45 mGal with half widths of about 20-45 km, and decrease with increasing spreading rate to about 5 mGal amplitude and 10-15 km half-width over fast paleo-spreading ridges. The amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers. Most previously proposed models of mid-ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid-ocean ridges. This research provides a further constraint on geometric models of accretion, as these models predict vastly different gravity fields. Results indicate that a frozen-in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis. This conclusion is consistent with the presence of a low-density gabbroic root situated at shallow mantle depths beneath the ridge axis. This root tends to narrow with increasing spreading rate, producing the required residual gravity field variation.