|Abstract or Summary
- Three-dimensional gravity analysis is the process of removing the predictable
components from the free-air gravity anomalies and has proven to be useful for
interpreting the subsurface structures and active processes at mid-ocean ridges. The
three-dimensional effects of the seafloor and Moho topography, assuming a constant
crustal thickness and constant crust and upper mantle densities, are subtracted from the
free-air anomalies, yielding the mantle Bouguer anomalies. Mantle Bouguer anomalies
at mid-ocean ridges are believed to be largely due to the three-dimensional thermal
structure, which can be predicted using a simple passive flow model. When the gravity
contribution from the predicted thermal structure is removed from the mantle Bouguer
anomalies, the residual mantle Bouguer anomalies are created, which represent lateral
variations in the crustal thickness and/or density variations from the assumed model.
Three-dimensional gravity analysis has been carried out over three areas along
the Pacific-Antarctic East Pacific Rise (EPR): (1) the eastern intersection of the Menard
transform with the EPR, (2) the overlapping spreading center (OSC) at 36.5°S and, (3)
the western intersection of the Raitt transform with the EPR. This geophysical analysis
provides an essential tool for understanding the subsurface crustal/upper mantle structure of the fast spreading EPR, and more specifically at transform and nontransform offsets along the EPR.
Several interesting features were observed at the eastern intersection between the
Menard transform and the EPR. The continuous nature of the residual mantle Bouguer
anomalies along the ridge axis suggests that the 60 km of ridge axis surveyed here has a
fairly uniform crustal/upper mantle structure. Significant features are not observed in
the residual mantle Bouguer anomalies at the ridge-transform intersection or along the
eastern 75 km of the Menard transform. At the ridge-transform intersection, fresh lavas
from the observed overshot ridge have filled in the transform valley and have
subsequently thickened the crust, eliminating any crustal thinning that is occurring there.
The large OSC at 36.5°S has a left-stepping offset of approximately 34 km. The
most significant feature in the gravity data from this study area is the observed low in
the mantle Bouguer anomalies which extends from the northern ridge segment, eastward
to the "inactive" rift and continued along the southern ridge segment. This gravity low
suggests that this region is underlain by thicker crust and/or hotter, less dense material.
No significant features are observed in the residual mantle Bouguer anomalies
associated with the overlap basin or the two smaller basins that border the "inactive" rift.
The western ridge-transform intersection (RTI) between the Raitt transform and
the EPR significantly differs from the Menard transform study area. A transform valley
is not observed at this RYE and neither is an overshot ridge. Instead, a transformparallel
median ridge is observed east of the RTI, and a fossil transform valley is
observed north of the RTI. A low in the residual mantle Bouguer anomalies is
associated with the fossil transform valley and the median ridge, suggesting that these
areas are underlain by thicker crust and/or less dense material. Positive residual mantle
Bouguer anomalies observed at the inside corner of the RYE suggest that this area is
underlain by thinner crust and/or colder, more dense material; while at the outside corner of the ridge-transform intersection, a residual anomaly low is observed which suggests
that the outside corner is underlain with thicker crust and/or hotter, less dense material.