Abstract
This study uses gravity data to investigate crustal structure in the
region of the Galapagos triple junction where the westward propagating
Cocos-Nazca rift (CNR) approaches the East Pacific Rise (EPR) and forms
the northern boundary of the Galapagos micro plate. Shipboard and global
gravity data are analyzed from 104˚W to 96˚W and 0˚ to 4˚N. In May 2018,
the high-resolution gravity data were collected along ship tracks that
run across the entire width of the Galapagos gore from the tip of the
CNR at ~101.7˚W to 98.5˚W. Residual mantle Bouguer
anomaly (RMBA) was calculated by removing the effects of water-crust,
crust-mantle, and lithospheric cooling from the free-air anomaly (FAA).
We also calculated a model of gravity-derived crustal thickness by
downward continuation of the RMBA, as well as a model of non-isostatic
topography by removing the topographic effects of thermal subsidence and
crustal thickness variations. The results reveal several distinctive
features in gravity and crustal structure: (1) The eastern flank of the
EPR has systematic shallower topography and more negative RMBA than the
conjugate western flank, reflecting regional density variations. (2) On
the eastern flank of the EPR, the region south of the Galapagos gore is
associated with more negative RMBA than the conjugate region to the
north, possibly reflecting closer proximity to the Galapagos hotspot in
the southern region. (3) The first ~100 km behind the
propagating CNR tip (~101.7˚W to 100.8˚W) is associated
with more positive RMBA (up to ~35 mGal) than the CNR
rift between ~100.8˚W and 98.5˚W, suggesting locally
thinner crust (up to ~1.5 – 2 km). East of 98.5˚W along
the CNR, RMBA decreases gradually towards the Galapagos hotspot. (4) A
region of local high topography on the southern boundary of the
Galapagos microplate, where fresh basalts were sampled, is associated
with negative RMBA centered at ~101.6˚W and 1.3˚N,
indicating local relatively thick crust. (5) Within our study area, the
CNR crust shows shallower average off-axis topography and more negative
average RMBA than the EPR crust of corresponding age, which is
consistent with a model of isostatic compensation of average thicker CNR
crust than the surrounding EPR crust, possibly reflecting Galapagos
hotspot effects.