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.

The Galapagos microplate formed at 1.4 Ma, initiating Nazca–Galapagos magmatic spreading along its southern and eastern borders. We examine in detail the formation and evolution of the microplate and its effect on the major rift boundaries of the Galapagos triple junction region. We show that the microplate originated by breaks along three pre-existing zones of structural weakness in the Nazca lithosphere: 1) to the south, an active ‘secondary rift’ located ~50 km south of the Pacific-Cocos-Nazca triple junction; 2) to the east, faults associated with the off-axis East Pacific Rise (EPR) abyssal hill fabric, and 3) to the north, the deep normal faults of the southern scarp of the Galapagos gore (the faulted boundary between the Pacific-Nazca and the Cocos-Nazca regimes). The breaks were likely forced by the appearance of a significant magmatic anomaly that crossed the EPR, flooded the ‘secondary rift’ in the south with lavas and shortly thereafter, created two large seamounts (~1500 m and ~1000 m in relief) on the southern boundary. This magmatic anomaly may also be associated with the unusually high elevation of Dietz Volcanic Ridge west of the seamounts, which resembles the rift zones of Axial Seamount on the Juan de Fuca Ridge in height, width and length. Dietz Volcanic Ridge is the present southern boundary of the Galapagos microplate and opens at ~33 mm/yr. It is ~900 m in relief and 7.5-8 km wide at its shallowest section. Rock samples dredged from the shallow section of the ridge in 2018 on the R/V Sally Ride support the idea of a magmatic anomaly in this area. The rocks are transitional MORB that are more enriched than any Cocos-Nazca lavas or the adjacent EPR that were sampled (see Wernette et al. 2019 abstract). The residual mantle Bouguer anomaly indicates thicker crust associated with the two seamounts and the eastern section of Dietz Volcanic Ridge (see Zheng et al. 2019 abstract). We also examine the response of the Cocos-Nazca rift and the EPR to the arrival of the magmatic anomaly and microplate formation. The Galapagos triple junction region is complex, but this complexity provides an opportunity to obtain a better understanding of how plates deform internally near their boundaries, and the relationship between this deformation and upwelling mantle material.