Formation of the Galapagos Microplate and its Effect on Rifting at the
Galapagos Triple Junction
Abstract
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.