Continental Rifting

In some continental rifts there is little magmatism and large areas of subcontinental lithospheric mantle are exhumed. However, the mechanism by which these rocks are exhumed remains controversial. Two of the main hypotheses are 1) the mantle is exhumed along large-scale detachment faults or 2) the mantle is exhumed during ultra-slow seafloor spreading. These two models make dramatically different predictions regarding the timing of mantle exhumation and magmatism on each margin. The detachment model predicts highly asymmetric record of exhumation and magmatism, while the ultra-slow seafloor spreading model requires a more symmetric record. Recently, I was able to work with a unique set of samples from ODP cores drilled along conjugate margins of the Jurassic-Cretaceous Newfoundland-Iberia rift, which I used to test these two hypotheses.

Cartoon of mantle exhumation along a lithospheric-scale detachment fault prior to plate rupture and initial seafloor spreading from Eddy et al. (2017). This model may explain why magmatism and mantle exhumation was time-transgressive from east to we… — Cartoon of mantle exhumation along a lithospheric-scale detachment fault prior to plate rupture and initial seafloor spreading from Eddy et al. (2017). This model may explain why magmatism and mantle exhumation was time-transgressive from east to west and the narrow region of exhumed mantle on the Newfoundland margin compared to the Iberia margin.

Newfoundland-Iberia Rift

The Newfoundland-Iberia rift was active from the Jurassic through the early Cretaceous. During this time, subcontinental lithospheric mantle was exhumed on both the Newfoundland and Iberian margins. These rocks have been sampled extensively through the use of submersibles, scientific drilling, and dredging. In our (Oli Jagoutz [MIT] and Mauricio Ibañez-Mejia[University of Rochester]) study, we dated magmatic veins that intruded the mantle as it was exhumed to the surface. This was done using zircon U-Pb geochronology and was complimented with Hf in zircon isotopic measurements to assess the magma's source mantle. The Hf data demonstrated that these magmas were generated from depleted Atlantic mantle and not a 'plume-like' source, while the dates were consistent with magmatism that was time-transgressive from east to west across the rift. This age progression in mantle exhumation and magmatism is most consistent with the highly asymmetric record expected during large-scale detachment faulting (Eddy et al., 2017). It is also consistent with the initiation of seafloor spreading in the rift up to 10-15 Myr after the commonly accepted age.