Both magmatic and tectonic processes contribute to the formation of volcanic continental margins. Such margins are thought to undergo short-lived extension across a narrow zone of lithospheric thinning (~100 km). New observations from the Eastern North American Margin (ENAM) contradicts this hypothesis. With ~64,000 km of 2D seismic data tied to 40 wells combined with published refraction, deep reflection, receiver function and onshore drilling efforts, we quantified along-strike variations in the distribution of rift structures, magmatism, crustal thickness, and early post-rift sedimentation on the shelf of Baltimore Canyon trough (BCT), Long Island Platform and Georges Bank Basin (GBB) of ENAM. Results indicate that BCT is narrow (80-120 km) with a sharp basement hinge and few rift basins. The Seaward Dipping Reflectors (SDR) there are ~50 km seaward of the hinge line. In contrast, GBB is wide (~200 km), has many syn-rift structures, and SDR there are about 200 km away from the hinge line. Early post-rift depocenters at the GBB coincide with thinner crust suggesting “uniform” thinning of the entire lithosphere. Models for the formation of volcanic margins do not explain the wide structure of the GBB. The different characteristics between BCT and GBB point to different modes of rifting. The BCT underwent little, or highly localized, thinning prior to the volcanic phase. Thinning of the GBB segment was broader. These variations result from either diachronous rifting, heterogenous rheology or a lateral asthenosphere temperature gradient.

Distributed faulting typically tends to coalesce into one or a few faults with repeated deformation. The 2020 seismic sequence in southwestern Puerto Rico (SWPR) was characterized however by rupture of several short intersecting strike-slip and normal faults. The deformation does not appear to have coalesced despite several lines of geological and morphological evidence suggesting repeated deformation since post early Pliocene (~>3 Ma). The progression of clustered medium-sized (≥Mw4.5) earthquakes, modeling shoreline subsidence from InSAR, and sub-seafloor mapping by high-resolution seismic reflection profiles, suggest that the earthquake swarm was distributed across several fault planes beneath the insular shelf and upper slope in the vicinity of Guayanilla submarine canyon. The deformation may represent the southernmost part of a diffuse boundary, the Western Puerto Rico Deformation Boundary, which accommodates differential movement between the Puerto Rico and Hispaniola arc blocks. This differential movement is possibly driven by the differential seismic coupling along the Puerto Rico – Hispaniola subduction zone. We propose that the compositional heterogeneity across the island arc retards the process of focusing the deformation into a single fault. Given the evidence presented here, we should not expect a single large event in this area but similar diffuse sequences in the future.