| Summary: | Variations in the topographic roughness of volcanic basement in the western North Atlantic basin and in the vicinity of the East Pacific Rise have been measured and related to the seafloor spreading rates and histories of these two regions. A feature known as the rough-smooth basement boundary has been mapped in both the western and eastern central North Atlantic basins of Mesozoic age. This boundary is here attributed to a primary seafloor spreading origin, involving changes in seafloor spreading rate and direction to account for its diachronous nature. The history of seafloor spreading recorded by the Keathley sequence magnetic anomaly pattern between 25(DEGREES)N and 40(DEGREES)N has been modeled using a new computer graphic technique and magnetic block modeling of selected profiles. The modeling shows changes in spreading rate are consistent in timing but not in magnitude among 8 profiles collected between and parallel to fracture zones, and several ridge jumping events not previously mapped. Interesting trends oblique to the spreading direction evident in aeromagnetic anomaly data are attributed to along ridge axis migration of the centers of oceanic crust accretion, and evidently are not predicted from absolute motion of the spreading center over the hotspot frame of reference. A triple ridge jump event near 36(DEGREES)N, 65(DEGREES)W is attributed to the passage of the Verde hotspot nearby the Mesozoic Mid Atlantic Ridge crest at 137 mybp. Avoiding fracture zone topography, ridge crest topography near ridge jumps, seamounts, and other non-steady state seafloor spreading rate areas of the ocean floor, several basement topographic profiles have been analyzed quantitatively to assess the relationship of their roughness to spreading rate. A spectral model of topographic roughness shows a significant threshold change at a spreading half rate of 12 km/m.y. This change is attributed to the presence of a steady state axial magma chamber at ridges spreading faster than this rate, and the absence of a long-lived magma chamber at more slowly spreading ridges.
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