Marine geology of the Hess Rise: 2. Basement morphology, sediment thickness, and structural geology

Abstract

The basement morphology and sediment thickness of the Hess Rise, an oceanic plateau in the central North Pacific, have been mapped on the basis of seismic reflection profiles. The acoustic stratigraphy on and around the rise is correlated with the lithostratigraphy at Deep Sea Drilling Project sites 464, 310, 465, and 466. A total sediment isopach chart of the rise reveals small‐scale departures from the expected sedimentary pattern (thick sediment in shallow areas; thin sediment in deep areas). Sediment‐filled basement depressions result from mass transport; thin sediment (<50 m) occurs on steep scarps, basement ridges, and areas affected by bottom currents. A pre‐Senonian sediment isopach chart shows a thickening from less than 50 m to more than 250 m of sediment from the northeast to the southwest. This trend seems explainable only in terms of the time‐transgressive nature of seafloor formed at a mid‐ocean ridge. The axial trend of the rise (N30°W) parallels nearby Mesozoic magnetic lineations and seems to be isochronous as deduced from the Deep Sea Drilling Project data. The Hess Rise began developing in late Aptian time along a segment of the Pacific‐Farallon Ridge. Important events in the history of the rise are late‐stage volcanism on the southern margin of the rise along the Mendocino Fracture Zone, tectonism and volcanism about 85 Ma that resulted in a major regional unconformity (reflector C), and another period of tectonism and volcanism between 65 and 43 Ma that coincided with the formation of the Emperor Seamounts and created structural benches on the western side of the rise. A significant change in the paleoenvironment that apparently occurred around the Paleogene‐Neogene boundary (∼25–20 Ma) caused pronounced changes in the depositional environment and resulted in another major regional unconformity (reflector A).