The Geisha Guyots: Multibeam bathymetry and morphometric interpretation

Multibeam bathymetric charts (100 fm = 183 m contour interval) are presented and analyzed for the ∼1100-km-long, 120° trending Geisha chain of eight guyots and at least 13 other seamounts (>1 km height) in the northwest Pacific; these guyots are compared with 23 others in the North Pacific. Edifice distribution is nonrandom and nonuniform and is best described as clusters (200 km apart), some of which are composed of subclusters (30-50 km apart) and edifices 10-20 km from their neighbors. Published radiometric dates from the two ends of the Geisha chain (94 Ma for Makarov in the southeast and 102 Ma for Seiko in the northwest) are consistent with rapid (14 cm/yr) plate motion over a fixed hotspot, but other mechanisms cannot be excluded. Crustal age (from magnetic lineations) increases south westward from 141 to 156 Ma along the chain, i.e., from 39 to 62 Ma at times of seamount volcanism. Total edifice volume is small (2×10⁴ km³), with Makarov guyot at 6000 km³ and the other guyots 700-1500 km³ each. Summit plateau depths (averaging 1420±60 m minimum depth and 1600±120 m break depth) are remarkably uniform along the chain; plateau areas (87±68 km², ranging from 14 to 246 km²) are the smallest for any guyot chain. Original island heights (0.5-1.3 km) and minimum volumes eroded (2-100 km³) are estimated from the height/area relation of modern volcanic islands. Summit plateau relief (∼100-200 m) is explained by simultaneous shoreline erosion (∼1 km/Ma) and subsidence of thermally rejuvenated lithosphere. There is no bathymetric evidence for guyot volcanism or faulting postdating subsidence below wave base. The Geisha guyots rise 4.5-4.9 km above the regional basement, implying a hotspot-generated swell height of 0.5-1 km and lithosphere thermally reset to about 45% of its age at time of volcanism. Average upper guyot slopes (21°±3°) exceed those of other guyots (13°+3°) but resemble small seamount slopes, suggesting that flank slopes decrease, above about 5000 km ³ edifice volume, due to higher effusive rates and larger flows. The Geisha guyots exhibit prominent but short radial protuberances interpreted as flank rift zones (FRZ). A relation between base-edifice differential magma pressure and FRZ length suggests three regimes: FRZ's are absent on seamounts <2 km in height, increase slowly (0.2 km/MPa) for large seamounts and small guyots like the Geishas, and above 100 MPa increase rapidly (3 km/MPa) with large scatter. FRZ's exceeding 100-150 km length on large edifices suggest hot, low-viscosity rift conduits along which magma can flow under low-pressure gradients.