Hotspot activity and plume pulses recorded by geometry of spreading axes

Anomalous plan view geometry (planform) of spreading axes is shown to be a faithful indicator of hotspot influence, possibly capable of detecting puls es of hotspot discharge. A planform anomaly (PA) occurs when the orientatio n of second-order ridge segments is prominently oblique to the spreading di rection. PA is found in the vicinity of hotspots at shallow ridges ( < 1.5 km), suggesting hotspot influence. In places the PA and shallow bathymetry are accompanied by geochemical anomalies, corroborating hotspot influence. This linkage is best expressed in the western Gulf of Aden, where the exten t of the PA from the Afar hotspot coincides with the extent of La/Sm and Sr isotopic anomalies. Using fracture mechanics we predict PA to reflect over pressurized melt that dominates the stresses in the crust, consistent with hotspot regime. Accordingly, the temporal variations of the planform previo usly inferred from magnetic anomalies around the Kolbeinsey Ridge (KR), nor th of Iceland, record episodes of interaction with the hotspot and major pu lses of the plume. This suggestion is corroborated by temporal correlation of episodes showing PA north of Iceland with plume pulses previously inferr ed by the V-shaped ridges around the Reykjanes Ridge (RR), south of Iceland . In contrast to the RR, the temporal correlation suggests simultaneous inc idence of the plume pulses at Iceland and KR, hundreds of kilometers to the north. A deep northward branch of the Iceland plume active during pulse-pe riods may explain these observations. (C) 2001 Elsevier Science B.V. All ri ghts reserved.