Two Time-Scales of Pulsation of the Iceland Plume Inferred From Magnetic Anomalies of the North-Atlantic

We have shown previously [Abelson and Agnon, EPSL, 1997, 2001] that small offsets 2nd-order MOR segments, oblique to spreading direction, may indicate a strong influence of a nearby hotspot. We term this anomalous plan-view-geometry of the ridge axis as a planform anomaly (PA). The PA-hypothesis was successfully tested on the ridge-plume system around Iceland, where ancient planforms of the Kolbeinsey Ridge (KR) and the Reykjanes Ridge (RR) are recorded by magnetic anomalies north and south of Iceland, respectively. The magnetic fabric off-KR axis displays variations between normal planform (orthogonal & segmented) and PA in the past 10 Myr. The PA recurrences of the KR correlate with the plume pulses previously inferred by the V-shaped ridges around RR, suggesting that PA records episodes of strong influence of Iceland on KR. Accordingly, the inferred time-scale of cycles of plume pulsation is ~5 Myr. On the other hand, the magnetic fabric around the RR displays variations between normal planform and PA in a time-scale of 30 Myr. The fluctuations between PA and normal planform are made by twist of the 2nd-order segments while preservation of obliquity of the general trend of the ridge on ~30° to spreading direction. From this magnetic record two major pulses of enhanced plume activity are inferred since the formation of the oceanic crust (anomaly #24). The first pulse is between 54 Ma to 39 Ma, compatible with thick igneous crust (18-9 km). The second pulse starts some 20-24 Ma, consistent with the disappearance of off-axis scars (i.e., disappearance of 2nd-order discontinuities along the RR). This long pulse is accompanied with the emergence of the Iceland Plateau. Between the two long pulses, a period of segmented and orthogonal planform suggests a significant decrease of the plume activity between anomalies #17 (38.1-36.6 Ma) to anomaly #6 (20.1-19 Ma), consistent with the deeper seafloor bathymetry. The inference of the PA-pulse linkage and the two time-scales of pulsation may assist in the detection of the plume structure and dynamics. For instance, we suggest that the short-time scale pulsation is due to fluctuations in the flux of plume material, supported by a pinch-and-swell structure of a vertical anomaly of seismic velocity. This structure implies ascent velocity of ~4 cm/y for 5 Myr interval between pulses.