History of rift propagation and magnetization intensity for the Cocos‐Nazca sspreading Center

Abstract

Analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the Cocos‐Nazca spreading center. Forward models of the magnetic anomalies accounting for spreading rate variations, ridge jumps, asymmetric spreading, magnetization intensity variations, and bathymetry show excellent agreement with observed anomalies. Spreading rates can be constrained to a common finite rotation history through anomaly 4A with three changes in rates. Rate changes at about 1.5 Ma and 4.1 Ma correspond to changes in rate gradients and occur during the well‐calibrated part of the reversal timescale, so they can unquestionably be identified as true changes in plate motion. A ∼15% rate decrease at about 5.2 Ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale. The change at 4.1 Ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0–6 Ma. Rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis. Propagation has been predominantly away from the hotspot, with jumps predominantly south‐ward. Propagation rates have ranged from 30 to 120 mm/yr, commonly near 70 mm/yr. Origin of most propagation sequences is difficult to interpret, but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot. Magnetic anomaly amplitude appears to be a reliable tracer of Fe content of lavas. Several generalizations can be drawn about along‐axis variations in magnetization intensities since 8 Ma: high magnetizations are only observed at the far ends (relative to the Galapagos hotspot) of segments at least 150 km long; offset at the end of a high‐magnetization segment is at least 15 km; and there are no offsets larger than 30–45 km between high‐magnetization segments and the reconstructed position of the hotspot. We interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along‐axis flow of evolved magma. The gradients in supply result from subaxial flow of hotspot‐derived asthenosphere in a narrow conduit. This flow is only partly obstructed by an offset of 20–30 km but entirely blocked by an offset of 50 km.

Patterns in Galápagos Magmatism Arising from the Upper Mantle Dynamics of Plume‐Ridge Interaction

“…Over the 65-km radius, the average excess temperature is approximately 130 °C, which is consistent with seismic constraints [Hooft et al 2003;Villagomez et al 2007]. The plume is centered at a location corresponding to just off the southeast shore of Fernandina at present day [Wilson and Hey, 1995]. Horizontal flow conditions on the top and bottom boundaries simulate the ridge geometry and absolute plate motions.…”

“…The model boundary conditions simulate a realistic GSC spreading rate (28 km/Myr), absolute plate motion [Gripp and Gordon, 1990], and ridge geometry as it evolved in time since 6.3 Ma [Wilson and Hey, 1995]. Potential temperature is 0 °C at the surface and 1,300 °C at the base, except in a circular patch where the plume is imposed.…”

“…Horizontal flow conditions on the top and bottom boundaries simulate the ridge geometry and absolute plate motions. Timeintegration spans 12 Myr of total model time with the first 5.7 Myr needed to allow the plume to rise and spread beneath the lithosphere, and the final 6.3 Myr simulating the realistic evolution of the ridge and motion of the plates over the hotspot [Wilson and Hey, 1995]. The last time step represents the present-day configuration and activity from which the solutions of observables are produced.…”

Subduction dynamics at the middle America trench : new constraints from swath bathymetry, multichannel seismic data, and 10BE

“…Rica. The magnetic anomaly data are compiled after Barckhausen et al (2001), Wilson (1996, Wilson and Hey (1995), Hardy (1991), and Lonsdale and Klitgord (1978). CNS-l, separated from CNS-2 by a ridge jump (Barckhausen et al, 2001).…”

Evolution of the Carlsberg Ridge between 60 and 45 Ma: Ridge propagation, spreading asymmetry, and the Deccan‐Reunion hotspot

“…This "moderate" distance seems to depend on the "strength" of the hotspot. Examples of major ridge jumps in relation to a hotspot include the already mentioned extinction of the Mascarene Basin spreading center and the opening of the Carlsberg Ridge in response to the inset of the Deccan-Reunion hotspot, which can be envisioned as a 1000 to 1500 km ridge jump; the progressive extinction of the Labrador Basin spreading center and the opening of the Reykjanes Ridge between Greenland and Europe [e.g., Srivastava, 1978] [Royer, 1985;Small, 1995]; and on the Coco-Nazca spreading center near the Galapagos hotspot [Wilson and Hey, 1995].…”