Spatial and temporal variations in the petrology, morphology and tectonics of a migrating spreading center: The Endeavour Segment, Juan de Fuca Ridge

The tectonic and volcanic consequences of ridge crest migration have been investigated at the Juan de Fuca Ridge, which migrates northwestward over the mesosphere rapidly compared with its spreading rate. The detailed morphology and neo-tectonics of the Endeavour Segment have been analyzed using Seabeam bathymetry, Sea MARC II images, magnetics data, seismic reflection records and deep-tow photographs. The ridge axis, a narrow ({dollar}<{dollar}1 km) rift valley, is segmented at a small (15-20 km) scale, and incised into a variety of structures along-strike. Lavas recovered from the region have been analysed with electron microprobe and X-ray fluorescence techniques. Samples from the axial valley and flanking ridges have enriched incompatible element signatures, whereas depleted n-MORB's comprise the adjacent seamounts.; Morphological variations are interpreted to reflect spatial and temporal variations in magma supply. The elongate, shallow Endeavour Ridge, built during a high magma supply period, is segmented. Magmas delivered to each segment appear to have been derived from an axially-discontinuous or poorly mixed magma chamber which has been resupplied from the mantle through a de-centralized geometry. The chaotic, deeper North and South Endeavour Valleys and West Valley result from lower over magma supply. Flanking ridges parallel to the axis show progressive lengthening to the south, indicating temporal variations in the amplitude of magma supply.; Ridge axis and seamount lavas can be derived by melting a source which varies between a depleted and enriched endmember, with superimposed fractional crystallization. The degree of enrichment has increased over the 2 million year period observed. Local trace element patterns require significant small scale heterogeneity in the mantle.; Spatial and temporal variations observed are explained with a model in which the Endeavour Segment axis has migrated toward and overridden the Heckle melting anomaly. Off-axis seamount volcanism results from preferential melting within heterogeneities embedded in the mantle ascending in advance of the migrating ridge axis. Eventually, melting anomaly products become incorporated in the ridge axis magmatic system, dominating the geochemical signature of ridge axis lavas. Corresponding increases in magma supply at the axis have triggered reversals in rift propagation direction at the Cobb Offset.