Publication Date:
2022-05-25
Description:
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 6 (2005): Q11009, doi:10.1029/2005GC001026.
Description:
The northern scarp of the western Blanco Transform (BT) fault zone provides a "tectonic
window" into crust generated at an intermediate-rate spreading center, exposing a ~2000
m vertical section of lavas and dikes. The lava unit was sampled by submersible during
the Blancovin dive program in 1995, recovering a total of 61 samples over vertical
distances of ~1000 m and a lateral extent of ~13 km. Major elements analyses of 40
whole rock samples exhibit typical tholeiitic fractionation trends of increasing FeO*,
Na2O, and TiO2 and decreasing Al2O3 and CaO with decreasing MgO. The lava suite
shows a considerable range in extent of crystallization, including primitive samples (Mg#
64) and evolved FeTi basalts (FeO〉12%;TiO2〉2%). Based on rare earth element and
trace element data, all of the lavas are incompatible-element depleted normal mid-ocean
ridge basalts (N-MORB;La/SmN〈1). The geochemical systematics suggest that the
lavas were derived from a slightly heterogeneous mantle source, and crystallization
occurred in a magmatic regime of relatively low magma flux and/or high cooling rate,
consistent with magmatic processes occurring along the present-day southern Cleft
Segment. The BT scarp reveals the oceanic crust in two-dimensional space, allowing us
to explore temporal and spatial relationships in the horizontal and vertical directions. As a
whole, the data do not appear to form regular spatial trends; rather, primitive lavas tend to
cluster shallower and toward the center of the study area, while more evolved lavas are present deeper and toward the west and east. Considered within a model for construction
of the upper crust, these findings suggest that the upper lavas along the BT scarp may
have been emplaced off-axis, either by extensive off-axis flow or off-axis eruption, while
the lower lavas represent axial flows that have subsided with time. A calculation based
on an isochron model for construction of the upper crust suggests that the Cleft Segment
requires at least ~50 ka to build the lower extrusive section, consistent to first order with
independent estimates for the construction of intermediate-spreading rate crust.
Description:
This work was supported by the US National Science Foundation (OCE 02-
22154 to E.K. and J.K. and OCE 9400623 to M.T.).
Keywords:
Accretion
;
Isochron
;
MORB
Repository Name:
Woods Hole Open Access Server
Type:
Article
Format:
1193749 bytes
Format:
application/pdf
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