End of Life Issues in a Back Arc
Abstract
Backarc basins have a finite, established life cycle of about 15 Ma. This differentiates them from mid-ocean ridges, which may spread in a steady state for hundreds of millions of years. In the Parece Vela Rift, the end of volcanism in the final few Ma of active rifting can be traced over several ridge segments. In most of the ridge segments of the PVR, volcanic spreading died at ~8 Ma and continued amagmatically. This is documented by numerous bathymetric surveys and dredge observations showing that on those ridge segments, ultramafic rocks are exposed on so-called "smooth crust" (Michael, et al., 2003; Dick et al., 2003; Cannat et al. 2006) over the entire length of the ridge segment. On other segments however, volcanism, or a mixture of active volcanism persisted in time all the way until the end of spreading at ~7 Ma. This is documented by dredging of abundant eruptive, hypabyssal and plutonic igneous rocks in the rift valley of these segments. Interestingly, the magmatically most robust ridge segments in the PVR at any given time are the ones that contain core complexes. The most spectacular of these is the Godzilla Mullion, the world's largest oceanic core complex, which occurs at about the center of the PVR. This ridge segment appears to encompass two active thermal cells (as seen in off axis bathymetry before the breakaway) and which produced 125x75km of striated core complex surface, about 2/3 the size of the state of Delaware. Oceanic core complexes are often seens as evidence of reduced magmatic activity. This is because they often occur along otherwise magmatically robust mid-ocean ridges. In this case, on an ultraslow, highly oblique, dying backarc basin, the core complexes are the last remnant of magmatic activity and are thus the most productive segments of the rift at the end of spreading. This is documented not only by the volcanic products dredged from the paleo-rift, but also from the intense hybridization of mantle rocks with MORB-type melt that occurred in the last portion of active spreading.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T54A..08S
- Keywords:
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- 3621 MINERALOGY AND PETROLOGY / Mantle processes;
- 8170 TECTONOPHYSICS / Subduction zone processes;
- 8416 VOLCANOLOGY / Mid-oceanic ridge processes