Milankovich Sea Level-Change Pumping of Fault Slip May Enhance Abyssal Hill Growth, with Spacing Control by Melt Pumping or Elastic Properties

Abstract:

Sea level-change pumping of mantle melt to create abyssal hills is a very innovative and elegant hypothesis. However, I see 2 issues: 1. Abyssal hill observations indicate that they are bounded by normal faults in most cases. 2. The resolution of the crustal accretion “tape recorder” and its ability to accurately record cycles as short as 20Kyr and 40Kyr is limited given the width of the zone of crustal accretion. Magnetic anomaly polarity transition widths give us one measure of estimating the resolution of this tape recorder. Based on these measurements, the half-width of the zone of crustal accretion ranges from 1-8 km, with 2-4 km being typical. At a half-spreading rate of 3 cm/yr, a width of 3km of crust is created during the 100Kyr Milankovich cycle, ~1.2 km during the 40Kyr cycle, and 600m during the 20Kyr cycle, so only the 100K signal is likely to be resolvable. In contrast, consider the effect of pressure changes on fault slip. In critically stressed lithosphere, a small change in pressure can produce nearly instantaneous slip. For example, pressure changes of only ~0.1 MPa near injection wells in OK, TX trigger earthquakes at depths of 3-5 km (Mark Zoback, personal comm.). This pressure change is equivalent to a 100 m sea level change. For the 20Kyr and 40Kyr cycles, sea level-change pumping of fault slip may dominate because fault uplift of abyssal hills is not smeared by the time-averaging effects of crustal accretion. The spacing of the abyssal hills in this scenario would be controlled by the elastic thickness and flexural rigidity of young lithosphere. Faulting also can accommodate variations in melt supply, with most of the melt freezing at depth; this would explain why we observe dominantly tectonic rather than constructional volcanic origins of abyssal hills. If abyssal hills are caused by variations in melt production, the crust should be measurably thicker beneath the axes of the hills. If faulting dominates, crustal thickness would be more uniform. This would be a key test.