Magnetic anisotropy and correction of paleomagnetic inclination shallowing in deep-sea sediments.

摘要

The inclination {dollar}(Isb{lcub}obs{rcub}){dollar} of natural remanent magnetization (NRM) in sediment may be shallower than the inclination {dollar}(Isb{lcub}H{rcub}){dollar} of the Earth's field in which it is acquired. This thesis investigates whether such inclination shallowing can be detected and corrected for by measuring the magnetic anisotropy of the sediment. Paleomagnetism and magnetic anisotropy were measured for 79 turbidite specimens from 3 cores on the Scotian Rise, 87 pelagic clay specimens from DSDP site 578, and 90 pelagic lime-mud specimens from DSDP site 606.; For the turbidites, {dollar}Isb{lcub}obs{rcub}{dollar} is on average 12{dollar}spcirc{dollar} shallower than the 61{dollar}spcirc{dollar} expected from the geocentric axial dipole (GAD) model. The turbidites are also highly anisotropic with the ratio of anhysteretic remanence (ARM) along the hard axis {dollar}(ARMsb{lcub}min{rcub}){dollar} to that along the easy axis {dollar}(ARMsb{lcub}max{rcub}){dollar} averaging 0.82. A significant correlation between tan {dollar}Isb{lcub}obs{rcub}{dollar} and {dollar}ARMsb{lcub}min{rcub}/ARMsb{lcub}max{rcub}{dollar} exists as Hodych and Bijaksana (1993) showed was expected from the theory of Jackson et al. (1991). The correlation line's estimate of tan {dollar}Isb{lcub}obs{rcub}{dollar} when {dollar}ARMsb{lcub}min{rcub}/ARMsb{lcub}max{rcub}=1{dollar} predicts {dollar}Isb{lcub}H{rcub}=69spcirc{dollar} (+5{dollar}spcirc{dollar}/{dollar}-{dollar}10{dollar}spcirc ){dollar} with 95% confidence agreeing with the expected 61{dollar}spcirc .{dollar}; The turbidites show a significant correlation between tan {dollar}Isb{lcub}obs{rcub}{dollar} and the degree of compaction {dollar}Delta V.{dollar} The correlation line's estimate of tan {dollar}Isb{lcub}obs{rcub}{dollar} when {dollar}Delta V=0{dollar} predicts {dollar}Isb{lcub}H{rcub}=71spcirc{dollar} (+6{dollar}spcirc{dollar}/{dollar}-{dollar}12{dollar}spcirc ){dollar} agreeing with the 61{dollar}spcirc{dollar} and suggesting that the inclination shallowing is compaction-induced. A significant correlation between {dollar}ARMsb{lcub}min{rcub}/ARMsb{lcub}max{rcub}{dollar} and {dollar}Delta V{dollar} suggests the ARM anisotropy is also compaction-induced. Compaction experiments on a few specimens further support a compaction origin for the ARM anisotropy and the inclination shallowing.; For the turbidites, the susceptibility anisotropy was similar to but smaller than the ARM anisotropy. The ratio of susceptibility along the hard axis {dollar}chisb{lcub}min{rcub}){dollar} to susceptibility along the easy axis {dollar}(chisb{lcub}max{rcub}){dollar} averaged 0.90. A significant correlation was found between {dollar}chisb{lcub}min{rcub}/chisb{lcub}max{rcub}{dollar} and {dollar}ARMsb{lcub}min{rcub}/ARMsb{lcub}max{rcub}{dollar} and hence between tan {dollar}Isb{lcub}obs{rcub}{dollar} and {dollar}chisb{lcub}min{rcub}/chisb{lcub}max{rcub}.{dollar} The latter correlation line's estimate of tan {dollar}Isb{lcub}obs{rcub}{dollar} when {dollar}chisb{lcub}min{rcub}/chisb{lcub}max{rcub}=1{dollar} predicts {dollar}Isb{lcub}H{rcub}=67spcirc{dollar} (+6{dollar}spcirc{dollar}/{dollar}-{dollar}13{dollar}spcirc ).{dollar} This prediction is as successful as that using {dollar}ARMsb{lcub}min{rcub}/ARMsb{lcub}max{rcub}{dollar} and is much faster to measure.; An attempt was made to study the pelagic clays and pelagic lime-muds in a similar fashion. However, inclination shallowing only averaged 1{dollar}spcirc{dollar} for the pelagic clays and 4{dollar}spcirc{dollar} for the pelagic lime-muds and ARM anisotropy was also very low. The low inclination shallowing and anisotropy in these pelagic sediments are probably due to relatively low compaction.