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doi:10.2204/iodp.proc.314315316.216.2012

Methods

Sample reorientation

Samples were azimuthally reoriented through stepwise demagnetization of the viscous remanent magnetization (VRM) acquired in an ambient magnetic field. Demagnetization was conducted at the École Normale Supérieure de Paris (France) for samples from Sites C0001, C0002, C0007, and C0008 and at CEREGE in Aix-en-Provence (France) for samples from Sites C0004 and C0006. Both facilities use a superconducting quantum interference device (SQUID) magnetometer with horizontal sensors, and the measurement protocol consists of demagnetization increments of 2–10 mT, up to 60 mT. The resulting data were processed using Zijderveld diagrams (Zijderveld, 1967). In most cases, samples exhibit low-coercivity magnetization between 0 and 10 mT with a subvertical slope, attributed to the rotary motion of the coring device, and medium-coercivity magnetization between 10 (or 20) and 40 mT with a mean inclination near 33°, which was used for reorientation.

Anisotropy of magnetic susceptibility

The AMS was measured with a susceptibility bridge AGICO KLY3S, which has a sensitivity of 3 × 10^–8 SI. The system measures the induced magnetization (M) of a specimen inside a coil that generates a low alternating field (H). If the magnitude of the magnetization field is on the same order as that of the Earth’s magnetic field, then M and H are approximately linearly related according to M_i = K_ij × H_j, where K_ij is a second-rank symmetric tensor representing the magnetic susceptibility (Nye, 1957; Daly, 1970). The eigenvalues of the magnetic susceptibility tensor (K_ij) are denoted by K₁, K₂, and K₃, with K₁ > K₂ > K₃. The anisotropy and shape of the susceptibility ellipsoid are quantified using the parameters P_j and T_j (Jelinek, 1981). Their mathematical expressions are as follows:

and

,

where K_m is the mean magnetic susceptibility. The anisotropy parameter (P_j) is typically a measure of the eccentricity of the AMS ellipsoid, and the shape parameter (T_j) is negative (–1 < T_j < 0) when the fabric is linear (or prolate) and positive (0 < T_j < 1) when the fabric is planar (or oblate) (Jelinek, 1981; Borradaile and Henry, 1997).

Anisotropy of P-wave velocity

Ultrasonic P-wave velocities were measured with a pulse generator Panametrics 5058 PR with up to 900 V output voltage, two P-wave transducers with a resonance frequency of 0.5 MHz, and a digital oscilloscope HP54603B connected to a PC for data collection. For every sample, the thickness across each of the seven sets of parallel facets was measured, and then the P-wave time-of-flight across each length was determined by picking the first arrival on the oscilloscope. Once the P-wave velocities were calculated, a best fitting second-order symmetric tensor was retrieved in the same fashion as for magnetic susceptibility, following the method proposed by Louis et al. (2004) and based on the assumption of elliptical anisotropy (Thomsen, 1986; Tsvankin, 1997). Because the inversion yields three principal directions and values, the same anisotropy and shape parameters as those used for the AMS data (i.e., P_j and T_j) were calculated, providing a direct and simple way of comparing both data sets. The eigenvalues of the P-wave velocity pseudotensor are denoted by V₁, V₂, and V₃, with V₁ > V₂ > V₃.

The results of the magnetic susceptibility and P-wave velocity measurements are reported in Table T1 as the mean, anisotropy parameter P_j, and shape parameter T_j for each property.

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