Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones

Abstract

[1] Mantle-derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid-Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep-seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1°+0.8°/ À1.4°) that is shallower than either the expected geocentric axial dipole inclination (40.7°) or present-day inclination (41.9°) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (>350°C) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5°) more closely approximates the time-averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350°C. Accounting for the anisotropyrelated deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional-scale fault zones on the western median valley wall.

Slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area

“…Taken together, these lines of evidence suggest that these gabbroic rocks experienced little tilt after cooling to temperatures of $500°C. We note that a similar conclusion (for temperatures less than $350°C) has been reached for the serpentinites exposed $20 km south of the gabbroic exposures studied here [Lawrence et al, 2002]. As suggested by these authors, slip along a relatively low-angle fault and subsequent differential motion on vertical faults across the massif may account for the relative uplift of the MARK area gabbros with relatively little rotation.…”

Source of Oceanic Magnetic Anomalies and the Geomagnetic Polarity Timescale

“…Temperature estimates for serpentinization based on oxygen isotope fractionation between magnetite and serpentine vary widely, though most serpentinites from Hess Deep and the MARK area yield temperatures conservatively estimated as >350 C (Agrinier and Cannat, 1997;Früh-Green et al, 1996). Based on these relatively high temperatures and the simple demagnetization behavior of MARK area peridotites, Lawrence et al (2002) suggested that much of remanence might be thermal in origin. While coarse-grained magnetite is abundant, values of M rs /M s as high as 0.4, as well as direct grain size estimates from image analysis indicate that relatively fine-grain magnetite is also present (Lawrence et al, 2002;Oufi et al, 2002).…”

“…Based on these relatively high temperatures and the simple demagnetization behavior of MARK area peridotites, Lawrence et al (2002) suggested that much of remanence might be thermal in origin. While coarse-grained magnetite is abundant, values of M rs /M s as high as 0.4, as well as direct grain size estimates from image analysis indicate that relatively fine-grain magnetite is also present (Lawrence et al, 2002;Oufi et al, 2002). These finer particles are presumably responsible for the relatively stable remanence in the MARK area peridotites, which was used to infer that little tilting occurred during uplift and exposure (Lawrence et al, 2002).…”

Anisotropy of (partial) isothermal remanent magnetization: DC-field-dependence and additivity

“…More recent separation methods exploit the different field-and temperature-dependencies of paramagnetic and ferromagnetic properties (Martín-Hernández and Ferré, 2007). Numerous studies have used combinations of AMS, AARM, AIRM and anisotropy of thermal remanence (ATRM) to better understand the carriers of magnetic fabrics, in aid of their interpretation (Tema, 2009, Raposo et al, 2004, Bilardello and Jackson, 2014, Agro et al, 2017, Lycka, 2017, Selkin et al, 2000, Biedermann et al, 2016, Borradaile and Dehls, 1993, Lawrence et al, 2002.…”