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doi:10.2204/iodp.proc.344.206.2015

Introduction

Magnetic properties of sediments and rocks can be used to identify magnetic carriers and evaluate the fidelity of the paleomagnetic record. This information can be used to determine whether the rocks and sediments have been deformed or chemically altered and whether they are appropriate for determining the magnetostratigraphy of a site.

This pilot study presents the rock magnetic properties of representative basalt, sediment, and tephra samples collected from the Costa Rica subduction margin off Osa Peninsula during Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344 (Expedition 334 Scientists, 2012a; see the “Expedition 344 summary” chapter [Harris et al., 2013a]). Measurements include magnetic susceptibility, hysteresis, and first-order reversal curves (FORCs). In addition, the natural remanent magnetization (NRM) of a few basalt samples was also measured to investigate the demagnetization behavior, which provides further insights into the rock magnetic properties of the samples as well as information about the reliability of the paleomagnetic signal for tectonic studies that are being conducted on the Expedition 334 and 344 cores.

A goal of both expeditions, which are part of the Costa Rica Seismogenesis Project (CRISP), was to understand the processes that control fault nucleation and seismic rupture for large earthquakes that occur along this convergent margin (Expedition 334 Scientists, 2012a; see the “Expedition 344 summary” chapter [Harris et al., 2013a]). This requires characterization of the lithology, including chemical, physical, and magnetic properties, and the stress, hydrologic, and thermal state of the upper plate. Rock magnetic studies provide part of the characterization by constraining compositional components and yielding information about alteration and magnetic remanence history. The latter is important for chronostratigraphic analysis (e.g., magnetostratigraphy) and for assessing past deformation, such as tilting and vertical-axis rotations.

We analyzed samples that come from both sides of the subduction zone, with sites on the subducting Cocos plate and the overriding Caribbean plate (Fig. F1). Sites U1381 and U1414 are located on the Cocos plate, over crust formed at the Cocos-Nazca spreading center (Barckhausen et al., 2001), and serve as reference sites for the material entering the subduction zone. Site U1381 is located at 2065 m below seafloor (mbsf) on the top of the Cocos Ridge, which was created when Galapagos magmatism intruded the Cocos plate (Walther, 2003). Site U1414 is located at 2459 mbsf on the northern flank of the Cocos Ridge, in what could be described as Cocos plate transitional crust. Sites U1380 (503 mbsf) and U1413 (540 mbsf) are located on the middle and upper slope of the Caribbean plate, respectively, and record the subsidence and erosion history of the margin (Ranero and Von Huene, 2000; Vannucchi et al., 2001).

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