Fabrics and mechanics of forearc deformation at the Costa Rican erosive convergent margin and implications for seismogenic fault zone behavior - Follow-up of IODP expedition 334 (Costa Rica Seismogenesis Project, CRISP)
Final Report Abstract
The Costa Rica Seismogenesis Project (CRISP) is the first drilling program established to investigate the genesis of megathrust earthquakes and related tsunamis at an erosive active continental margin. The localization of major plate boundary faults within frictionally unstable material is precondition for earthquake nucleation and propagation. Hence, the material properties of sediments and rocks at or close to the plate boundary are crucial for the development and activity of subduction zone megathrust faults. Surface rupturing offsetting the seafloor through plate boundary or overriding plate faulting is also controlled by the overlying rocks and sediments. To test material properties, drill cores from the International Ocean Discovery Program expeditions 334 and 344 of CRISP have been studied mechanically by ring shear and triaxial testing as well as by composition and microfabric analysis. Dynamic and static friction of gouge-material was tested using a ring shear deformation apparatus. The only sediment type that produced frictional instabilities (i.e. laboratory earthquakes) was the calcareous ooze carried by the incoming Cocos Plate, which by virtue of its slip weakening behavior is also a likely candidate for triggering slow slip events. In contrast to the silty clay sediments, calcareous ooze with high calcite contents (> 80 vol.-%) shows a dramatic change in frictional behavior as a function of temperature and pore fluid pressure. This material is frictionally strong at room temperature with no significant dependency on effective normal stress or sliding velocity. At 140 °C, however, it exhibits unstable slip, i.e. “stick-slip” events. Dramatic weakening is linked to increasing pore fluid pressures. The internal friction coefficient drops below ~0.6 at Pf = 60 MPa and to 0.27 at Pf = 120 MPa, when calcareous ooze becomes clearly weaker than hemipelagic clayey sediment. The combined effects of thermally induced frictional instability and fluid pressure related weakening of calcareous sediments could be a fundamental mechanism of earthquake nucleation on subduction zone megathrusts at low latitude, where carbonates make up to >90% of marine sediment. Transitions between different fault slip modes, such as aseismic fault slip, shallow slow earthquakes and the emergence of plate boundary ruptures, i.e. the generation of regular earthquakes at the upper limit of the seismogenic zone, can be explained by the presence of these calcareous sediments in the subduction channel, and their variability in mechanical behavior during progressive deformation. Triaxial deformation experiments were carried out on samples from the incoming plate as well as from the lower, middle and upper slope of the overriding plate to test fracture/creep strength and overall deformation. Despite the compositional variability the overriding plate sediments off Costa Rica display remarkably similar deformation behavior, consolidation state, cohesion, internal friction angles and low peak strength. This contrasts to, for example, the situation at the Nankai accretionary margin where large differences between samples from the lower and the upper slope have been observed. The reason why the mechanical properties differ considerably between these two active continental margins is not yet understood. Interestingly for the incoming oceanic plate, also the triaxial experiments show a significant variation between the calcareous ooze and the silty clay sample. Peak stresses of calcareous ooze are higher, cohesion is lower and the internal friction coefficient of 0.72 is the highest value of the entire data set corresponding to what has been found at room temperature and low pore pressure in the ring shear tests and by that confirming these results. Crystallographic preferred orientation (texture) analysis using synchrotron radiation displays pronounced phyllosilicate alignment, especially for middle slope samples. The crystal basal planes are preferentially parallel to the (sub)horizontal bedding. Predominant smectite, however, is less well aligned in small-circle distributions at a low angle to the drill core axis normal to the sea floor. The weak phyllosilicate textures from the frontal prism are ~30-40° inclined with respect to a horizontal orientation and hence significantly steeper than any slope suggesting tectonic tilting after sedimentation. Incoming plate samples display (sub)horizontal kaolinite bedding plane orientations whereas smectite deviates from this alignment. Causes for the general misalignment of smectite require further investigation. Overall, the samples from different drill cores across the Costa Rica erosive continental margin are only weakly preferentially oriented. Ongoing compaction during burial and diagenesis has not led to a progressive texture strengthening with depth, probably due to rapid sedimentation and effects of smectite on texture formation. The differences in smectite content and texture compared to data from the Nankai accretionary margin might correspond to differences in the mechanical behavior at the two continental margins. Further investigations need to elucidate differing interrelationships between deformation, texture formation, composition and tectonic setting.
Publications
- IODP Expedition 334: An investigation of the sedimentary record, fluid flow and state of stress on top of the seismogenic zone of an erosive subduction margin. Scientific Drilling, vol.15. 2013, pp. 23-30.
Vannucchi, P., Ujiie, K., Stroncik, N., et al.
- Earthquake nucleation in weak subducted carbonates.
Nature Geoscience, Vol. 9. 2016, pp. 717–722.
Kurzawski, R.M., Stipp, M., Niemeijer, A.R., Spiers, C.J., Behrmann, J.H.
(See online at https://doi.org/10.1038/NGEO2774) - Frictional properties of subduction input sediments at an erosive convergent continental margin and related controls on décollement slip modes - the Costa Rica Seismogenesis Project (CRISP). Journal of Geophysical Research: Solid Earth, Vol. 123. 2018, Issue 10, pp.8385-8408.
Kurzawski, R.M., Niemeijer, A.R., Stipp, M., Charpentier, D., Behrmann, J.H., Spiers, C.J.
(See online at https://doi.org/10.1029/2017JB015398)
