Carol A. Stein- TicoFlux Research

My co-Principal Investigators (Andy Fisher and Eli Silver from the University of California at Santa Cruz, Rob Harris from the University of Utah, Geoff Wheat from the University of Alaska, Fairbanks, and Kelin Wang from Pacific Geoscience Centre, Geological Survey of Canada) and I gathered data during two cruises on the R/V Maurice Ewing (2001) and the R/V Melville (2002) to examine the thermal state of the oceanic crust offshore of the Nicoya Peninsula, Costa Rica.

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Participants from TicoFlux1 (aboard the R/V Maurice Ewing 2001; left) and TicoFlux2 (aboard the R/V Melville 2002; right).

The 18-24 million year old lithosphere on the Cocos Plate near the Nicoya Peninsula, Costa Rica has significantly variable heat flow. Seafloor formed at the East Pacific Rise (EPR) has heat flow ~70% less than expected from conductive lithospheric cooling models but also ~50% of average measured values for this age. In contrast, heat flow through seafloor formed at the Cocos-Nazca Spreading Center (CNS) is consistent with conductive lithospheric models but ~50% higher than average values for this age [Fisher et al., 2003]. To explore this issue 360 new heat flow measurements, seismic reflection, and sediment cores were taken during two cruises in the "TicoFlux" study on the R/V Maurice Ewing and R/V Melville.

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TicoFlux study area with heat flow measurements, bathymetry, and tectonic boundaries. For its crustal age, heat flow on the EPR-generated crust (NW region) is unusually low but that on the CNS-generated crust (SW region) is unusually high. The thermal boundary near the trench is located near the fracture zone trace separating CNS and EPR crust, but occurs closer to the seamounts on EPR crust farther away from the trench [after Hutnak et al., in press].

The closely-spaced heat flow measurements show that the transition, from anomalously low heat flow on the Cocos plate formed at the East Pacific Rise to unusually high heat flow on the lithosphere formed at the Cocos-Nazca spreading ridge, is quite abrupt (over a 2-5 km distance) and appears to be controlled by the location of outcrops and topographic highs. Within the colder area heat flow measurements indicate substantial recharge and discharge of fluids at basement highs. The modeling results (below) suggest this abrupt change results from a shallow (<600 m) hydrothermal origin, rather than an overall change in the plate's thermal structure [Fisher et al., 2003].

[Image] Conceptualization, heat flow data, and thermal models for thermal transition in the TicoFlux area. (top) Cartoons show the expected variation in heat flow depending on a deep (left) or shallow mechanism (right). (bottom) Heat flow measurements show an abrupt increase, consistent with models of shallow (<600 m) hydrothermal circulation. Location of this profile is shown on the above map.

The thermal state of the incoming crust controls the updip limit of the seismogenic zone of the Middle America Trench near the Nicoya Peninsula. The updip limit for the low heat flow EPR-generated subducted crust is deeper than for the high heat flow CNS-generated subducted crust [Newman et al., 2002] as expected if approximately the same temperature is required. Thermal modeling [Harris and Wang, 2002] shows how the thermal structure and temperatures along the main subduction thrust are affected assuming different depths of hydrothermal cooling in the incoming plate. Comparisons with Newman et al.'s [2002] hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100-150 degrees C, but there are large along-strike variations in both the seismicity and thermal regime.

References:

Fisher, A. T., C. A. Stein, R. N. Harris, K. Wang, E. A. Silver, M. Pfender, M. Hutnak, A. Cherkaoui, R. Bodzin, and H. Villinger, Abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the Cocos Plate, Geophys. Res. Lett., 30, doi10.1029/2002GL016766, 2003. For pdf click here

Harris, R. N., and K. Wang, Thermal models of the Middle America Trench at the Nicoya Peninsula, Costa Rica, Geophys. Res. Lett., 29, doi:10.1029/2002GL015406, 2002.

Hutnak, M., A. T. Fisher, C. A. Stein, R. Harris, K. Wang, E. Silver, G. Spinelli, M. Pfender, H. Villinger, P. C. Pisani, H. Deshon, B. MacKnight, The thermal state of 18-24 Ma upper lithosphere subducting below the Nicoya Peninsula, northern Costa Rica margin, SEIZE Margin volume, in press.

Newman, A.V., S. Y. Schwartz, V. Gonzalez, H. R. DeShon, J. M. Protti, and L. M. Dorman, Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica, Geophys. Res. Lett., 29, (20), DOI 10.1029/2002GL015409, 2002.

This material is based upon work supported by the National Science Foundation under Grant No. 0001941. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.