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.
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.
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].
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  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.
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.