A detailed heat flow, topographic, and magnetic survey across the Galapagos Spreading Center at 86°W

Abstract

The Galapagos spreading center connects the triple junction at the equator in the eastern Pacific to the Panama fracture zone. The spreading center passes north of the Galapagos Islands and into the Panama basin between the Cocos and Carnegie ridges. Between 88°W and 85°30′W symmetric high-amplitude magnetic anomalies out to anomaly 2′ (3 m.y.), giving a uniform rate of 3.25 cm/yr (32.5 mm/yr), have been observed over the crest of the spreading center. A large short-wavelength negative within the central anomaly and associated with the sharp bathymetric peak at the axis is used to determine the magnetization within the central anomaly. A uniform magnetization of 0.025 emu/cm3 (25 A/m) was used for the central block to match the anomaly associated with the bathymetric peak. An intensity of 0.0125 emu/cm3 (12.5 A/m) elsewhere and a block thickness of 0.5 km give the best match to the rest of the magnetic profiles close to 86°W. To the south of the spreading center anomaly 2′ abuts the Carnegie ridge, whereas to the north there is a change in character of the anomalies at anomaly 2′. Then, further to the north, anomalies 3 (4 m.y. B.P.) through the beginning of 5 (9 m.y. B.P.) terminate at the base of the Cocos ridge. These differences in age are evidence that the Carnegie and Cocos ridges may be tectonically unrelated. Fifty-seven heat flow measurements are presented. Thirty-eight of these were taken during a detailed survey on either side of the Galapagos spreading center. In the survey area the heat flow is high (4.08 ± 2.40 μcal/cm2 sec (171 ± 101 mw/m2) but variable. Temperature versus depth profiles in the bottom water taken during the heat flow stations show a 0.04°C increase in potential temperature over the spreading center. Hydrothermal circulation is invoked to account for the scatter in the heat flow data. In the simple plate model, vertical intrusion at a uniform temperature is assumed at the ridge crest. We show that loss of heat by hydrothermal circulation can explain the difference between theoretical and observed heat flow and that this loss of heat can be hidden in the thermal anomaly in the bottom water over the spreading center. This process not only removes the singularity in the heat flow in the original model but also flattens the temperatures near the origin. A much-improved fit to the uniform increase of depth with age of the oceanic crust observed in the North Pacific results.