Day 11: Anomalies Abound

Something very exciting is happening....we are crossing the boundary between the Cocos and Nazca plates (see image below), which is a deep zone at the Panama transform fault, a very active active earthquake zone.  This marks the end of our transit and the beginning of the marine geological survey that we set out to do.  It all began with the Farallon plate, which underwent its final episode of plate fragmentation in the Miocene, when the Cocos plate split off and the remainder of the Farallon plate became the Nazca.  The Cocos plate is spreading in a northeasterly direction, while the Nazca plate follows a easterly course; this spreading began at 23 Ma.  A small portion of the northern rifted margin of the Farallon plate exists today, but is being subducted at about 50 km/m.y.

Beyond that, we were studying a map of magnetic anomalies earlier today, with ship tracks (and their magnetic records) from the 1950's to the present.  On this cruise, we have a a magnetometer that trails 700 m behind the ship and records magnetic anomalies.  This record is preserved in the crust at the time of formation and can be used to date it.  We mostly view it in the context of a spreading axis, where parallel and symmetric (reflected) bands of known magnetic anomalies form, though they are often shifted and offset by plate movement thereafter.  As we approach the equator, the local magnetics reverse, resulting in positive anomalies being recorded as negative.  And as of late, the magnetic readings have been all over the place because of the Cocos Ridge....more on that later...

~Jasmeet