The Influence of the Panama Fracture Zone on Arc Magmatism

Over the past 25 Ma the Panama magmatic arc has undergone multiple changes in tectonic regime, each of which has produced recognizable events in the character and chemistry of the volcanic arc. At 23 Ma, the Cocos and Nazca plates split from the Farallon with the formation of spreading ridge CNS-1. This plate reconfiguration, along with the subsequent formation of ridges CNS-2 (19.5Ma) and CNS-3 (14.7Ma) caused incremental counter-clockwise northward rotation of the Cocos plate motion vector, and led subduction to become near margin perpendicular. During this time interval standard arc magmatism in Panama was dominant and was similar to Miocene magmatism in Nicaragua.

At 9.5 Ma the Panama Fracture Zone became active and NE corner of the Cocos plate was captured by the Nazca plate. This event is associated with ≈90^o rotation of the outboard plate motion vector offshore of Panama. Outboard plate motion changed from near perpendicular to near margin parallel. At the same time the arc experienced greatly increased silicic magmatic activity.

The ≈10 Ma silicic volcanic event is observed at multiple volcanoes including El Valle, La Yeguada and possibly in the Talamanca Range of southern Costa Rica. This event is characterized by SiO₂ values up to 75%, K₂0 > 3%, Mg#(mol) < 30, increased Ba/La (≈80), Sr/Y (≈50) and La/Yb (≈10) ratios. The rocks are also peraluminous with A/CNK ratios of 1.0-1.44, which is suggestive of crustal assimilation.

The 10 Ma event exhibits similarities with younger (<3Ma) adakitic rocks that occur after Cocos ridge began to subduct, but lack the extreme Sr/Y and La/Yb ratios and therefore must have formed by a different process.

One potential explanation of the 10 Ma tectonic and magmatic events is that they mark the initiation of the collision between South America and Panama.