TRANSTENSIONAL DEFORMATION ALONG THE GAILLARD CUT, PANAMA CANAL

Recent field work in the Panama Canal, Gaillard Cut has revealed numerous small and several large left-lateral normal oblique faults with ≈500 meters of vertical offset and ≈2 km of left lateral offset. These faults cut sedimentary and volcanic units indicating significant post-Miocene trans-tensional deformation. Two large left-lateral normal oblique faults are well exposed along the canal and dip 60 degrees to the southeast . The footwall in the northern fault is generally undeformed, but the southern one has some normal faults that are syn- and antithetic to the main fault. Due to the greater density of normal faults in the hanging wall, we propose that these faults become listric at depth. Rough calculations of depth to detachment were performed on a cross section along the west bank of the Panama Canal from Gamboa to just north of Puente Centenario. Using the balanced bed length method, we calculated the detachment to be shallow at about 800 meters. Such a shallow depth to detachment is inconsistent with well documented studies of listric normal faults which generally detach at the brittle-ductile transition. However, there are substantial rheological differences between the welded agglomerates of the Bas Obispo Formation and the overlying ignimbrites and volcaniclastics of the Las Cascadas Formation which may accomodate detachment. It is proposed here that extension occurred in the canal zone along a NW/SE trend with listric faulting on faults dipping to the SE. The extension is estimated to be about 10-15% through this portion of the Gaillard Cut. This extension may be explained by termination of left lateral faults in Eastern Panama as proposed by Mann and Corrigan or it may be extension related to the fragmentation and translation of micro blocks of Panamanian crust during the collision between Panama and Colombia.