Seismic reflection profiles and SeaMARC II imagery from the southwest Panama margin demonstrate that oblique convergence is presently occurring along what had previously been thought of as a transform margin. Our seismic profiles image landward-dipping thrust faults and seaward-verging folds at the toe of the slope. The frontal deformation zone as imaged on the SeaMARC II mosaic is 12 to 15 km wide with individual east-west-trending folds and thrusts that are laterally continuous for 5 to 10 km. Much of the terrigenous trench sediment is offscraped and accreted, forming an accretionary prism (South Panama deformed belt). Three linear ridges (part of the Panama Fracture Zone complex) are being obliquely subducted along the southwest Panama margin. The oblique convergence causes the ridges to sweep eastward along the trench. The SeaMARC II mosaic shows that the regional structure of the South Panama deformed belt is dominated by east-west-trending trench segments that are separated by the north-south fracture zone ridges. The trench shallows where the ridges intersect the trench, and the deformation front is warped around the ridges. On the east side of each ridge the accretionary complex bends to a northwest-southeast trend, suggesting that the ridges are deforming the accretionary complex. As the accretionary prism rides up over each ridge, it thickens markedly. By the time the prism reaches the top of the ridge, its surface slope has been greatly oversteepened and large portions of accreted material slump into the trench. After passage of the ridge, the system returns to its “normal” state, and accretion resumes, adding the slumped material back into the accretionary prism. The accretionary prism is thus only temporarily disrupted by the subduction of the Panama Fracture Zone system ridges.

This study examines the interplay between sediment dispersal along the Panama margin and the evolution of the North Panama deformed belt (NPDB) using SeaMARC II swath-mapping data and migrated seismic profiles. The NPDB is composed of folded and thrusted strata of the Colombian basin that rise to within 800 m of the sea surface along the San Bias ridge. The ridge extends for more than 200 km along strike, thereby acting as an important barrier to sediment dispersal in the eastern region of the NPDB and the Colombian basin. Sediments derived from eastern Panama are trapped in the San Bias basin, located along the landward margin of the ridge, and deflected westward before traversing the lower slope along the San Bias canyon in the central region of the NPDB. Sediment entrapment within the San Bias basin and the westward deflection of transport paths by the San Bias ridge have contributed to a recent decrease in sediment thickness along the thrust front in the eastern region of the NPDB. Sediment thickness on the underthrusting basement of the Colombian basin also decreases in the region as a result of the blockage of Magdalena fan deposits by a basement ridge located less than 20 km from the thrust front. The decrease in sediment thickness is accompanied by a decrease in the width of the NPDB in this region, suggesting a concomitant decrease in the rate of frontal accretion as compared to other regions of the NPDB. A marked decrease in the thickness and fold wavelength of accreted strata from the interior of the belt to the thrust front may result from the diversion of terrigenous sediments away from the eastern region of the belt. Convergence rates along the frontal thrust are also significantly less in this region than elsewhere in the belt. Decreased frontal accretion along the eastern region has, however, been partially offset by the incorporation of thick deposits of the San Bias basin into the rear of the belt along backthrusts. Recent accretionary growth is therefore two-sided but may have varied considerably in the past in response to shifting patterns of sediment dispersal.

The western part of the North Panama deformed belt (NPDB) undergoes a number of changes in the orientation of the frontal thrusts, in marked contrast to the eastern part of the NPDB, which shows very little change over distances of 150 km. In the western part of the belt, vergence and structural style of the frontal thrusts vary rapidly along the belt, again in contrast to the eastern part of the belt. The northeast-trending part of the western NPDB appears to be constrained in orientation by the slope of the Panama arc, which lies parallel to the thrust front. Farther west the belt trends northwest, and the thrust front migrates across the slope to the shelf area, where it has been identified by studies of the April 22, 1991, earthquake. Seismic data offshore from the surface location of this earthquake show no signs of crustal deformation, further constraining the location of the thrust front. The northwest orientation of the thrust belt on- and offshore of Costa Rica is consistent with a kinematic mechanism related to collision of Costa Rica by the Cocos Ridge. A proposal to explain the fact that the thrust front is located on the shelf, rather than at the base of the slope in its westernmost position, is impedance of thrusting as a result of the heavy load of sediment deposited by the Costa Rica fan. Thus several kinematic mechanisms are operating around the NPDB, producing very different directions of thrust belt orientation. In addition, several more mechanisms, such as slope stress, sediment loading, and possibly the structure of the lower plate, are operating to modify the orientation of the frontal thrusts.