Exhuming an Accretionary Prism: A Case Study of the Kodiak Accretionary Complex, Alaska, USA

We have carried a structural study across Kodiak accretionary complex in Alaska, USA, in order to describe its thermal structure and decipher the processes of exhumation. The accretionary complex consists of a stack of tectonic melanges and coherent units. Melanges are characterized by simple shear deformation with a pervasive network of top-to-the-trench shear zones, whereas coherent units are affected principally by horizontal shortening, except for spatially limited outcrops in Kodiak Central Belt. Peak temperatures estimated using Raman spectroscopy of carbonaceous material range from 220 to 400 degrees C through most of the complex. These temperatures coincide with estimates from metamorphic parageneses and are interpreted as temperatures of deformation achieved under a high gradient due to contemporaneous ridge subduction. The highest temperatures are recorded in the central part of the complex, pointing to a dome-like structure. Lower temperatures (similar to 120 degrees C) are recorded along the southeastern border of the complex, in slope sediments unconformably overlying accreted units. Based on the combination of structural and thermal data, we conclude that the rocks constituting the Kodiak complex experienced at least two stages of burial and then exhumation, with vertical motions reaching up to similar to 13 km from the Paleocene to the present. Given the pervasive horizontal shortening within the wedge, exhumation resulted from prism thickening contemporaneous with surficial erosion. Recorded subsidence episodes may be local phenomena associated with thrusting or large-scale processes associated with basal erosion. The rates of vertical motion range from 0.2 to 1.3 mm/yr, comparable with estimates in modern margins. Plain Language Summary In this study, we examined several units from the Kodiak accretionary complex, an archetypal example of the accretionary wedge in southern Alaska, to improve our understanding of the processes that influence convergent margins. These units were subducted to significant depths, but the mechanisms responsible for their exhumation back to the surface remain unclear. Estimated peak-burial temperatures in basally accreted units across the complex are in the range 220-400 degrees C, with a temperature gap between basally accreted units and slope sediments (100-140 degrees C). Temperatures and pressures suggest a burial down to depths of similar to 7-13 km. Based on deformation kinematics and temperature estimates in basally accreted units and slope sediments, we propose that the exhumation periods result from basal accretion, horizontal shortening, and surficial erosion. In contrast, subsidence most likely resulted from thrusting in the upper plate or basal erosion.

Automated summary

We conducted a structural study on the Kodiak accretionary complex in Alaska, USA to understand its thermal structure and the processes of exhumation. The complex consists of tectonic melanges and coherent units, with melanges characterized by shear deformation and coherent units affected by horizontal shortening. Our findings indicate peak temperatures ranging from 220 to 400 degrees C, with the highest temperatures recorded in the central part of the complex. Based on structural and thermal data, we conclude that the rocks in the Kodiak complex experienced stages of burial and exhumation, with vertical motions reaching up to 13 km. The exhumation resulted from prism thickening and surficial erosion.