Seismic reflection data on the shelf offshore Guatemala reveal the stratigraphy of the forearc basin that has developed as the result of Tertiary subduction on the site of a Cretaceous passive margin. Abyssal Cretaceous and Paleocene sequences that presently dip landward to the northeast probably originally downlapped across a seaward-dipping continental rise surface. The termination of the thick Paleocene sequence at the shelf edge is probably a postdepositional tectonic truncation. A bathyal Eocene-Oligocene sequence deposited in the forearc subsequent to the initiation of subduction is truncated by a sharp angular unconformity, which indicates severe tilting and erosion of the forearc basin prior to subsidence and the deposition of the seaward-thickening early Miocene sequence. A late Miocene and Pliocene unit thins seaward, indicating renewed uplift of the shelf edge. A Quaternary sequence progrades across the gently tilted upper Miocene-Pliocene sequence toward San José Canyon.Major unconformities of the forearc basin were coeval with major plate reorganizations. A major reorganization of the Pacific-Farallon spreading center, which occurred in the late Paleocene, may have coincided with or preceded the uplift of the Guatemala shelf edge and the inception of subduction in this region. A late Oligocene disruption of the Farallon Plate forming the Nazca and Cocos plates coincided with renewed subsidence of the shelf edge associated with the seaward-thickening early Miocene sequence. In the middle Miocene another plate reorganization, which led to a much shortened Pacific-Cocos plate boundary, may have coincided with renewed uplift of the shelf edge. The regional Pliocene-Pleistocene unconformity may be associated with an eastward jump in the Pacific-Cocos spreading center from the Mathematician spreading center to the present East Pacific Rise.The causal mechanism relating plate reorganizations to ocean margin crustal dynamics may involve the changing pattern of forces driving an oceanic plate as the size and shape of the plate change.
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