Large-scale rotations of the Chortis Block (Honduras) at the southern termination of the Laramide flat slab

Abstract

Paleogeographic reconstructions place the Chortis block adjacent to southern Mexico in the Late Cretaceous and earlier time, forming the tectonically disturbed southwestern tip of the North American plate. This study assesses the relative motion between Chortis and North America during the Laramide orogeny, later development of the North America-Caribbean plate boundaries. We carried out paleomagnetic analysis of rotations and latitudinal displacement using a total of 90 sites from the Átima Limestone and the Valle de Ángeles Formation redbeds sampled at seven localities in Honduras. Two secondary but ancient magnetizations post-date Late Cretaceous folding at most localities. The oldest magnetization is a ChRM of high laboratory unblocking temperature in the redbeds (>650°C), and a moderate temperature in the limestones (500–600°C). This component is WNW directed at five localities, varying somewhat in declination, having a consistent moderate positive inclination (mean I=39.7°; α95=6.8°, n=53 sites). The age of the ChRM is bracketed between 85 and 63Ma, based on youngest zircon age and the age of a cross-cutting dike. An intermediate unblocking temperature component present in both units (Dec=146.2°, Inc=−28.5°; k=13.1, α95=6.8°, n=37 sites), is interpreted as an overprint. The ChRM indicates an average counterclockwise rotation of 55.4±5.7°, with respect to the expected direction. The younger overprint was acquired between ~50 and 15Ma; its mean is also discordant with the expected North America direction, implying some Neogene rotation of Chortis. Abundant late Albian-Turonian zircons in Valle de Ángeles redbeds explicitly require extending the Laramide arc along western Mexico into Chortis. A reconstruction of Chortis south of Mexico that takes into account 55 degrees of rotation shows a gap between both continental blocks at about 70Ma. We propose a model where this gap formed by back-arc extension creating the Late Cretaceous Chontal basin at the southern termination of the Laramide flat-slab.