--> Geological Research on the Isthmus of Panama (GRIP)

Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins

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Geological Research on the Isthmus of Panama (GRIP)

Abstract

The formation and emergence of the Isthmus of Panama (Fig. 1a) is among the most fundamental geological events in Earth’s recent history, with significant implications for the connection of North and South American ecosystems and global palaeoceanographic conditions during the Neogene. Until recently, it was almost universally accepted that formation of the Isthmus, connecting terrestrial environments in the Americas and disconnecting the Atlantic and Pacific oceans in subequatorial latitudes, occurred in the Pliocene (ca. 3 Ma) in response to collision of the Panama volcanic arc with South America (Coates et al., 2004). Emergence of the Isthmus is thought to have triggered the onset of a new (still on-going) climatic regime in the northern hemisphere (Haug and Tiedemann, 1998), and facilitated migration of terrestrial organisms between the Americas (i.e., the Great American Biotic Interchange or GABI) (Marshall et al, 1982). However, detrital zircons recently dated in Neogene sedimentary basins in Colombia record earlier collision of the Panama volcanic arc with South America in the middle Miocene (13-15 Ma) (Montes et al., 2015). This suggests earlier closure of the inter-American seaway, and casts doubt on the validity of causal relationships between the formation of the Isthmus and the onset of glaciation in the northern hemisphere and the GABI. The work by Montes et al. (2015) sparked a much needed scientific debate on the exact timing of formation of the Isthmus (O’Dea et al., 2016; Jaramillo et al., 2017). However, it is significant to note that most data used to constrain the formation of the Isthmus are indirect environmental, biologic and tectonic proxies, whereas field constraints required to characterise the palaeogeographic evolution of the Isthmus lack over considerable areas of the Panama volcanic arc. To address this problem, we are conducting regional field work in Panama and Colombia, to explore and characterise geologic units collectively grouped as Òpre-Tertiary basementÓ on national geological maps (Fig. 1b) (https://panamageology.wordpress.com/). Our preliminary lithostratigraphic, geochemical, biostratigraphic and geochronological results suggest that the formation of the Isthmus is not a simple result of the collision of the Panama volcanic arc with South America, but that constructional volcanic processes associated with the formation of volcanic fronts (some uncharted) actively contributed to the Isthmus emergence. This effect seems particularly significant in the Panama Canal area, which constitutes a topographic low in the middle of the Isthmus. In addition, new and previous regional geological constraints support a complex tectonic evolution of the Panama volcanic arc, with notably: (1) subduction initiation along the edge of the Caribbean oceanic plateau in the Late Cretaceous (Buchs et al., 2010; this study); (2) localised tectonic events presumably related to seamount collisions during most of the Tertiary (Buchs et al., 2011); and (3) regional tectonic events in the middle Eocene and early Miocene (Barat et al., 2014) that could relate to large-scale plate reorganisations in the Pacific (e.g., break-up of the Farallon plate ca. 23 Ma). Significantly, regional tectonic events correlate to migration and/or abandonment of volcanic fronts along the Panama volcanic arc (Lissinna, 2005; Wegner et al., 2011; this study). Overall, this supports a link between topography and subduction dynamics/magmatism, rather than emergence of the Isthmus purely controlled by collision with South America. On-going work will help better characterise this complex interplay of tectonic and magmatic processes and their link to the palaeogeographic evolution of south Central America.