Abstract
Since the initial proposal of the past existence of a southward-directed mid-ocean ridge-subduction interaction in the Andes during Late Cretaceous-Paleogene times, several studies have been devoted to uncover the tectonomagmatic evidence of this process. The collision of a spreading ridge against a subduction margin provoke important tectonomagmatic changes; including between them variations in arc-related magmatic activity and in the plate-margin stress regime. However, the cryptic nature of the geological record often hampers assessing the influence and along-strike evolution of this process. In this study, we integrate new isotopic data with previous field and geochemical data on Andean arc-related magmatism, together with seismic tomography to track the main tectonic changes that affected the Andes between 35° and 42oS from Latest Cretaceous to early Miocene times. In particular, we carry out a new tomotectonic analysis combining the regional bedrock record of the Late Cretaceous-early Miocene arc with upper-lower mantle seismic tomography. This analysis allowed us to unravel the main geodynamic changes that affected the Andean active-margin when the Farallon-Aluk spreading ridge was subducting. Besides, new isotopic analyses reveal the variable nature of the mantle source that fed the Late Cretaceous-early Miocene arc. Hence, the integration of geological, geochemical and geophysical data, together with new isotopic data studying the geochemical composition of the main Andean arc-related magmatic units in three main periods: 1) Latest Cretaceous-early Paleocene; 2) Early Paleocene-late Eocene; 3) Late Eocene-early Miocene, allow us to understand with an unprecedented detail the geochemical and spatio-temporal evolution of the passage of this spreading ridge along the Andean margin
Highlights
The subduction of oceanic spreading ridges causes a significant impact on active margins producing multiple thermal, deformational, and magmatic effects and leaving a unique geological signature on overriding plates (e.g., DeLong et al, 1979; Thorkelson, 1996)
Our model shows the interaction of the Farallon–Aluk spreading ridge with the South American margin, whereas, integrated in a regional magmatic context, it provides robust evidence about the existence of a slab window event by latest Cretaceous–early Paleocene times and its southward migration along the Southern Central Andes till Eocene times
In the Southern Central Andes (∼35◦S), the interaction of the Farallon–Aluk mid-ocean ridge with the Andean margin would have promoted extensional deformation during Latest Cretaceous–early Paleocene times (80–59 Ma), the development of the collapse calderas magmatism in the north (23–28◦S), the transitional intraplate-like magmatic character recognized in specific arc-related magmas (35◦30 S), and the presence of adakite-like intrusions in the fore-arc area (39◦S)
Summary
The subduction of oceanic spreading ridges causes a significant impact on active margins producing multiple thermal, deformational, and magmatic effects and leaving a unique geological signature on overriding plates (e.g., DeLong et al, 1979; Thorkelson, 1996). Sublithospheric divergence of oceanic plates provokes the unzipping of the mid-ocean ridge leading to the formation of slab windows (e.g., Dickinson and Snyder, 1979; Groome and Thorkelson, 2009) This phenomenon produces a slab gap that allows the sub-oceanic asthenosphere to well up in a complex 3-D mantle flow (e.g., Guillaume et al, 2010). Identifying the precise location of past slab window event is a challenging task as these effects are time transgressive and are controlled by triple-junction kinematics (Thorkelson, 1996) This is further complicated in settings where mid-ocean ridge subduction took place obliquely to the subduction zone producing a time–space migration of the slab window process along the active margin. The latter tectonic configuration has been suggested in plate kinematic reconstructions in the southeast Pacific Ocean indicating that the Farallon–Aluk mid-ocean ridge should have subducted obliquely beneath the Andes sometime during Late Cretaceous to Paleogene times (Cande and Leslie, 1986; Ramos and Kay, 1992; Aragón et al, 2011; Somoza and Ghidella, 2012; Eagles and Scott, 2014; Iannelli et al, 2018; Fennell et al, 2019)
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