Bending-related faulting and mantle serpentinization at the Middle America trench

Abstract

The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50-300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes.

Rupture Process During the Mw 8.1 2017 Chiapas Mexico Earthquake: Shallow Intraplate Normal Faulting by Slab Bending

“…The downward rupture propagation and the limited width of the main rupture may reflect lateral changes in the slab geometry and thermal structure (e.g., Manea & Manea, 2008) along the strike direction, and the down-dip edge of the main rupture area may correspond to the brittle-ductile transition in the oceanic lithosphere. Earthquake swarms around the epicenter (Figure 1) detected by Nishikawa and Ide (2017) may be independent evidence of slab bending around the source region, because fracturing and hydration in the plate due to bending may manifest as high seismicity rates (Nishikawa & Ide, 2015;Poli et al, 2017;Ranero et al, 2003;Shillington et al, 2015), and such a relation between earthquake swarms and subduction of a fracture zone has been documented in the Coquimbo-Illapel region of central Chile (e.g., Poli et al, 2017).…”

Distribution of hydrous minerals in the subduction system beneath Mexico

“…Such processes determine geophysical and mineralogical properties of the oceanic lithosphere with significant consequences for the mechanical behavior of subduction zones (Bach and Früh-Green, 2010). In particular, bending-related faulting affects porosity and permeability structure of the oceanic crust and consequently creates a pathway for fluids down to mantle depth (Ronero et al, 2003;ContrerasReyes et al, 2007). However, there is little constraint on in situ abundance and distribution of hydration within subducting crust near the trench to fully assess the strength of the plate interface and nature of mega earthquakes.…”

Low heat flow from young oceanic lithosphere at the Middle America Trench off Mexico

“…Young oceanic lithosphere is relatively weak and therefore is likely to bend readily over relatively short wavelengths in response to loading by the downgoing slab, promoting such faulting. Faults attributed to plate bending and penetrating to 18-20 km depth have been imaged in 14-24 Ma lithosphere off Costa Rica and Nicaragua [25]. In our survey area, fault blocks are clearly imaged in the oceanic basement beneath the trench (Figs.…”

“…The observed low heat flow may indicate that heat exchange continues between the ocean and the subducted crust, or alternatively that the subducting lithosphere is hydrothermally cooled to much greater depth. For example, the thermal time constant for 20 km of lithosphere is N1 Myr, so if faults extended as deep in our survey area as off Costa Rica and Nicaragua [25] and hydrothermal cooling reached similar depths, a conductive geotherm would be reestablished only several tens of kilometres landward of the trench.…”