Evidence and models for lower crustal flow beneath the Galápagos platform

Abstract

The volcanic Galápagos Islands are constructed upon a broad platform, with their active westernmost islands marking the current position of the hotspot. Built upon young oceanic lithosphere (<15 Ma), this platform exhibits unique morphologic features including a system of stepped terraces on the southwestern escarpment with 3 km relief, contrasting with gentle slopes off the eastern platform toward the Carnegie Ridge. Considering horizontal lithostatic pressure differences associated with this relief, along with high temperatures within the young, hotspot‐affected lithosphere, it is likely that lower crustal flow contributes significantly to crustal deformation within the Galápagos platform. Using a 2‐D, isostatic, thin‐sheet approximation for the Stokes flow equation with (Newtonian) space‐time‐dependent viscosity, we suggest that the bathymetric rim along the eastern platform region (where gravimetry indicates Airy isostasy) near Española Island may be the expression of a mature lower crustal flow front developed over the last ∼3 Myr; horizontal mass displacements (∼50 km) associated with this crustal flow episode may have advected mantle plume geochemical signatures toward the southeast, and in directions not necessarily parallel to the hotspot track. Also, the stepped terraces along the southwestern platform may be explained by lower crustal flow‐associated backward tilting of the bathymetric surface that, although resulting in small angular changes (∼0.1°), effectively hinders the horizontal flow of lava sheets. This backward‐tilting process was likely restricted to the last ∼1 Ma or less, and may be a unique event involving extrusion of lavas from within the southwestward‐marching lower‐crustal flow front.

Rough versus smooth topography along oceanic hotspot tracks: Observations and scaling analysis

“…Orellana-Rovirosa and Richards, 2015]. Deviating from this trend (red bars) are the Ninetyeast Ridge (with HTL up to 300 km), and to some extent Reunion (with HTL up to 180 km) and the transition region along Nazca-Easter (HTL as low as 30 km), all of which were influenced by preexisting fracture zones (see section 4).…”

“…We hypothesize that smooth hotspot topography occurs when the overlying lithosphere is weak and thin (young), unable to support large volcanic complexes [Feighner and Richards, 1994], thus allowing for broadly intrusive magmatism in the lower crust to dominate over extrusion [Richards et al, 2013;Orellana-Rovirosa and Richards, 2015]. Accordingly, smooth topography should occur when the thermal energy available Figure 2).…”

Emergence/Subsidence Histories Along the Carnegie and Cocos Ridges and Their Bearing Upon Biological Speciation in the Galápagos

“…However, horizontal pressure gradients make the thickened crust subject to postemplacement deformation. Relaxation of lithostatic pressure gradients broaden the thickened crust (formed by magmatic intrusion and extrusion) on timescales of a few million years, reducing its height, as shown by Orellana‐Rovirosa and Richards (). For example, their model for the Eastern platform region having an initial 2‐km thickness suggests ~0.25 km subsidence, most of it occurring during the first ~1 Ma span.…”

“…Following the models of Orellana‐Rovirosa and Richards (; their code being based on incompressible mass conservation, momentum conservation [Stokes' equation], and temperature‐dependent viscosity), we have estimated the amount of height reduction due to lower crustal flow at each one of the target regions by (i) forming a transect that cuts across the hotspot track at each region; (ii) measuring the present‐day elevation contrasts ( p.d. h in Table ) along the transect, from the hotspot track toward the two basin sides; and (iii) measuring the background crustal age difference between the track and the transect's extremes.…”

“…The formulation uses a symmetric description (it models one side, but assumes reflection symmetry), thus we chose age gradients equal to zero in order to conform with the net topographic subsidence resulting from relaxation on both sides. Common parameters for all the target‐regions models were mass densities: {ρ c , ρ m } = {2,800, 3,300} kg/m 3 ; viscosities of the system: { η min , η ref , η max } = {10 17 , 10 18 , 5.5 × 10 22 } Pa·s; background crustal thickness = 6 km; system‐transect length L = 350 km; Age grad = 0 Ma/km; N steep = 6; x 0 = 0.35 L; L Tx = 0.45; all of which are properly defined in the work of Orellana‐Rovirosa and Richards ().…”