Opening of Hess Deep Rift at the Galapagos Triple Junction

Abstract

At the Galapagos triple junction, the westward propagating Cocos-Nazca (C-N) Rift breaks intõ 0.5 Ma crust accreted at the East Pacific Rise. Rifting transitions to full magmatic seafloor spreading in the wake of the propagating tip. The 25-km-long Hess Deep rift is the transitional segment from rifting to spreading. Intrarift ridge (IRR), located within Hess Deep rift, is interpreted as a detachment fault, which exhumes deep-seated rocks to the seafloor. Although transitional segments must have occurred throughout the westward propagation of C-N Rift, IRR is the only obvious detachment fault along the base of the Rift scarps in the last~5 Ma of its propagation. IRR formation may be in response to a decrease in spreading rate (~40 to <20 mm/yr) and presumed lower melt supply, resulting from the formation of the Galapagos microplate~1.4 Ma, which now controls the opening at the C-N Rift tip. Plain Language SummaryAt the Galapagos triple junction in the equatorial Pacific three midocean ridges come together: North East Pacific Rise (EPR), South EPR, and Cocos-Nazca (C-N) Rift. The C-N Rift, however, stops short of intersecting the EPR. The tip of the C-N Rift propagates westward toward the EPR, breaking apart~0.5-million-year-old crust built at the EPR. Rifting of the crust changes to full magmatic seafloor spreading in the wake of the propagating C-N Rift tip, and the segment that is transitional from rifting to spreading has been identified. The intrarift ridge (IRR) is located within the transitional segment. IRR is interpreted as a long-ived fault, which has brought deep-seated rocks to the seafloor. The IRR is the only long-lived fault identified in the last~5-million years of C-N Rift propagation implying that a long-lived fault is not a common stage in the transition to spreading. The IRR fault may be a result of the slowdown in spreading rate, and presumed decrease in the amount of magma rising beneath the segment, caused by the formation of the Galapagos microplate~1.4-million years ago.

The Evolution of Seafloor Spreading Behind the Tip of the Westward Propagating Cocos‐Nazca Spreading Center

“…Since these early tectonic studies, the western end of the Galapagos gore has been the focus of numerous efforts, including Ocean Drilling Project (ODP) Leg 147 and Integrated Ocean Drilling Program (IODP) Leg 345, which drilled the Intrarift Ridge (IRR) in Hess Deep rift (e.g., Ferrini et al, 2013;Francheteau et al, 1990;Gillis et al, 1993Gillis et al, , 2014Karson et al, 2002;MacLeod et al, 1996;Stewart et al, 2003). The IRR has been interpreted as a south facing, low-angle, long-lived normal fault (i.e., detachment fault) (e.g., Ballu et al, 1999;Francheteau et al, 1990;Lonsdale, 1988;Smith & Schouten, 2018;Wiggins et al, 1996), and studies have been conducted primarily to explore the geochemistry and architecture of the crust and upper mantle exposed at the seafloor. Near-axis values decrease toward the east and become more centered under the axis.…”

“…Near-axis values decrease toward the east and become more centered under the axis. Smith and Schouten (2018) examined the three initial rift basins developing in the Galapagos gore and suggested that the basin identified by Searle and Francheteau (1986) west of Hess Deep rift represents the earliest stage of rifting (s1, Figure 2a). The next segment to the east, the 24-km-long Hess Deep rift (s2), was identified as a transitional segment from rifting to magmatic seafloor spreading, displaying the growth of an axial volcanic ridge (AVR) in its eastern section and flanked on the north by an active low-angle detachment fault (the IRR).…”

“…The next segment to the east, the 24-km-long Hess Deep rift (s2), was identified as a transitional segment from rifting to magmatic seafloor spreading, displaying the growth of an axial volcanic ridge (AVR) in its eastern section and flanked on the north by an active low-angle detachment fault (the IRR). The IRR is the only active detachment fault that Smith and Schouten (2018) could identify along the base of the gore scarps from the existing data, and they suggested that IRR formation was in response to a slow-down in spreading rate and presumed lower melt supply, resulting from the formation of the Galapagos microplate. Smith and Schouten (2018) concluded that full magmatic spreading (i.e., along the entire length of a segment) currently begins at Segment s3.…”

“…The IRR is the only active detachment fault that Smith and Schouten (2018) could identify along the base of the gore scarps from the existing data, and they suggested that IRR formation was in response to a slow-down in spreading rate and presumed lower melt supply, resulting from the formation of the Galapagos microplate. Smith and Schouten (2018) concluded that full magmatic spreading (i.e., along the entire length of a segment) currently begins at Segment s3.…”

No significant boron in the hydrated mantle of most subducting slabs

“…IODP Expedition 345 32 , 33 drilled several holes towards the base of a ~3 km south-facing fault scarp exposing lower crustal gabbros and upper mantle rocks with an age of 1.27–1.42 Ma 34 . This fault has recently been interpreted as a detachment fault 35 . Three holes at IODP Site U1415 recovered intact sections of layered gabbro including gabbronorite, olivine gabbro and significant intervals of troctolite containing up to 30% olivine.…”