Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

Abstract

[1] The escarpments that bound the Pito Deep Rift (northeastern Easter microplate) expose in situ upper oceanic crust that was accreted $3 Ma ago at the superfast spreading ($142 mm/a, full rate) southeast Pacific Rise (SEPR). Samples and images of these escarpments were taken during transects utilizing the human-occupied vehicle Alvin and remotely operated vehicle Jason II. The dive areas were mapped with a ''deformation intensity scale'' revealing that the sheeted dike complex and the base of the lavas contain approximately meter-wide fault zones surrounded by fractured ''damage zones.'' Fault zones are spaced several hundred meters apart, in places offset the base of the lavas, separate areas with differently oriented dikes, and are locally crosscut by (younger) dikes. Fault rocks are rich in interstitial amphibole, matrix and vein chlorite, prominent veins of quartz, and accessory grains of sulfides, oxides, and sphene. These phases form the fine-grained matrix materials for cataclasites and cements for breccias where they completely surround angular to subangular clasts of variably altered and deformed basalt. Bulk rock geochemical compositions of the fault rocks are largely governed by the abundance of quartz veins. When compositions are normalized to compensate for the excess silica, the fault rocks exhibit evidence for additional geochemical changes via hydrothermal alteration, including the loss of mobile elements and gain of some trace metals and magnesium. Microstructures and compositions suggest that the fault rocks developed over multiple increments of deformation and hydrothermal fluid flow in the subaxial environment of the SEPR; faults related to the opening of the Pito Deep Rift can be distinguished by their orientation and fault rock microstructure. Some subaxial deformation increments were likely linked with violent discharge events associated with fluid pressure fluctuations and mineral sealing within the fault zones. Other increments were linked with the influx of relatively fresh seawater. The spacing of the faults is consistent with fault localization occurring every 7000 to 14,000 years, with long-term slip rates of <3 mm/a. Once spread from the ridge axis, the faults were probably not active, and damage zones likely played a more significant role in axial flank and off-axis crustal permeability.

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

“…The third rotation about an EPR-parallel horizontal axis also decreases the dip of the mode of NW-dipping dikes (Figure 15b). Crosscutting relationships in the sheeted dike complex and common dike margin parallel faults at Pito Deep Rift [Hayman and Karson, 2009], support this third rotation in both study areas that represents a rotational-planar, or bookshelf, model [e.g., Mandl, 1987] and accomplishes the rotation of dikes to their commonly observed outward dips. This mechanism for accommodating block rotations is well documented in sheeted dikes of the Troodos Ophiolite [e.g., Varga, 1991] and inferred from dikes at Hess Deep [Varga et al, 2004].…”

“…Although that interpretation supports bending and inwardtilting of the lava unit, Schouten and Denham [2000] suggest that the underlying sheeted dikes undergo vertical compensation so as to maintain the steep, vertical orientations of the dikes. The common observation of outward-dipping dikes with numerous dike-margin-parallel faults and cataclastic zones in other orientations that both isolate panels of dikes Hayman and Karson, 2009], suggests that rotational normal ("bookshelf") faulting of panels of sheeted dikes is the dominant mode of deformation and is a significant structural element of the uppermost crust. Dikeparallel slip and associated block rotation are well documented in extensional regimes such as ophiolites [e.g., Varga, 1991Varga, , 2003].…”

“…Major fault scarps along the rift walls oriented at a high angle to spreading-generated structures provide an ideal cross-sectional view of the upper oceanic crust. Although most of the structures exposed in the escarpment appear to be related to spreading processes at the EPR [e.g., Hayman and Karson, 2009], overprinting of ridge-related structures is related to the tectonic evolution of the Easter Microplate and rifting of Pito Deep [Engeln and Stein, 1984; Schilling et al, 1985;Searle et al, 1989;Naar and Hey, 1991;Schouten et al, 1993;Searle et al, 1993;Rusby and Searle, 1995;Hey et al, 2002].…”

“…Dikes within the sheeted dike complex at Pito Deep Rift dip primarily to the southeast, that is outward from the EPR, with dips varying from 90°to 46°. Cataclastic fault zones primarily focused between individual dikes or separating panels of subparallel dikes, suggest an inseparable kinematic relationship of the damage zones and the tilting of dikes Hayman and Karson, 2009; A. Chutas, Structures in upper oceanic crust: Perspectives from Pito Deep and Iceland, unpublished Masters thesis, 122 pp., Duke University, 2007]. The mechanical anisotropy of the uppermost crust imparted by subparallel dikes and faults provides planes of weakness that preferentially slip allowing tilting to occur, similar to block rotations in other extensional settings including ophiolites and other tectonic windows [Varga, 1991;Varga et al, 1999Varga et al, , 2004.…”

Tectonics: 50 years after the Revolution

“…Direct observations of oceanic crust (Karson, 2002) at three separate windows, the Hess Deep Rift (HD), a hole through fast-spreading crust that formed the Galapagos Rift; the Blanco Transform escarpment, an exposure of intermediate-rate spreading along the Juan de Fuca Rift; and the Pito Deep (Hayman and Karson, 2009), an exposure in the small Easter microplate Figure 7. Digital elevation models of the world's major continental rifts, illustrating large elevation variations.…”