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Link to original content: https://pubmed.ncbi.nlm.nih.gov/27821756
Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology - PubMed Skip to main page content
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. 2016 Nov 22;113(47):E7359-E7366.
doi: 10.1073/pnas.1609999113. Epub 2016 Nov 7.

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

Timothy E Keenan  1 , John Encarnación  2 , Robert Buchwaldt  3 , Dan Fernandez  4 , James Mattinson  5 , Christine Rasoazanamparany  6 , P Benjamin Luetkemeyer  1
Affiliations

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

Timothy E Keenan et al. Proc Natl Acad Sci U S A. .

Abstract

Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence.

Keywords: Philippines; geochronology; ophiolite; subduction initiation; tectonics.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Models of subduction initiation that explain similar ages between the formation of metamorphic soles and associated ophiolites (in cross-section and map view). The high temperature metamorphic sole (shown as a thick, black line) is generated from the crust of the subducting plate during subduction initiation. It may then be preserved at the base of the upper plate (future ophiolite, shown in cross-hatched pattern). Each model predicts a different age relation between the initially subducted crust, the overlying ophiolite, and the time of subduction initiation. Plate ages are schematically shown with darker shades representing older lithosphere. White arrows on subducting plate indicate relative plate motion. (A) Sinking of the subducting plate along a transform fault (TF) or fracture zone (FZ) drives extension in the upper plate, generating the future ophiolite (10). (B) Subduction initiation of distinctly older lithosphere near an active spreading center (27, 28). (C) Subduction initiation along an oceanic detachment fault near a spreading center (12). (D) Subduction initiation very close to or at a spreading center axis (13). Hacker et al. (25) proposed a variant of this in which subduction initiates across a transform or fracture zone with underthrusting directed parallel to an active spreading center axis. Both options are shown in map view. (E) Schematic of the Palawan ophiolite, its metamorphic sole, and the dated lensoid pods preserved within the sole. U-Pb zircon ages of the pods and ophiolite obtained from this study are displayed along with the metamorphic cooling age of the sole (18).
Fig. 2.
Fig. 2.
(A) Present tectonic setting of Palawan island, Philippines (18, 29, 30). Rectangle outlines the area shown in B. (B) General geology of central Palawan showing locations of sample sites. The general structure consists of an ∼34-Ma ophiolite (the Central Palawan ophiolite) thrust over deformed Cretaceous-Eocene turbiditic sedimentary rocks of the NPCT. Remnants of the older Early Cretaceous proto-SCSB ophiolite are found as occasional pillow lavas in tectonic windows in the younger Palawan ophiolite (geology from our field observations and refs. and 31). (C) Geologic map of the metamorphic sole at Dalrymple Point. See B for location. Background image from Google Earth (Digital Globe, CNES/Astrium). Apparent metamorphic grade decreases away from the mantle peridotite. (D) Schematic NW-SE cross section of Palawan in the Ulugan Bay area after ref. .
Fig. 3.
Fig. 3.
Photographs of outcrops from the central Palawan ophiolite (A) and its metamorphic sole (B–F) and cathodoluminescence images of extracted zircons from selected samples (G–L). (A) Magma-mingling structures exhibited by light-colored tonalite (plagiogranite) and diorite-gabbro (darker) at Penacosa Point. The tonalite yielded zircons with a crystallization age of 34 Ma. Pencil for scale. (B) Layered chert/quartzite and amphibolite showing sheath folds. (C) Amphibolite gneiss with hornblendite domains exhibiting isoclinal folding. (D) Foliated and lineated epidote amphibolite, looking ∼west; mountains across Ulugan Bay are mantle harzburgite of the Palawan ophiolite structurally overlying metamorphic sole rocks; strike and dip symbol indicates foliation. (E) Smaller, foliation-parallel, light-colored lensoid pods of amphibolite (sample PL-14-07). (F) Competent, light-colored pod of epidote amphibolite (with cumulate gabbro-like REE signatures; sample PL-14-05) enclosed within the strongly foliated amphibolite. These pods yielded zircons with crystallization ages of 35.242 Ma. Hammer for scale. (G and H, I and J, and K and L) Cathodoluminescence images of zircons, showing magmatic oscillatory zoning (samples PL-14-05, PL-14-06, and PL-14-07, respectively).
Fig. 4.
Fig. 4.
Geochemical data on samples from the central Palawan ophiolite and its metamorphic sole. Palawan ophiolite samples plotted in A–C include pillow lavas, mafic dikes, gabbroic intrusions, and (A) felsic intrusions (plagiogranite). Metamorphic sole samples plotted in A–C include amphibolites, epidote amphibolites, and garnet amphibolites. (A) Chondrite normalized REE concentrations in the samples. Samples that were selected for U-Pb zircon geochronology are symbolized by diamonds and their ages are indicated next to the data. The majority of samples have REE patterns resembling MORB and possible differentiates of MORB. Two samples (PL-14-06 and PL-14-05) display positive Eu anomalies indicating cumulate plagioclase in the samples. The geochemistry of the plagiogranite (PW-00-18) is consistent with simple fractional crystallization from the MORB-like basaltic magmas. (B) Ti-V-Sm and (C) Ti-V-Sc tectonic discrimination diagrams (38). Basaltic samples from the ophiolite and sole are similar and plot as MORB or transitional MORB-IAB.
Fig. S1.
Fig. S1.
U-Pb Concordia diagram for samples from the metamorphic sole. (A) Data from PL-14-05. Weighted mean age is reported as 35.242 ± 0.062 Ma. (B) Data from PL-14-06. Weighted mean age is reported as 35.862 ± 0.048 Ma. (C) Data from PL-14-07. Three externally discordant, but internally concordant, ages are 37.00 ± 0.16, 35.97 ± 0.11, and 35.25 ± 0.15 Ma.
Fig. 5.
Fig. 5.
Subduction initiation (∼34–35 Ma) at the spreading center that generated the Palawan ophiolite. (A) Schematic map view of the area showing the timing of initial strike-slip movement on the Red River shear zone (48), seafloor spreading in the South China Sea (30), and initiation of subduction at the Palawan ophiolite spreading center. Palawan ophiolite (yet to be obducted onto the rifted continental crust) shown in cross-hatched pattern. (B) Cross-sectional schematic view of the area along A-A′ during the onset of subduction. Weak zones where subduction had the potential to initiate, but did not, are also shown.
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References

    1. Forsyth D, Uyeda S. On the relative importance of the driving forces of plate motion. Geophys J R Astron Soc. 1975;43(1):163–200.
    1. Gurnis M, Hall C, Lavier L. Evolving force balance during incipient subduction. Geochem Geophys Geosyst. 2004;5(7):Q07001.
    1. McKenzie DP. 1977. The initiation of trenches: A finite amplitude instability. Island Arcs, Deep Sea Trenches and Back-Arc Basins, Maurice Ewing Series, eds Talwani M, Pitman WC IIII (American Geophysical Union, Washington, DC), Vol 1, pp 57–61.
    1. Mueller S, Phillips RJ. On the initiation of subduction. J Geophys Res. 1991;96(B1):651–665.
    1. Toth J, Gurnis M. Dynamics of subduction initiation at preexisting fault zones. J Geophys Res. 1998;103(B8):18053–18067.

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