What are common geological settings for subduction initiation, and what tectonic events precede the development of self- sustaining subduction?

Mark Reagan University of Iowa With thanks to: Julian Pearce and the IODP Expedition 352 Scientific team Outline

• Magmas in subduction initiation (SI) settings o FAB and boninite • Examples of SI o Eocene: Izu-Bonin-Mariana (IBM) o Puysegur o Matthew & Hunter • SI through geologic time o Boninites over Earth history o Implications for changes in dynamics of SI

Sheeted dike complex Troodos Ophiolite, Cyprus olivine olivine

Boninites opx orthopyroxene

Low-Si High-Si (LSB) (HSB) boninite komatiite komatiite

picrite picrite High-Mg Andesite basalt

picro- basalt bas. (HMA) basalt HMA & BADR and. andesite dacite

Siliceous High-Mg Basalt series BADR (SHMB)

LSB HSB

Low-Ti basalt series (LOTI)

After: Pearce and Reagan (submitted – ST2B-2) Mantle melting

opx opx

HSB komatiite komatiite Harzburgite LSB dry, 1.5 GPa 1550oC ( )

~2 GPa ~cpx Hz ~2 GPa picrite wet, 1.2 GPa picrite 1240oC

DMM ~0.5 GPa boninite basalt ~0.5 GPa melting HMA HMA & BADR basalt bas. and. and.

Based primarily on equilibrium melting experiments by: Hirose & Kawamoto (1995); Hirose & Kushiro (1998); Falloon & Danyushevsky (2000); Parman & Grove (2004); Wood & Turner (2009); Mitchell & Grove (2015) Izu-Bonin-Mariana (IBM) subduction system

• IBM forearc recognized as early subduction terrane since ~ 1980

• Stern and Bloomer (1992) suggested boninites erupted at arc inception during near-trench rifting resulting from slab rollback

• Recent work has shown that the IBM forearc preserves an ophiolitic rock record attributed to a large scale, but short duration, subduction initiation event

Reagan et al. (2013) Drill Sites for IODP Expedition 352

Perid

Site U1440

Site U1441

Site U1439 Site U1442 Gab

Bon FAB Dol

geology after Ishizuka et al. (2011) (Reagan et al. 2017; Shervais et al. 2019; Li et al. submitted) LSB HSB bon komatiite komatiite

picrite picrite

basalt

HMA basalt HMA & BADR

Chichijima Marubewan

IODP Site U1439 BADR SHMB IODP Sites U1440, U1441 FAB

HSB LOTI Data sources: Taylor et al. (1994); LSB Shervais et al. (2019) Godard et al. (in prep) ? • Transform boundary before 52.5 Ma? • Rapid, SI and near-trench sea floor spreading beginning at 52.0-52.5 Ma • Flux melting and LSB to HSB generation begins within 1 m.y. • Why? • Spontaneous? (e.g. Stern, 2004) • Induced by India-Asia collision? by plume?

1st “normal” arc volcanism at 45-46 Ma

after Reagan et al. (2019) (Whattam and Stern 2015) Early Eocene events

?

50-47 Ma

52 Ma (from: Dilek and Furnes, 2009) Ongoing subduction initiation I 160oE 162oE 164oE 166oE. 168oE. 170oE

44oS New Zealand

46oS

Puysegur Solander trench volcano

48oS

Map base: 50oS GeoMapApp geology: Sutherland et al. (2006) Puysegur arc Solander volcano

komatiite LSB HSB komatiite

boninite picrite picrite

basalt basalt HMA bas. and. BADR/HMA and. dacite

Data: Foley et al. (2013) Ongoing subduction initiation II

Monzier rift Matthew-Hunter subduction system Monzier rift

komatiite LSB HSB boninite komatiite

picrite picrite

basalt

HMA basalt bas. BADR/HMA and. and. dacite

Data: Patriat et al. (2019) BADR SHMB Modern Ophiolitic Archean+ oceanic terranes arcs(?) arcs and (< 2 Ga) basins Monzier Nuvvuagittuq

Mata (Lau Basin)

LOTI LSB HSB Flin Flon

Modern Continental continental rifts (< 3 Ga) “Boninitic” magmas arcs Shasta through time

Red lettering – arc-like trace Stillwater element abundance patterns

Phanerozoic Archean plumes plumes Black lettering – MORB/OIB- like trace element abundance patterns Manihiki Plateau Commondale (LIP) (Kaapvaal) After Pearce and Reagan (submitted) Archean Proterozoic Phaner- ozoic Subduction-related boninites LSB/HSB IBM-type SI terranes ophiolites - protoarcs

LSB/SHMB Other arc-basin systems ? arc-basin systems and localized SI

Intraplate boninites

Plume- LSB/LOTI/SHMB sourced ? komatiitic terranes, LIP

LSB/SHMB SHMB only SCLM- derived intracratonic rifting

4 3 2 1 0 Age (Ga)

after Pearce and Reagan (submitted) Conclusions

• Organized FAB to LSB to HSB volcanism in IBM related to a large-scale SI event that resulted near-trench sea-floor spreading associated with slab rollback and extreme mantle depletion before establishment of the volcanic arc.

• In IBM, FAB to HSB transition took ~ 1 Myr beginning about 52 Ma. “Normal” arc volcanism began at 46 Ma.

• SI in W. Pacific was a relatively rare significant tectonic and magmatic event; approximately synchronous with India-Asia collision and initiation of Manus plume; preceded bend in Hawaii-Emperor seamount chain by ~ 2 Myr.

• Recent SI events (Puysegur and Matthew-Hunter) have localized causes and effects. Similar events may be common.

• Evidence for subduction goes back to >3.8 Ga, modern strong plate SI could be <2 Ga.