A multi-sill magma plumbing system beneath the axis of the East Pacific Rise

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1 A multi-sill mgm pluming system eneth the xis of the Est Pcific Rise Milen Mrjnović 1*, Suznne M. Crotte 1, Helene Crton 1, Mlden Nedimović 2,1, John C. Mutter 1, Jun Plo Cnles 3 1 Lmont-Doherty Erth Oservtory, Columi University, Plisdes, New York, USA 2 Deprtment of Erth Sciences, Dlhousie University, Hlifx, Nov Scoti, Cnd 3 Woods Hole Ocenogrphic Institution, Woods Hole, Msschusetts, USA *Corresponding uthor. E-mil: milen@ldeo.columi.edu NATURE GEOSCIENCE 1

2 Supplementry Discussion Possile origins for seismic events rriving t two-wy trvel times lter thn the AML re descried here in more detil. These include internl multiples, sefloor side scttering, nd out-of-plne imging of the AML or other intr-crustl horizons. Internl multiples. To explore the possiility tht the SAML events could e internl multiples, we clculte predicted pths, ssuming 1D ridge structure nd zero-offset, nd using twtt picks for sefloor, lyer 2A nd AML picked long xis2r1, for 1 st nd 2 nd order intred multiples within lyer 2A (extrusive slt lyer), the 1 st intred multiple within lyer 2B (dyke lyer) nd 1 st intered multiple within the whole upper crust (slts nd dykes) (Fig. S4-S5). The predicted lyer 2B intred nd lyer 2 intered multiples rrive t twtts > s, ut do not mtch ny of the deeper SAMLs we oserve, which elimintes these multiples s possile explntion. For the shllowest SAML events locted t s elow the AML, lthough there re regions where the twtt of these events coincides with either the 1 st or 2 nd order lyer 2A intred multiple, no consistent mtch is oserved. For exmple, while the twtt to the SAML t 9º51-53 N roughly coincides with the predicted twtt of the 1 st order intred multiple, north of 9º54 N the SAML event deepens nd its twtt cn e more closely mtched y the predicted pth of the 2 nd order lyer 2A intred multiple (Figs S4-S5). Further evidence tht the shllowest SAML events re not multiples comes from the source-receiver offset t which they re detected (Fig. S6). Multiples should e oserved t source-receiver offsets similr to or greter thn the offset rnge within which the primry events involved in the genertion of the multiple re oserved. Our dt show tht the SAMLs stck s prominent events t source-receiver offsets <1000 m (Fig. S6c) wheres the lyer 2A event, which is not true reflection 1, is not present t these ner offsets (Fig. S6). Side scttering from sefloor topogrphy nd intr-crustl events. Side-sctter rrivls from rough sefloor topogrphy locted outside the imging plne cn e recorded in mrine 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION seismic sections nd potentilly misinterpreted s true structure 2. As illustrted in Fig. S7, side-scttered energy rising from the prominent fult scrps locted on the west flnk of the ridge xis re predicted to rrive t twtts ~ s, which is greter thn the trveltimes of the oserved SAMLs (Figs S7c, d). Arrivls in our seismic sections tht mtch the clculted trveltimes over long-xis distnces of severl kilometers cn e confidently ttriuted to sideswipe from these fults (Fig. S7d). The genertion of sideswipe from out-of-plne intr-crustl reflectors (here the AML) my lso occur. This is common prolem of 2D crustl-scle seismic reflection studies 3. Some of the most prominent SAML events re imged within the re for which full 3D survey ws conducted (Fig. S1) nd hence their 3D geometry cn e exmined (Fig. S8). Profiles extrcted from the 3D seismic volume 4 show tht the SAMLs imged in the longxis direction (Fig. S8) re present in cross-xis sections (Figs S8-g) s distinct events elow the AML, t roughly the sme trveltimes s recorded on the long-xis dt. While the geometry of the internl multiples is expected to mimic the shpe of the generting primries, the cross-xis profiles show tht the geometry of the SAML events differs in chrcter from the AML event ove (e.g. in shpe, complexity, dip of diffrction tils, Figs S8-d). For instnce, in the cross-xis stcked section shown in Fig. S8 the AML ppers s complex reflection for which two nd possily three distinct diffrction hyperols (rising from the edges of possily composite AML) cn e identified extending over ~1800 m in cross-xis direction. In contrst, the SAML in this section is much nrrower (~500 m) single event with poorly imged edge diffrctions. NATURE GEOSCIENCE 3

4 Supplementry Methods Picking seismic horizons. Seismic rrivls re digitized from the 2D sections using Prdigm s 2D interprettion tool Section with n estimted picking uncertinty of ±0.008 s. Additionl uncertinties in rrivl trveltimes rise from the choice of stcking velocities which re derived from the ESP5 velocity function for the ridge xis t 9 34 N (ref. 5) with dditionl velocity nlysis performed vi semlnce nd constnt velocity stcking methods. From the rnge of stcking velocities tht est imge the events of interest, we estimte stcking errors of ±0.016 s for lyer 2A nd ±0.008 s for the AML nd SAML events. For the swth 3D volumes the sme seismic horizons re identified nd digitized using the Propgtor tool ville in VoxelGeo (Prdigm s 3D interprettion softwre), with the sme picking uncertinties s for the 2D interprettion. CMP supergthers. In CMP gthers, the AML nd SAML rrivls s well s converted sher phses from the AML (P AML S) re oscured t mid to fr source-receiver offsets y lower velocity reflection nd refrction events rising in the shllower crust (i.e. sefloor nd lyer 2A). To improve imges of the deeper intrcrustl events, f-k dip filtering is used to eliminte the contminting steeply-dipping sefloor nd lyer 2A energy. To void sptil lising when pplying n f-k dip-filter, filtering should e conducted on shot gthers or on interleved CMP gthers 6. For the ltter, the geometry of the reflectors must e uniform (i.e. horizontlly lyered) with no significnt lterl chnges in velocity in the region over which composite CMP gther is defined. We ssume tht these pproximtions re vlid for the long-xis crustl structure, especilly over short distnces (<200 m) nd thus pply frequency-wvenumer (f-k) filtering on 24-fold CMP gthers (or CMP supergthers), ech spnning n long-xis distnce of ~150 m. 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION References 1. Hrding, A. J., Kent, G. M. & Orcutt, J. A., A Multichnnel Seismic Investigtion of Upper Crustl Structure t 9 N on the Est Pcific Rise: Implictions for Crustl Accretion. J. Geophys. Res. 98, 13,925 13,944 (1993). 2. Günther, R.H., Klemperer, S.L. & Goodliffe, A., Modeling sideswipe in 2D ocenic surveys from sonr dt: Applictions to the Mrin rc. Tectonophysics 420, (2006). 3. Nedimović, M. R. & West, G. F., Crooked-line 2D seismic reflection imging in crystlline terrins: Prt 1, dt processing. Geophysics, 68, (2003). 4. Crton, H. D. et l. Three-dimensionl seismic reflection imges of xil melt lens nd seismic lyer 2A etween 9 42 N nd 9 57 N on the Est Pcific Rise. Astrct OS21C- 1514, Fll Meeting, AGU, Sn Frncisco, Cliforni, (2010). 5. Ver, E. E. et l. The Structure of 0- to 0.2-m.y.-Old Ocenic Crust t 9 N on the Est Pcific Rise From Expnded Spred Profiles. J. Geophys. Res. 95, 15,529 15,556 (1990). 6. Yilmz, Ö., Seismic Dt Anlysis, Processing, Inversion, nd Interprettion of Seismic Dt (Society of Explortion Geophysicists, 2001). 7. White, S. M., Hymon, R. M., & Crotte, S. M., A new view of ridge segmenttion nd ner-xis volcnism t the Est Pcific Rise, 8-12ºN, from EM300 multiem thymetry. Geochem. Geophys. Geosyst. 7, Q12O05 (2006). NATURE GEOSCIENCE 5

6 Depth (m) 2,400 2,600 >2,800 xis 2r1 lv flow xis 2r1 hydrotherml vent SAMLs twtt elow AML (s) c S 5.0 9º20 5 km º25 9º30 9º35 9º40 9º45 9º50 9º55 e 1 3 N d 9º22 9º24 9º26 9º28 f ? 9º38 9º40 9º42 9º44 9º32 9º34 9º44 9º46 9º48 9º50 9º52 9º54 9º56 Figure S1. SAML events long the EPR N., Bthymetry 7 showing two-wy-trvel-time (twtt) to the SAML events elow the AML with indicted long seismic line xis2r1, which roughly follows the eruptive fissure zone of the EPR., Migrted seismic section xis2r1. Verticl dshed lines indicte CMP supergthers shown in Fig. S2. Horizontl rrows show extent of the cross-xis 3D survey. Numered lck oxes 1-4 highlight res shown in detil in pnels c, d, e nd f. Red rrowheds indicte AML nd green rrowheds indicte SAMLs. The rest of the symols re s in Fig NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION offset (m) 1,000 2,000 3,000 4,000 5,000 6,000 SuperCMP ,000 2,000 3,000 4,000 5,000 6,000 offset (m) SuperCMP PAMLP PSAMLP PAMLP PSAMLP PAMLS 5.0 c 5.0 d offset (m)1,000 2,000 3,000 4,000 5,000 6,000 SuperCMP offset (m) 1,000 2,000 3,000 4,000 5,000 6,000 SuperCMP PAMLP PSAMLP PAMLP PSAMLP PAMLS Figure S2. Exmple CMP supergthers., (centred t ~9º43 N) nd, CMP (centred t ~9º52 N), oth extrcted from xis2r1 (loctions shown in Fig. S1). Arrivls include P wves reflecting from the AML (P AML P), P wve reflecting from the SAML (P SAML P) nd P-to-S converted wve off the AML (P AML S). c, nd d, re the sme s nd, respectively, with modelled twtt of the ove events superimposed (dshed red P AML P, green P SAML P nd lue P AML S). Predicted pths re clculted using ESP5 velocity model 5. For SAML we ssume the sme twtt ehviour s for the AML. PAMLS º20 9º25 9º º35 9º40 9º45 9º50 9º55 `P S P sefloor lyer 2A AML Figure S3. P-to-S converted phse (P AML S) imge. Prtil offset stck for the P AML S (see Supplementry Methods). Verticl dshed lines represent loctions of CMP supergthers shown in Fig. S2. Block model of lyered ocenic crust shows schemtic ry pth for P AML S converted phse: `P down-going compressionl wve, S up-going sher wve nd P up-going compressionl wve. NATURE GEOSCIENCE 7

8 xis 2r1 lyer 2A sefloor AML S 5 km 9º20 9º25 9º N 9º35 9º40 9º45 9º50 9º55 1st lyer 2A intred 2nd lyer 2A intred 1st lyer 2B intred 1st AML intered 1 sefloor lyer 2A AML 2 sefloor lyer 2A AML 3 sefloor lyer 2A lyer 2B AML 4 sefloor lyer 2A lyer 2B AML Figure S4. Predicted trvel times for internl multiples., Migrted seismic section (s shown in Fig. S1) with digitized picks of primry horizons, nd predicted multiples superimposed. Primry events: sefloor (yellow), lyer 2A (green) nd AML (red); multiples: 1 st order lyer 2A intred multiple (drk lue), 2 nd order lyer 2A intred multiple (light lue), intred lyer 2B multiple (purple), intered AML multiple (ornge). Predicted pths for multiples re clculted using twtt of the primry events nd ssuming zero sourcereceiver offset. Verticl dshed lines represent loctions of CMP supergthers shown in Fig. S2., Block models showing schemtic ry pths for intr/intered multiples included in (sme numering nd colour code). Note tht non-zero offset ry pths re used for illustrtion clrity. 8 NATURE GEOSCIENCE

9 SUPPLEMENTARY INFORMATION º22 9º26 9º32 9º34 9º38 9º42 9º44 9º48 9º52 9º56 c d 3.7 9º22 9º26 9º22 9º26 9º22 9º26 9º32 9º º32 9º34 9º32 9º34 9º38 9º42 9º38 9º42 9º38 9º42 9º44 9º48 9º52 9º56 9º º48 9º52 9º56 9º44 9º48 9º52 9º56 Figure S5. Comprison etween oserved seismic rrivls nd predicted trveltimes for lyer 2A intred multiples. Close-up sections from seismic line xis2r1 corresponding to oxes 1-4 shown in Fig. S1., Seismic sections showing sefloor nd lyer 2A events with digitized horizons superimposed., Seismic sections showing AML nd SAML with AML horizon picks nd predicted trveltimes for 1 st nd 2 nd order intred multiples within lyer 2A superimposed. Horizon picks re shifted ~0.01 s for etter visiility of the reflection events. Colour code s in Fig. S5. c, Amplitude envelope seismic ttriute for lyer 2A clculted within the time window to exclude sefloor rrivl. d, Amplitude envelope seismic ttriute clculted for the AML nd SAML events. For the lst row of pnels, sections 1 nd 2, higher gin is needed due to low reflection signl of the events. The rrowheds mrk loctions of the AML (red) nd SAML (green). NATURE GEOSCIENCE 9

10 V=1560 m/s offset m 9º20 9º25 9º30 9º35 9º40 9º45 9º50 9º º44 9º48 V=1560 m/s offset m 9º20 9º25 9º30 9º35 9º40 9º45 9º50 9º º44 9º48 c V=2600 m/s offset m 9º20 9º25 9º30 9º35 9º40 9º45 9º50 9º º44 9º48 Figure S6. Constnt velocity, prtil-offset stcks., Ner source-receiver offset ( m) stck for stcking velocity V=1560 m/s, pproprite for lyer 2A., Mid- source-receiver offset stck ( m), stcked with V=1560 m/s. Lyer 2A (pink rrowheds) is imged in this mid-offset stck t ~0.1 s elow the sefloor. c, Ner source-receiver offset stck ( m), stcked with AML velocity (V=2600 m/s). AML nd SAML events pper oth in ner offset stck nd re highlighted y red nd green rrowheds, respectively. Blck oxes outline re centred t ~9º45 N tht is shown in pnels to the right. 10 NATURE GEOSCIENCE

11 SUPPLEMENTARY INFORMATION 10º06 10º04 10º02 xis 2r1 10º00 sefloor fult scrp ~ x ~ 9º58 9º56 9º54 9º52 SAML 9º50 9º48 c -104º24-104º18-104º12 d km 6.5 9º48 9º50 9º52 9º54 9º56 9º58 10º00 10º02 10º04 10º06 Figure S7. Side-scttered energy from sefloor topogrphy., Schemtic digrm illustrting side-scttered energy from fult scrp locted t horizontl distnce x from the survey line, projected onto the verticl imging plne (sefloor profile is shown in rown line)., Bthymetry mp for the EPR N showing prominent sefloor fult scrps on the west flnk of the ridge xis (mrked in red) s possile sources of side-sctter energy. The fult scrps re locted t horizontl distnces of ~2 km from the seismic profile. c, Migrted xis2r1 section extending etween 3.9 nd s twtt showing presence of AML (indicted with red rrowheds) nd SAMLs (green rrowheds). d, Modelled twtt for side-scttered energy from the fult scrps mpped in (red line) projected onto wter velocity stcked xis2r1 seismic section extending etween nd 6.5 s two-wy-trvel time. Side-scttered energy identified in the seismic section is mrked in white rrows. Grey shded region is to indicte low fold in seismic dtset. NATURE GEOSCIENCE 11

12 º44 9º46 9º48 9º50 9º52 9º54 9º56 1 e 1 c -104º18-104º17-104º16-104º18-104º17-104º16 2 f 2 d -104º18-104º17-104º18-104º17 3 g 3-104º19-104º18-104º17-104º19-104º18-104º17 Figure S8. 3D geometry of the SAML events., Migrted long-xis seismic section xis2r1 showing AML (in red rrowheds) nd SAML (green rrowheds) within the eruption re (s in Fig. S1f). Dshed red lines show loctions of three cross-xis profiles (1-3), centred t 9º49.3, 9º50.4 nd 9º53.25 N, extrcted from the 3D seismic volume ville in this region. Crossxis seismic sections in pnels -d re stcked sections, nd e-g with migrtion velocities pproprite for the AML. See Supplementry Discussion for further detils. 12 NATURE GEOSCIENCE