Controlled-Source Electromagnetic Imaging of the Middle America Trench O"Shore Nicaragua

Home , Middle America Trench

Controlled-source electromagnetic imaging of the Middle America Trench o"shore Nicaragua

Kerry Key Scripps Institution of Oceanography

Collaborators: Samer Naif (SIO, now LDEO), Steven Constable (SIO), Rob L Evans (WHOI) Central America tectonic setting

20º N

Middle America Trench 15º N

58 mm/yr Nicaragua EPR

10º N 72 mm/yr

Cocos 83 mm/yr 5º N

Cocos Ridge

CNS 0º N Pacific Nazca

105º W 100º W 95º W 90º W 85º W Plate bending 20

Middle America Trench 15

58 mm/yr Nicaragua EPR

10 reactivated 72 mm/yr ! Cocos 83 mm/yr 5

normal faults Cocos Ridge

CNS 0 Pacific Nazca plate motion

trench articles Nicaragua’s bending faults I. Grevemeyer et al. / Earth and Planetary Science Letters 236 (2005) 238–248 245

a) Grevemeyer et al. (2005) are very likely related to increased hydrothermal cir-

35 culation of cold seawater in the upper portion of the incoming plate caused by bend-faulting. However, ] 30 -2 like on the flanks of mid-ocean ridges, fluid circula- 25 tion should cause both fluid recharge and discharge [28]. Recharge is characterised by low heat flow 20 values while discharge is indicated by high heat 15 flow values. We believe that high-permeability pathways support low-temperature vent sites in the area of Heat flow [mW m 10 bending related faulting which may occur elsewhere. 5 In order to understand differences in the pattern observed in our case studies off Nicaragua and Central b) Chile and the constraints from the global data set, we 10 investigated two other areas using the global data base 1000 [29] and previously published geothermal measure- 8 ments from the Peru trench [14] and Japan trench 800 [30]. Off Peru, low heat flow in the trench and sea- 6 600 ward of the trench axis are characteristic features (Fig. 4c). The low heat flow near 13 S offshore Peru was 4 8 # Faults 400 already investigated by Yamano and Uyeda [14]. They 2 200 suggested that water migrating along normal faults may act as cooling agent and were the first to propose 0 0 Cumulative fault offset [m] that bend-faulting at trenches may reactivate hydro- 30 25 20 15 10 5 0 Ranero et al. (2003) Distance from trench [km] thermal circulation in the oceanic lithosphere prior to subduction. In the Japan trench offshore Honshu and Fig. 5. a) Heat flow data from the incoming plate off Central the Kurile trench offshore Hokkaido, detailed multi- Mantle penetrating normal faults observed at theNicaragua Nicaragua (see Figs. 1 and 4a) outer and b) statistics rise of active normal beam bathymetric surveys show that bending causes • faults on the incoming plate [6]. The histogram displays the fault density as a function of distance from the trench axis. The number pervasive normal faulting in the incoming plate [11]. of faults increases towards the axis, indicating that new faults Although only a few heat flow values are available, • Density of faults correlates with decreasing (unusuallycontinue to form low) as the plate heat approaches ﬂow the trench axis. Lines they support the results from the Stein [1] study. Thus, indicate the cumulative fault offset [6] measured at the top basement heat flow is not significantly different from the global _ of multi-channel seismic line BGR99 39 (black line) and mean or the prediction of the plate cooling model for BGR99_41 (grey line). There is a tendency to larger fault offsets • Suggests faults are porous pathways for ﬂuid transport,towards the trench. Notefueling that the measured heat flow decreases with crust older than 100 Myr. Consequently, bend-faulting increasing number of faults and fault throw. We interpret this is not the sole factor governing the reactivation of hydrothermal circulation, hence hydrothermal alterationphenomenon in terms of increased hydrothermal activity when the hydrothermal circulation at subduction zones. plate approaches the deep sea trench. It has long been recognised that hydrothermal circulation systems are inherently affected by the sedi- However, no published evidence shows faults are Inpathways both detailed study areas chosen for this case mentary blanket [31]. The forces available to drive • study the measured heat flow is only one-third of the hydrothermal flow through the oceanic crust are mod- expected global mean for crust of that age (Fig. 4a and est, being limited mainly to pressure differences at the b); thus, much lower than suggested by the global base of recharging and discharging columns of water approach of Stein [1]. Our closely spaced, carefully [32]. These forces are insufficient to drive fluids at located measurements show low values that are clear- thermally significant rates through N200 m of sedi- ly linked to reactivated or new faults created in the ments. Thus, basement outcrops are required to allow outer rise. Off Nicaragua, a trenchward decrease in hydrothermal fluids to bypass the sediment and mine heat flow on the incoming plate correlates well with lithospheric heat efficiently [28,33]. In the NW Pacific an increase in the number and offset of faults cutting about 600 m of sediments cover the incoming plate. into the igneous basement (Fig. 5). Thus, low values Although escarpment heights on the incoming plate

Figure 2 Poststack finite-difference time migration of various lines. a, Line 39 and into the mantle where they are masked by the acoustic energy of the seafloor multiple. b, line 41 of cruise BGR99 and c, line 1331 of Ewing cruise 9503. Lines 39 and 41 show Arrows at the sea floor indicate faults used for Figs 3 and 4. Seafloor mass-wasting the structure of the Cocos plate across the ocean trench slope. The crust–mantle processes are evidenced by truncation of strata at many fault scarps and by turbidite infill boundary (CMB) is defined by a series of gently dipping reflections, delineated by white of half-grabens. White arrows in black boxes indicate faults marked by arrows in the lower filled circles, at ,1.7 s two-way time (,5 km) beneath the top of the igneous basement. inset of Fig. 1. Line Ewing 9503-1331 shows the structure of the ocean plate formed at A pervasive fabric traverses the mantle: sometimes this fabric appears to continue up the East Pacific Rise at latitude 168 N (ref. 27). The crust–mantle boundary is defined by into reflections that cross the crust projecting into top-basement and seafloor offsets. discontinuous reflections at ,1.8 s two-way time, the crust and upper mantle are Several reflections align forming individual features—indicated by the black arrows—that featureless. The ‘smiles’ underneath the spreading centre axis are created by cross much of the lithosphere. We propose that these reflections represent normal overmigration of side-scattered energy in the rough axial topography. FD, finite difference. faults which slip during plate bending. The reflections cut to ,18–20 km across the crust

NATURE|VOL 425|25 SEPTEMBER 2003|www.nature.com/nature © 2003 Nature Publishing Group 369 Geochemistry Geophysics 3 ranero et al.: bend-faulting and seismicity 10.1029/2005GC000997 Geosystems G Motivation

Ranero et al. (2005)

Figure 13. Cartoon showing a conceptual model of the structure and metamorphic evolution of subducting lithosphere# Mantle formed penetrating at a fast spreading normal center.faults The could topography provide of theﬂuid plate pathway in the outer-rise/trench for serpentinization region has been exaggeratedSubducted to show hydrous better the deformationminerals become associated tounstable plate bending. at high Scale pressure/temperature, is approximate everywhere else. Fault plane# solutions of earthquakes are projected into the top of the slab and the plane of the cross section. Black filled circlesleading in oceanic to crustdehydration indicate hydration. reactions See sectionthat release 6 for discussion free water of model. # Water promotes melting and is critical driver of arc volcanism Water weakens the plate interface, modulates megathrust seismicity lower# crust and mantle are comparatively unde- Bock et al., 2000; Yuan et al., 2000]. There, fluids formed and dry. (2) At the trench, extensional may reach the mantle wedge at the appropriate bend-faulting leads to partial hydration of the upper temperatures to produce partial melting leading to 20 km of the incoming plate [Ranero et al., arc volcanism. If mantle dehydration starts at 2003a; Ranero and Sallares, 2004]. Deeper thrust depths where eclogitization is not completed it faults do not reach shallow levels and faults are not may lead to a double seismic zone [Comte et al., hydrated, and may not be reactivated as intraslab 1999]. events. (3) Plate bending and faulting continues underneath the overriding plate [Mikumo et al., 2002; Valle´e et al., 2003] and hydration might still 7. Conclusions be active there. (4) Progressive metamorphism [41] The observations described above lead to dehydrates most of the water chemically bound in several conclusions regarding the deformation of subducting sediment [Ruepke et al., 2002] leading oceanic plates during bending, the relationships to upper plate mantle hydration [Walther et al., between bend-faulting and intraslab seismicity 2000; ANCORP Working Group, 1999]. (5) Intra- and the hydration of oceanic plates. slab seismicity initiates between 60–80 km when dehydration of the oceanic crust [Ruepke et al., [42] 1. Bending-related deformation at trenches 2002; Hacker et al., 2003] leads to reactivation of involves a complex faulting structure that seems the upper segment of bend-faults and earthquakes to be mainly controlled by the relative orientation concentrate in the upper part of the slab [Bock et of the tectonic fabric formed at the paleo-spreading al., 2000; Yuan et al., 2000]. (6) Deeper than center with the axis of bending, by crustal thick- 100 km eclogitization is completed [Hacker et ness, and possibly by the presence of preexisting al. , 2003]. At similar depths or slightly deeper the weak zones caused by lithospheric deformation slab mantle dehydrates [Ruepke et al., 2002] and linked to hot spot magmatism. The style of defor- bend-faults will be reactivated producing seismic- mation of the incoming plate may dramatically ity within the upper slab mantle [Tibi et al., 2002; change over along-strike distances of several hun-

20 of 25 Electrical resistivity of oceanic plates

Resistivity ρ (ohm-m) 0.1 1 10 100 1,000 10,000

water seawater fresh water porous sediments

basalt Dependent on porosity

gabbro

cold mantle

serpentinite depends on magnetite

basaltic melt pure melt partial melt

10 1 0.1 0.01 0.001 0.0001 Conductivity σ (S/m)

inversion models use same colorbar Controlled Source Electromagnetic Method

ocean (conductive)

sediments (conductive) oil or gas reservoir (resistive)

3D Vector Diffusion Equation:

Resistors Conductors Frequency Range: 0.1 - 10 Hz Attenuation: Low High

Phase Velocity: Fast Slow

EM Receiver Deployment Movie: CSEM Transmitter Deployment Movie: The Serpentinite, Extension and Regional Porosity Experiment Across the Nicaraguan Trench (SERPENT)

• First CSEM survey of a subduction zone • Single 28 day cruise produced 54 broadband MT / EM stations • 4 long-wire EM (LEM) receiver deployments • 800 km of CSEM tows • 96% data recovery rate Results from Anisotropy Circles

# Polarization ellipse maxima show anisotropic fabric aligned with faults # Anisotropy signiﬁcantly stronger beneath the outer rise ! Data well ﬁt with fault parallel conductive plates, 5:1 anisotropy Key, K., S. Constable, T. Matsuno, R. L. Evans, and D. Myer (2012), Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench, Earth Planet Sc Lett, 351-352, 45–53. CSEM Data Examples

a −12 10 −40 0.25 Hz Abyssal Plain site s08 (100 km) 0.75 Hz −80 1.75 Hz ) −13 flat phase = highly resistive 2 10 −120

−160 −14

10 Phase (degrees) Amplitdue ( V/Am

−200 Abyssal Plain

−15 southwest northeast 10 −240 −15 −10 −5 0 5 10 15 −15 −10 −5 0 5 10 15 Tx-Rx Range (km) Tx-Rx Range (km) b

−12 10 0.25 Hz Outer Rise −40 site s27 (20 km) 0.75 Hz 1.75 Hz ) −80 2 −13 10 −120 decreasing phase = faults less resistive crust −14 −160 Phase (degrees)

Amplitdue ( V/Am 10 faults −200

−15 Outer Rise 10 −240 −15 −10 −5 0 5 10 15 −15 −10 −5 0 5 10 15 Tx-Rx Range (km) Tx-Rx Range (km) EM receivers fluid seeps seafloor depth (km) CSEM Inversion Model 1

North 5

tectonic plate motion Depth below seasurface (km) seasurface below Depth 4 4

8 outer rise 8

12 12 -100 -80 -60 -40 -20 0 20 40 Distance from trench (km)

-1 0 1 2 3 4 10 10 10 10 10 10 # 32,500 data Resistivity (Ω-m) # 28,000 model parameters # Regularized non-linear inversion with adaptive ﬁnite element forward solver # Fit to RMS 1.0 with 2% error ﬂoor # Ran on 320 processors for 16 hours Outer rise fault scarps

conductive faults

# Dashed lines: P-wave velocity anomalies from Ivandic et al. (2008) # Fault scarps correlate with steeply dipping conductive channels # Porous channels along the fault traces drive ﬂuids into the slab # Mantle stays resistive Naif, S., K. Key, S. Constable, and R. L. Evans (2015), Water-rich bending faults at the Middle America Trench, Geochem Geophy Geosy, 16(8), 2582–2597. -0.4 Porosity Estimated from Resistivity

2.5 VE 7.5:1 25 5 15

10 Porosity (%) 7 7.5 4 3 Depth (km) 2 Moho 1.5 10 1 0.7 0.5 12.5 100 80 60 40 20 0 Distance from trench (km)

Archie’s Law: cementation exponent bulk resistivity m = 2.0 ﬂuid resistivity porosity Porosity Evolution with Plate Bending

Distance from Extrusives, Dikes, Gabbros, trench m= 2 m= 2 m= 2

24 Ma crust* 10.4 3 0.7

80-100 km 12.2 2.7 0.7

40-60 km 13.5 4.7 1.3

5-20 km 14.3 4.8 1.7 * from Jarrard's (2003) ocean drilling compilation study

• Crustal porosity increases 60%, doubles in the lower crust

• Signiﬁcantly more crustal pore water is subducted than previously thought Seismic evidence for uppermost mantle serpentinization

van Avendonk et al (2011) Outer Rise Summary:

• Bending faults are porous ﬂuid pathways

• More pore water subducts than previously thought • Crust is heterogeneously hydrated • Mantle remains resistive • seismic data requires signiﬁcant serpentinization • high resistive compatible with low magnetite content, implies low degree of serpentinization (<15%) CSEM Inversion Model EM receivers fluid seeps seafloor depth (km) 1

North 5

tectonic plate motion Depth below seasurface (km) seasurface below Depth 4 4

forearc 8 8

12 12 -100 -80 -60 -40 -20 0 20 40 Distance from trench (km)

-1 0 1 2 3 4 10 10 10 10 10 10 Resistivity (Ω-m) Depth (km) 9 8 7 6 5 4 3 2 1 0 0 10 5 i 0 10 Distance fromtrench (km) Resistivity 15 10 Forearc Resistivity ii 1 ( Ωm) 20 iii

25 10 plate interface plate

2 30 10 35 3 iii. ii. i. upper plate Conductor penetrates with basementrock low porosity, consistent Resistive upperplateis interface along megathrustplate Sediment subduction Sediment Subduction

Le Pichon et al. (1993)

Accreting Margin: • thicker sediments on incoming plate • most sediments accreted onto margin • compacted deeper sediments subducted • less water subducted

Non-accreting Margin: • thinner sediments on incoming plate • all sediments subducted • sediments have higher porosity • more water subducted

Nicaragua is non-accreting Resistivity (Ωm) 0 1 2 3 10 10 10 10

1 Subducted sediment 2 porosity 3

4 Porosity estimated with an 5 # Depth (km) empirical relationship that 6 accounts for surface conduction

7 in clays

8 (Sen & Goode, 1998)

9 0 5 10 15 20 25 30 35 80 estimated 70 compaction model # Compaction model from lab 60 studies and drilling data:

50 (Spinelli et al., 2006; Bray & φ

40 m=2.6 Karig, 1985) Qv =0.3

Porosity 30

10 m=2 Qv =0.5 0 0 5 10 15 20 25 30 35 Distance from trench (km) Additional ﬂuids from clay transformation

Sa"er and Tobin (2011)

# Example for Nankai Trough # Compaction is largest source of fluids in first 20 km # Clay transformation is largest source of fluids beyond 20 km Abundant seeps 20-40 km from trench axis

SERPENT CSEM

Sahling et al (2008)

Ranero et al. (2008) Geochemistry 3 FluidsGeophysics onranero et al.:the convergent margin margins and tectonics 10.1029/2007GC001679 slope GeosystemsConceptualG model of shallow forearc ﬂuid processes

Ranero et al. (2008)

Figure 2. Structure and interpretation ofRanero processes et active al. (2008) at erosional plate boundaries. A conceptual cross section showing active processes projected on bathymetry (50 m grid) illustrates the relationships between subsurface processes and seafloor structures. Fluid abundance along plate boundary is the main factor governing the distribution of processes with depth. Fluid abundance appears constrained by water release during temperature-controlled dehydration of subducting minerals. Overpressured water hydrofractures the base of the upper plate; fractured material is collected into the subduction channel and removed, causing the subsidence that forms the continental slope. Upper plate subsidence is partially accommodated by extensional faulting over a weakly coupled plate boundary. Faulting promotes upward migration of plate-boundary fluid that triggers mud diapirism of slope sediment and seepage at the seafloor, forming mounds. Under the slope, hydrofracturing probably creates a broad plate- boundary fault zone with distributed slip. At temperatures > 150°C, water release declines, causing areas of high fluid pressure to reduce in size and effective stress to increase on the plate-boundary fault. Here, the fault zone narrows, localizing slip, and a little-fractured upper plate accumulates elastic energy. Interplate earthquakes nucleate in this ‘‘drier’’ fault zone, but large events may rupture into the shallower portions of the plate boundary.

[Kimura et al., 1997]. The source of deep fluids where dehydration reactions should release water detected at slope seeps is the smectite and biogenic (Figure 1). This association supports the inference opal in subducting sediment [Kimura et al., 1997; that subducting minerals are the source of seep Spinelli and Underwood, 2004]. These minerals fluids. Further significant dehydration does not dehydrate at 50°–160°C, releasing water. Bio- occur until clay transforms to muscovite [Frey, genic opal transforms to quartz at 50°–100°C, 1987] at 250°–300°C. The plate interface does and smectite transforms to illite at 60°–160°C not reach this temperature until beneath the land- [Pytte and Reynolds, 1988; Moore and Vrolijk, mass at >40 km depth [Harris and Wang, 2002; 1992]. Most seeps occur above the plate boundary Ruepke et al., 2002]. where temperatures are >60°C to 140°C and thus

6 of 18 Depth (km) 9 8 7 6 5 4 3 2 1 0 0 porous pathwayforﬂuidtransport? Conductor aboveplateinterface: 10 5 0 10 Distance fromtrench (km) Resistivity 15 10 1 ( Ωm) 20

ﬂuid seeps

25 10 plate interface plate

2 30 10 35 3 Depth (km) 9 8 7 6 5 4 3 2 1 0 0 10 5 0 Conductor aboveplateinterface-whatisit? 10 Distance fromtrench(km) eitvt ( Resistivity 15 10 1 Ωm) 20

ﬂuid seeps 25 10 constrained

2 not well 30 10 35 3 along axis? How localizedisthis feature # # # hydrofracturing? Locus ofpersistent Subducted seamount? Underthrust sediments? $ $ $ subsidence no evidenceinbathymetry uplift Resistivity (Ωm) 0 1 2 3 McIntosh et al. (2007) 10 10 10 10

5 Depth (km)

9 0 5 10 15 20 25 30 35 Distance from trench (km) Conclusions 1. Outer rise faults are porous ﬂuid pathways ! Water-rich heterogeneously hydrated crust

2. Incoming ﬂuid-rich sediment layer fully subducted ! First estimate of in-situ subducted sediment porosity ! Seeps fed by subducted sediment ﬂuids