Splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone. Prestack depth migration of a multichannel seismic line across the north Ecuador–south ...Colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 (Mw 7.7) tsunamogenic subduction earthquake. The splay fault separates 5–6.6 km/s velocity, inner wedge basement rocks, which belong to the accreted Gorgona oceanic terrane, from 4 to 5 km/s velocity outer wedge rocks. The outer wedge is dominated by basal tectonic erosion. Despite a 3‐km‐thick trench fill, subduction of 2‐km‐high seamount prevented tectonic accretion and promotes basal tectonic erosion. The low‐velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic, décollement thrust. Subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks. Conversely, duplexes are interpreted to form from and above subducting sediment, at ∼14‐ to 15‐km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic décollement. Coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget, which comes out negative. Intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the Gorgona terrane from Cretaceous extension to likely Eocene oblique compression. The splay faults could have resulted from tectonic inversion of listric normal faults, thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis.
The Matakaoa Debris Flow (MDF) is a 200‐km‐long mass‐transport deposit resulting from the failure of the Matakaoa continental margin, northeast New Zealand, ca. 38–100 ky ago. In this study, ...high‐quality bathymetric and seismic reflection data are used to identify the morpho‐structural characters that reflect the kinematics of the MDF, as well as its interactions with basin sediments. We demonstrate how the transport energy, together with the local topography led to the present geometry and complex structure of the MDF deposits. The remarkable transport energy of the MDF is demonstrated by its dynamic impact on adjacent sedimentary series, including erosion of the substratum, shearing and compressional deformation. In the proximal zone of transport, momentous substratum erosion, demonstrated by giant tool marks and truncated sediments at the base of the debrite, triggered the excavation of a large volume (>200 km3) of basin sediments. The size of transported blocks (up to 3‐km long) is used to estimate the matrix yield strength in an early stage of transport. In the distal zone of transport, 100 km north of the source, seismic profiles show the propagation of thrust structures from the MDF into adjacent basin sediments. This study highlights that the remarkable volume of 2000 km3 of deposits partly resulted from the propagation of compressive structures within the basin sedimentary series to the front of the debrite.
A dense GPS network deployed in Ecuador reveals a highly heterogeneous pattern of interseismic coupling confined in the first 35 km depth of the contact between the subducting oceanic Nazca plate and ...the North Andean Sliver. Interseismic models indicate that the coupling is weak and very shallow (0–15 km) in south Ecuador and increases northward, with maximum found in the rupture areas of large (Mw>7.0) megathrust earthquakes that occurred during the 20th century. Since the great 1906 Mw=8.8 Colombia–Ecuador earthquake may have involved the simultaneous rupture of three to six asperities, only one or two asperities were reactivated during the large seismic sequence of 1942 (Mw=7.8), 1958 (Mw=7.7), 1979 (Mw=8.2) and 1998 (Mw=7.1). The axis of the Carnegie Ridge, which is entering the subduction zone south of the Equator, coincides well with the location of a 50 km wide creeping corridor that may have acted as persistent barrier to large seismic ruptures. South of this creeping region, a highly locked asperity is found right below La Plata Island. While this asperity may have the potential to generate an Mw∼7.0–7.5 earthquake and a local tsunami, until now it is unknown to have produced any similar events. That region is characterized by the presence of slow slip events that may contribute significantly to reduce the long-term moment deficit accumulated there and postpone the failure of that asperity. At the actual accumulation rate, a characteristic recurrence time for events such as those in 1942, 1958 and 1979 is 140±30 yr, 90±20 yr, 153±80 yr respectively. For the great 1906 event, we find a recurrence time of at least 575±100 yr, making the great 1906 earthquake a rare super cycle event.
•We model heterogeneous interseismic coupling along the Ecuadorian subduction zone.•The ruptures of large megathrust earthquakes correlate with discrete locked asperities.•Subduction of geomorphologic features promotes creeping on the megathrust interface.
Tectonic processes that control the transition from poorly consolidated sediment entering the subduction channel (SC) to the seismogenic zone are documented using seismic imaging. We applied ...pre‐stack depth migration and a post‐processing sequence to a seismic reflection line acquired across the Ecuador convergent margin to obtain a 2D‐quantitative image of the first ∼24 km of the SC. Structural interpretation shows that the SC consists of a 630–1150‐m‐thick, low‐velocity, continuous sheet of sediment that dips ∼6° landward and undergoes shear deformation. The long sheet is bounded at top and bottom by décollement thrusts, and developed over time Riedel shears and basal thrust faulting and folding downdip, pointing to a dynamic mega‐shear zone. Modeling the strong uppermost and basal SC reflectors reveals that they are associated with 40–80‐m‐thick, 50–350 m/s, low‐velocity perturbations layers inferred to be fluid rich and mechanically weak. A fine‐scale velocity model shows two anomalously low‐Vp areas in the long sheet, advocating patches of over‐pressured fluids. Evidence for Vp variations along the upper‐plate foundation suggests either underplated bodies or a fluid‐damage zone. A simple temporal reconstruction indicates that underthrusting the long sheet initiated >450 kyr ago and interrupted ∼54 ± 13 kyr ago, when frontal accretion resumed. During this transient evolution, the SC boundaries revealed highly unstable as most of the SC was underplated while down going plate material may have been sheared off and incorporated to the SC.
Key Points
First time investigation of internal structures of a modern subduction channel
Novel mechanism for basal erosion
Timing for alternation between long sheet underthrusting and frontal accretion
Deciphering the migration pattern of the Esmeraldas submarine Canyon (EC) and its history of cut‐and‐fill allows constraining the Pliocene‐Pleistocene tectonic evolution of the Ecuador‐Colombia ...convergent margin. Swath bathymetry, multichannel seismic reflection, and chronological data show that the EC is a 143‐km‐long, shelf‐incising, river‐connected canyon that started incising slope apron deposits in the Manglares fore‐arc basin ~5.3 Ma ago. The EC inception appears contemporaneous with the subduction of the Carnegie Ridge that is believed to have initiated 5–6 Myr ago and is considered an indirect cause of the EC formation. During its two‐stage left‐lateral migration, the EC upper‐half scoured deep incisions providing evidences for uplift episodes in the Manglares Basin that are correlated with mid‐Pliocene and Pleistocene regional tectonic events. Glacioeustatic variations contributed significantly to shape the EC and its upslope tributaries by increasing the rate of canyon incision during rapid sea level falls. Faults, folds, and diapirs have structurally controlled the location of the EC and of its tributary canyons, including the Ancon Canyon, which served as the main spillway of the Manglares Basin prior to be cut from its source ~170 kyr ago by the growth of a fault‐related anticline. The margin wedge that hosts the EC is highly unstable as it is cut by active faults and shaken by large subduction earthquakes. Several mass transport deposits have dammed the EC, one of them between >~65 and ~37 kyr causing an impoverishment of detrital material in the trench sedimentation and a possible interruption of the paleoseismological record.
Key Points
The Esmeraldas Canyon (EC) started incising the Manglares Basin ~5.3 Ma ago contemporarily with the Carnegie Ridge subduction initiation
The EC migration and incisions reveal Plio‐Quaternary fore‐arc uplift episodes and the conspicuous influence of glacioeustatic variations
The Ancon Canyon, the main spillway of the Manglares fore‐arc basin, was abandoned ~170 kyr ago favoring the basin infill
The 2015 VESPA voyage (Volcanic Evolution of South Pacific Arcs) was a seismic and rock dredging expedition to the Loyalty and Three Kings Ridges and South Fiji Basin. In this paper we present 33 ...40Ar/39Ar, 22 micropaleontological, and two U/Pb ages for igneous and sedimentary rocks from 33 dredge sites in this little‐studied part of the southwest Pacific Ocean. Igneous rocks include basalts, dolerites, basaltic andesites, trachyandesites, and a granite. Successful Ar/Ar dating of altered and/or low‐K basalts was achieved through careful sample selection and processing, detailed petrographic and element mapping of groundmass, and incremental heating experiments on both phenocryst and groundmass separates to interpret the complex spectra produced by samples having multiple K reservoirs. The 40Ar/39Ar ages of most of the sampled lavas, irrespective of composition, are latest Oligocene to earliest Miocene (25–22 Ma); two are Eocene (39–36 Ma). The granite has a U/Pb zircon age of 23.6 ± 0.3 Ma. 40Ar/39Ar lava ages are corroborated by microfossil ages from associated sedimentary rocks. The VESPA lavas are part of a >3,000 km long disrupted belt of Eocene to Miocene subduction‐related volcanic rocks. The belt includes arc rocks in Northland New Zealand, Northland Plateau, Three Kings Ridge, and Loyalty Ridge and, speculatively, D’Entrecasteaux Ridge. This belt is the product of superimposed Eocene and Oligocene‐Miocene remnant volcanic arcs that were stranded along and near the edge of Zealandia while still‐active arc belts migrated east with the Pacific trench.
Plain Language Summary
Samples of lava from the seabed between New Zealand and New Caledonia have been dated using atomic clocks and fossils. Most lavas erupted in a big pulse of volcanic activity between 25 and 22 million years ago. They are part of a belt of now‐extinct undersea volcanoes that stretches for more than 3,000 km between New Zealand and the Solomon Islands. These volcanoes were formed by subduction of the Pacific Plate under the Australian Plate.
Key Points
A major pulse of 25–22 Ma volcanism is documented on the Loyalty and Three Kings Ridges, southwest Pacific Ocean
The ridges are part of a more than 3,000 km long belt of Eocene to Miocene remnant volcanic arcs, stranded along the edge of Zealandia
With care in sample selection, and petrological work, meaningful Ar/Ar ages can be obtained from altered and/or very low‐K submarine basalts
The north Ecuador/south Colombia convergent margin is affected by recurrent subduction earthquakes with magnitudes >7.5, like the 1906, 1942, 1958, 1979 and 2016 events. The subduction trench is ...characterized by the construction of the Esmeraldas Turbidite System (ETS) fed by the large Esmeraldas Canyon that deeply incises the continental slope and that connects directly onshore with the Esmeraldas River. The detailed description of cores collected in the left-hand (western) proximal levee of the ETS and in two lobes allowed discriminating two types of coarse-grained deposits: (1) “classical” flood-generated turbidites are normally graded beds with structureless, laminated and cross-laminated intervals and high organic-matter content, while (2) earthquake-induced deposits consist of amalgamated normally-graded laminated/cross-laminated intervals separated by erosive surfaces. These latter are interpreted to be deposited by quasi-synchronous flows generated during a single earthquake. Organic matter is absent in such beds while ferromagnesian minerals and pumices are abundant, suggesting remobilization of the slope deposits. When two amalgamated beds are superimposed, the interbedded clayey interval is not bioturbated, suggesting a short time period between the beds deposition, and thus the impact of a major earthquake shock and following earthquakes on the triggering of landslides.
Along the ETS, core-to-core correlation based on 210Pb excess revealed that 20th Century sedimentation occurred mainly in the proximal levee. There, a temporal relationship was established between the 1906, 1942, and 1979 earthquakes, and three coarse-grained beds showing features of earthquake-induced turbidites, suggesting the Esmeraldas Canyon was the main source for sediments to be remobilized during these earthquakes. The fining and thinning observed between the 1906, 1942 and 1979 turbidites correlate with the increasing distance of the rupture zone of each earthquake with the Esmeraldas Canyon. Earthquakes with magnitudes lower than 7 also affected the margin during the 20th Century but were not recorded in the trench sedimentation, suggesting that the turbidite levee acts as a natural filter so that potentially the highest the levee the strongest the earthquake magnitude recorded. At least ten earthquakes with the highest magnitudes were recorded on the turbidite levee within the last 800years with a recurrence time ranging from about 268years to 42–82years, or less for the 20th Century earthquakes. The comparison of the main features of the 1906 turbidite with older earthquake-triggered turbidites identified in a core collected in the trench suggests that one or two earthquakes similar to the 1906 event might have occurred ~600years ago.
We investigate the relationship between the long‐term (Quaternary) interplate coupling and the short‐term geodetically derived interseismic coupling at the Central Ecuador subduction zone. At this ...nonaccretionary margin, the Cabo Pasado shelf promontory and coastal area are associated with two inter‐plate geodetically locked patches. The deepest patch ruptured co‐seismically during the Mw7.8‐2016 Pedernales earthquake, while the shallowest underwent dominantly after‐slip. Marine geophysical and chronostratigraphic data allow reconstructing the Quaternary tectonic evolution of the shelf promontory and substantiating variation of the long‐term inter‐plate coupling that led to the geodetically locked patches. Prior to ∼1.8 Ma, the outer‐wedge inter‐plate coupling was strong enough to activate trench‐subparallel strike‐slip faults. Then, between ∼1.8 and 0.79 Ma, shortening and uplift affected the shelf promontory, implying a locally increased inter‐plate coupling. After a short, post‐0.79 Ma period of subsidence, shortening and uplift resumed denoting a high inter‐plate coupling that endured up to the present. The synchronicity of the structural evolution of the shelf promontory with the subduction chronology of two reliefs of the Carnegie Ridge crest suggests that the locked patches are caused by a geometrical resistance to subduction that propagates landward causing permanent deformation. In 2016, the deepest subducted relief localized stress accumulation and high seismic slip, while the shallowest relief, which is associated with a weakened outer‐wedge, prevented updip rupture propagation. Thus, at nonaccretionary margins, active outer‐wedge strike‐slip faults might be considered a proxy of near‐trench coupling, and subducted relief a cause of plate coupling but an obstacle to the tsunami genesis when the relief is shallow.
Plain Language Summary
The 2016‐Ecuador earthquake ruptured a subduction fault segment previously locked for decades beneath the coastline. The rupture was arrested updip by another locked fault segment called locked patch, which instead slipped slowly. To understand the cause of the locked patches, their rupture behaviors, and whether the decadal fault locking and long‐term subduction processes are related, we reconstructed the Quaternary tectonic evolution of the margin offshore Central Ecuador using geophysical data. We consider that tectonic deformation reflects the long‐term inter‐plate coupling, which is the ability of the fault to transfer long‐term stress and strain to the margin. Prior to ∼1.8 Ma, a trench‐subparallel fault accommodating lateral displacement indicates a shallow plate coupling, which increased locally between ∼1.8 and 0.79 Ma as shown by margin shortening. After a brief subsidence, shortening resumed, denoting a strong coupling that persisted until today in the form of the locked patches. Although many physical factors have been proposed to control plate coupling, here we find that the locked patches are caused by the subduction of two reliefs of a submarine ridge. Remarkably, in 2016, the deepest relief released high elastic strain, while the shallower relief, thrust under a weakened outer‐margin, damped updip rupture propagation, impeding a significant tsunami.
Key Points
A trench‐parallel strike‐slip fault and its earthquake‐controlled fault scarps substantiate a pre‐1.8 Ma, outer‐wedge inter‐plate coupling
From 1.8 Ma, a robust shelf uplift caused by subducted reliefs highlights a long‐term coupling that led to geodetically locked patches
The shallowest subducted relief likely impeded the generation of a major tsunami during the Mw 7.8, 2016 event
Norfolk Ridge bounds the northeastern edge of the continent of Zealandia and is proximal to where Cenozoic Tonga‐Kermadec subduction initiation occurred. We present and analyze new seismic ...reflection, bathymetric and rock data from Norfolk Ridge that show it is composed of a thick sedimentary succession and that it was formed and acquired its present‐day ridge physiography and architecture during Eocene to Oligocene uplift, emergence and erosion. Contemporaneous subsidence of the adjacent New Caledonia Trough shaped the western slope of Norfolk Ridge and was accompanied by volcanism. Neogene extension along the eastern slope of Norfolk Ridge led to the opening of the Norfolk Basin. Our observations reveal little or no contractional deformation, in contrast to observations elsewhere in Zealandia, and are hence significant for understanding the mechanics of subduction initiation. We suggest that subduction nucleated north of Norfolk Ridge and propagated rapidly along the ridge during the period 40‐35 Ma, giving it a linear and narrow shape. Slab roll‐back following subduction initiation may have preserved the ridge and created its eastern flank. Our observations suggest that pre‐existing structures, which were likely inherited from Cretaceous Gondwana subduction, were well‐oriented to propagate rupture and create self‐sustaining subduction.
Plain Language Summary
Plate tectonic theory established and proved that the surface of Earth is composed of rigid moving plates, but it remains unclear how and why these plates sometimes re‐configure their boundaries and motions. Subduction zones are places where two plates converge and one plunges deep into the Earth beneath the other one. As the plate sinks, it drags the rest of the plate with it and acts as an engine that “pulls” the plate and drives horizontal motion. This is what drives the dynamics of plate tectonics. How are subduction zones created? This remains an open question, but we know from geological observations that new subduction zones do get created: more than half of all active subduction zones were created after the dinosaurs died out 65 million years ago. We present new observations from northern Zealandia (a submerged continent between New Zealand and New Caledonia) that document how one of the largest subduction zones on Earth, the Tonga‐Kermadec system, started.
Key Points
We present new marine geophysical and geological data of Norfolk Ridge located along the northeastern edge of the Zealandia continent
We show that the ridge is not inherited from Cretaceous rifting that led to isolation of Zealandia but from the TECTA Cenozoic tectonic event
Analysis of the structure and evolution of Norfolk Ridge underpins our understanding of tectonic processes of subduction initiation
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