The Matata Earthquake Sequence of 1977, Bay of Plenty, New Zealand

<p>An ML. 5.4 earthquake and an associated sequence of smaller earthquakes, including foreshocks, were well recorded in 1977 by a network of 10 seismographs set for a microearthquake survey in the Bay of Plenty region, which is transitional between back-are spreading regions of the Havre Trough and the continental North Island. Upper crustal aftershock origins clustered and migrated within an area 7 km by 15 km elongated east-west. The aftershocks were relatively swarm-like, producing a b- value of 1.29 [plus or minus] 0.13, and were apparently of long sequence duration, with decay coefficient p = 0.67 [plus or minus] 0.03. A northeast-trending rupture fitted for the mainshock, originating close to where foreshocks were centred, and passing between tight concentrations of later aftershock activity to either side. Teleseismic waveforms, in addition to providing a 10.5 km estimate of focal depth, helped to constrain the solution of focal mechanism for the mainshock. The preferred solution is for mainly right-lateral slip on a northeast striking plane but with a normal component. The slip trend parallels the front of recent volcanism. Mechanisms for related events range from normal to strike slip, on parallel and intersecting planes, and are indicative of the complexity of geological structure where north-trending faults of the North Island shear belt meet with the front of recent volcanism. as well as of a prevailing traction across the volcanic front. The volcanic region is characterised by a low Poisson's ratio, suggested by the Wadati method to be v= 0.19 [plus or minus] 0.01 in contrast to v =0.27 [plus or minus] 0.01 for the greywacke region to the southeast; this difference is attributed to contrasting rock types and other conditions either side of the volcanic front. The multiplicity of earthquake sequences in the volcanic region indicates a high degree of heterogeneous structure. A low stress drop of 2.8 MPa inferred for the Matata mainshock suggests that the faulting occurs on pre-existing planes. Off-fault aftershocks occurred where the failure stress increased as a result of the mainshock rupture. A concurrent sequence of earthquakes originating near 50km depth indicated thrusting on the lithospheric plate interface underlying the North Island; thrusting on the interface apparently extends to about 70km depth, where the plates become decoupled. Oblique plate convergence and stick-slip motion on the weakly coupled interface provides the regional dextral shear component observed in the volcanic region for the Matata mainshock. An extensional component is therefore a necessary addition for the observed normal component of faulting, which predominated for the 1987 Edgecumbe mainshock. Wave mode conversions inferred for subcrustal earthquakes and the Matata sequence mainshock indicate that the Moho shallows from 28.5 km to 22 km northwestwards across the volcanic front, suggesting that new crust in the Bay of Plenty region is being created over a wide region rather than by active rifting along a sharp margin.</p>

The Matata Earthquake Sequence of 1977, Bay of Plenty, New Zealand

<p>An ML. 5.4 earthquake and an associated sequence of smaller earthquakes, including foreshocks, were well recorded in 1977 by a network of 10 seismographs set for a microearthquake survey in the Bay of Plenty region, which is transitional between back-are spreading regions of the Havre Trough and the continental North Island. Upper crustal aftershock origins clustered and migrated within an area 7 km by 15 km elongated east-west. The aftershocks were relatively swarm-like, producing a b- value of 1.29 [plus or minus] 0.13, and were apparently of long sequence duration, with decay coefficient p = 0.67 [plus or minus] 0.03. A northeast-trending rupture fitted for the mainshock, originating close to where foreshocks were centred, and passing between tight concentrations of later aftershock activity to either side. Teleseismic waveforms, in addition to providing a 10.5 km estimate of focal depth, helped to constrain the solution of focal mechanism for the mainshock. The preferred solution is for mainly right-lateral slip on a northeast striking plane but with a normal component. The slip trend parallels the front of recent volcanism. Mechanisms for related events range from normal to strike slip, on parallel and intersecting planes, and are indicative of the complexity of geological structure where north-trending faults of the North Island shear belt meet with the front of recent volcanism. as well as of a prevailing traction across the volcanic front. The volcanic region is characterised by a low Poisson's ratio, suggested by the Wadati method to be v= 0.19 [plus or minus] 0.01 in contrast to v =0.27 [plus or minus] 0.01 for the greywacke region to the southeast; this difference is attributed to contrasting rock types and other conditions either side of the volcanic front. The multiplicity of earthquake sequences in the volcanic region indicates a high degree of heterogeneous structure. A low stress drop of 2.8 MPa inferred for the Matata mainshock suggests that the faulting occurs on pre-existing planes. Off-fault aftershocks occurred where the failure stress increased as a result of the mainshock rupture. A concurrent sequence of earthquakes originating near 50km depth indicated thrusting on the lithospheric plate interface underlying the North Island; thrusting on the interface apparently extends to about 70km depth, where the plates become decoupled. Oblique plate convergence and stick-slip motion on the weakly coupled interface provides the regional dextral shear component observed in the volcanic region for the Matata mainshock. An extensional component is therefore a necessary addition for the observed normal component of faulting, which predominated for the 1987 Edgecumbe mainshock. Wave mode conversions inferred for subcrustal earthquakes and the Matata sequence mainshock indicate that the Moho shallows from 28.5 km to 22 km northwestwards across the volcanic front, suggesting that new crust in the Bay of Plenty region is being created over a wide region rather than by active rifting along a sharp margin.</p>

Molybdenum isotopes unmask slab dehydration and melting beneath the Mariana arc

How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts.

Tracing the subducting Pacific slab to the mantle transition zone with hydrogen isotopes

Hydrogen isotopes have been widely used as powerful tracers to understand the origin of terrestrial water and the water circulation between the surface and the deep interior of the Earth. However, further quantitative understanding is hindered due to a lack of observations about the changes in D/H ratios of a slab during subduction. Here, we report hydrogen isotope data of olivine-hosted melt inclusions from active volcanoes with variable depths (90‒550 km) to the subducting Pacific slab. The results show that the D/H ratio of the slab fluid at the volcanic front is lower than that of the slab fluid just behind the volcanic front. This demonstrates that fluids with different D/H ratios were released from the crust and the underlying peridotite portions of the slab around the volcanic front. The results also show that the D/H ratios of slab fluids do not change significantly with slab depths from 300 to 550 km, which demonstrates that slab dehydration did not occur significantly beyond the arc. Our estimated δD‰ value for the slab materials that accumulated in the mantle transition zone is > − 90‰, a value which is significantly higher than previous estimates.

Attenuation contrast in mantle wedge across the volcanic front of northeastern Japan that controls propagations of high-frequency S-wave later phases

Distinct later phases of waves with rich high-frequency (> 8 Hz) components were observed for intraslab earthquakes that occurred at intermediate depths, particularly at depths exceeding 100 km, in the northeastern (NE) Japan subduction zone. These high-frequency later phases (HFLPs) showed anomalously large peak-amplitude delays, up to ~ 50 s after direct S-wave arrivals at stations in the backarc region. Using a source-scanning algorithm, we investigated the locations of passing points affecting the propagation of HFLPs. The passing points were estimated to be in the forearc region in the entire NE Japan, indicating that HFLPs are scattered waves that pass through the forearc region. The propagating HFLPs seem to bypass the backarc mantle wedge, as a consequence of the distinct attenuation contrast in the mantle wedge across the volcanic front in NE Japan. These HFLP observations suggest that the high-attenuation zone in the backarc mantle wedge controls propagations of the high-frequency waves of intraslab earthquakes, in addition to the scatterers possibly locate in the forearc region.

Architectural and Tectonic Control on the Segmentation of the Central American Volcanic Arc

Central America has a rich mix of conditions that allow comparisons of different natural experiments in the generation of arc magmas within the relatively short length of the margin. The shape of the volcanic front and this margin's architecture derive from the assemblage of exotic continental and oceanic crustal slivers, and later modification by volcanism and tectonic activity. Active tectonics of the Cocos-Caribbean plate boundary are strongly influenced by oblique subduction, resulting in a narrow volcanic front segmented by right steps occurring at ∼150-km intervals. The largest volcanic centers are located where depths to the slab are ∼90–110 km. Volcanoes that develop above deeper sections of the subducting slab are less voluminous and better record source geochemical heterogeneity. Extreme variations in isotopic and trace element ratios are derived from different components of the subducted oceanic lithosphere. However, the extent that volcanoes sample these signatures is also influenced by lithospheric structures that control the arc segmentation. ▪ The architecture of Central America derives from the assemblage of exotic continental and oceanic crustal slivers modified by arc magmatism and tectonic processes. ▪ Active tectonics in Central America are controlled by oblique subduction. ▪ The lithospheric architecture and tectonics define the segmentation of the volcanic front, and thus the depth to the slab below a volcanic center. ▪ The composition of the subducted material is the main control of the along arc geochemical variations observed in Central American volcanoes. ▪ Geochemical heterogeneity in each segment is highlighted by extreme compositions representing the smaller centers with variations up to 65% of the total observed range. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Subduction-Zone Fluids

Fluids are essential to the physical and chemical processes in subduction zones. Two types of subduction-zone fluids can be distinguished. First, shallow fluids, which are relatively dilute and water rich and that have properties that vary between subduction zones depending on the local thermal regime. Second, deep fluids, which possess higher proportions of dissolved silicate, salts and non-polar gases relative to water content, and have properties that are broadly similar in most subduction systems, regardless of the local thermal structure. We review key physical and chemical properties of fluids in two key subduction-zone contexts—along the slab top and beneath the volcanic front—to illustrate the distinct properties of shallow and deep subduction-zone fluids.

LATE EOCENE BIOSTRATIGRAPHIC AGE FOR ANDESITIC VOLCANIC HOST ROCKS FORMED IN AN ISLAND ENVIRONMENT AT THE COBRE PANAMA PORPHYRY Cu-Mo-Au-Ag DEPOSIT, PANAMA

The giant Cobre Panama porphyry Cu-Mo-Au-Ag deposit in western Panama is hosted by an undated andesitic volcanic sequence, the Petaquilla batholith (32.20 ± 0.76–28.28 ± 0.61 Ma), and porphyry stocks (28.96 ± 0.62–27.48 ± 0.68 Ma). Here we present a biostratigraphic age for the volcanic sequence based on stratigraphically diagnostic large foraminifera from thin limestone beds within kilometer-thick andesitic rocks. These yield a late middle to late Eocene biostratigraphic age (41.2–33.9 Ma), with a probable late Eocene age (Priabonian stage, 37.8–33.9 Ma), which is slightly older than the age of the batholith and porphyry intrusions. The volcanic sequence is dominated by fine-grained, massive basalt to andesite lavas with subordinate volcaniclastic deposits. A preliminary description of volcanic textures based on macroscopic observation of drill core and quarry/road exposures supports the occurrence of lavas, fallout tuffs, volcanic breccias, and possible pyroclastic density current deposits. Rare polymictic conglomerates with well-rounded clasts of igneous rocks attest to minor sedimentary reworking from a nearby subaerial volcanic environment. The dated limestone that is interbedded with the submarine volcanic sequence was deposited in an estimated water depth of 50 to 80 m, probably in a middle- to outer-shelf large foraminiferal shoal. These results support deposition on the flank of an active volcanic island during early shallowing of the Isthmus of Panama. The Cobre Panama volcanic center is interpreted to have formed in the final stages of the latest Cretaceous-Eocene volcanic arc before, or possibly during, the 175-km sinistral offset of the Panama volcanic front in the late Eocene-Oligocene. However, it remains unclear whether the volcanic center formed on the western continuation of the San Blas-Chagres arc segment or the eastern termination of the Azuero-Soná arc segment and whether it was emplaced during broadening of the pre-Oligocene volcanic front or in a back-arc setting.