Geology topics

Roland E. von Huene

Publications and source records attributed to Roland E. von Huene.

At least 19 recordsLinked to original sources

USGS2024

Collision structures of the Prince William terrane and Chugach terrane docking along the Shumagin and Unimak convergent margins, Alaska, USA

Western Alaska’s convergent margins are composed of tectonostratigraphic terranes. On land, terrane assembly is recognized along boundaries or sutures between neighboring geologic elements with distinctly different origins. In marine areas where rock outcrops are covered by sediment, recognizing terrane sutures is problematic. A fault in seismic dip line 5 of the ALEUT project has been interpreted as a terrane suture. It is imaged intermittently down to the 30+-km-deep plate interface. Processing of ALEUT strike line 7 revealed the suture at ~18 km depths extending 300 km along the margin. Upper structures in line 5 are like the structures of adjacent seismic transects where imaging is only 8−10 km deep. They were previously not recognized as the upper reaches of terrane sutures and show structural details obscured at greater depths. The composite data are the basis for a simple tectonic model of terrane docking.

Alaska↗

USGS2021

Subducting oceanic basement roughness impacts on upper plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the 2nd largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977 – 1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Further surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ~3 to 5 km thick sediment. Sediment on the subducting plate modulates the plate interface relief. The accreted prism’s imbricate thrust faults have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Further upslope, another fault extends >100 km along-strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5 D plate interface images show modest relief whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading/landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Alaska↗

USGS2019

The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin

The potential for a major earthquake in the Shumagin seismic gap, and the tsunami it could generate, was reported in 1971. However, while potentially tsunamigenic splay faults in the adjacent Unimak and Semidi earthquake segments are known, such features along the Shumagin segment were undocumented until recently. To investigate margin structure and search for splay faults, we reprocessed six legacy seismic records and also processed seismic data acquired by RV Langseth during the ALEUT project (cf. Bécel et al., 2017). All records show splay faults separating the frontal prism from the margin framework. A ridge uplifted by the splay fault hanging wall extends along the entire segment. At the plate interface, the splay fault cuts across subducted sediment strata in some images, whereas in others, the plate interface sediment cuts across the fault. Splay fault zones are commonly associated with subducting lower-plate relief. Along the upper slope, beneath a sediment cover, major normal faults dipping landward and seaward border a ridge of basement rock. The faults displace a regional unconformity that elsewhere received Oligocene–Miocene sediment. Low seafloor scarps above some normal faults indicate recent tectonism. The buried ridge is a continuation of the Unimak Ridge structure that extends NE of the Unimak/Shumagin segment boundary. Some geological characteristics of the Shumagin segment differ from those of other Alaskan earthquake segments, but a causal link to the proposed Shumagin creeping seismic behavior is equivocal.

Alaska↗

USGS2016

A possible source mechanism of the 1946 Unimak Alaska far-field tsunami, uplift of the mid-slope terrace above a splay fault zone

In 1946, megathrust seismicity along the Unimak segment of the Alaska subduction zone generated the largest ever recorded Alaska/Aleutian tsunami. The tsunami severely damaged Pacific islands and coastal areas from Alaska to Antarctica. It is the charter member of “tsunami” earthquakes that produce outsized far-field tsunamis for the recorded magnitude. Its source mechanisms were unconstrained by observations because geophysical data for the Unimak segment were sparse and of low resolution. Reprocessing of legacy geophysical data reveals a deep water, high-angle reverse or splay thrust fault zone that leads megathrust slip upward to the mid-slope terrace seafloor rather than along the plate boundary toward the trench axis. Splay fault uplift elevates the outer mid-slope terrace and its inner area subsides. Multibeam bathymetry along the splay fault zone shows recent but undated seafloor disruption. The structural configuration of the nearby Semidi segment is similar to that of the Unimak segment, portending generation of a future large tsunami directed toward the US West coast.

Alaska↗

USGS2016

A possible transoceanic tsunami directed toward the U.S. west coast from the Semidi segment, Alaska convergent margin

The Semidi segment of the Alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby Unimak segment in 1946. Reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami. A 200 km long ridge or escarpment with crests >1 km high is the surface expression of an active out-of-sequence fault zone, recently referred to as a splay fault. Such faults are potentially tsunamigenic. This type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism. Seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone. The greater slip may derive from Quaternary subduction of the Patton Murray hot spot ridge that extends 200 km toward the east across the north Pacific. Estimates of tsunami repeat times from paleotsunami studies indicate that the Semidi segment could be near the end of its current inter-seismic cycle. GPS records from Chirikof Island at the shelf edge indicate 90% locking of plate interface faults. An earthquake in the shallow Semidi subduction zone could generate a tsunami that will inundate the US west coast more than the 1946 and 1964 earthquakes because the Semidi continental slope azimuth directs a tsunami southeastward.

Geochemistry, Geophysics, Geosystems↗

USGS2015

Great (≥Mw8.0) megathrust earthquakes and the subduction of excess sediment and bathymetrically smooth seafloor

Using older and in part flawed data, Ruff (1989) suggested that thick sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling. This circumstance was conjectured to favor rupture continuation and the generation of high-magnitude (≥Mw8.0) interplate thrust (IPT) earthquakes. Using larger and more accurate compilations of sediment thickness and instrumental (1899 to January 2013) and pre-instrumental era (1700–1898) IPTs (n = 176 and 12, respectively), we tested if a compelling relation existed between where IPT earthquakes ≥Mw7.5 occurred and where thick (≥1.0 km) versus thin (≤1.0 km) sedimentary sections entered the SZ. Based on the new compilations, a statistically supported statement (see Summary and Conclusions) can be made that high-magnitude earthquakes are most prone to nucleate at well-sedimented SZs. For example, despite the 7500 km shorter global length of thick-sediment trenches, they account for &sim;53% of instrumental era IPTs ≥Mw8.0, &sim;75% ≥Mw8.5, and 100% ≥Mw9.1. No megathrusts >Mw9.0 ruptured at thin-sediment trenches, whereas three occurred at thick-sediment trenches (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2). However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of high-magnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation.

Geosphere↗

USGS2014

The destructive 1946 Unimak near-field tsunami: New evidence for a submarine slide source from reprocessed marine geophysical data

The M w 8.6 earthquake in 1946 off the Pacific shore of Unimak Island at the end of the Alaska Peninsula generated a far-field tsunami that crossed the Pacific to Antarctica. Its tsunami magnitude, 9.3, is comparable to the 9.1 magnitude of the 2011 Tohoku tsunami. On Unimak Island's Pacific shore, a runup of 42 m destroyed the lighthouse at Scotch Cap. Elsewhere, localized tsunamis with such high runups have been interpreted as caused by large submarine landslides. However, previous to this study, no landslide large enough to generate this runup was found in the area that is limited by the time interval between earthquake shaking and tsunami inundation at Scotch Cap. Reworking of a seismic reflection transect and colocated multibeam bathymetric surveys reveal a landslide block that may explain the 1946 high runup. It is seaward of Scotch Cap on the midslope terrace and within the time-limited area.

Geophysical Research Letters↗

USGS2014

The 1946 Unimak Tsunami Earthquake Area: revised tectonic structure in reprocessed seismic images and a suspect near field tsunami source

In 1946 at Unimak Pass, Alaska, a tsunami destroyed the lighthouse at Scotch Cap, Unimak Island, took 159 lives on the Hawaiian Islands, damaged island coastal facilities across the south Pacific, and destroyed a hut in Antarctica. The tsunami magnitude of 9.3 is comparable to the magnitude 9.1 tsunami that devastated the Tohoku coast of Japan in 2011. Both causative earthquake epicenters occurred in shallow reaches of the subduction zone. Contractile tectonism along the Alaska margin presumably generated the far-field tsunami by producing a seafloor elevation change. However, the Scotch Cap lighthouse was destroyed by a near-field tsunami that was probably generated by a coeval large undersea landslide, yet bathymetric surveys showed no fresh large landslide scar. We investigated this problem by reprocessing five seismic lines, presented here as high-resolution graphic images, both uninterpreted and interpreted, and available for the reader to download. In addition, the processed seismic data for each line are available for download as seismic industry-standard SEG-Y files. One line, processed through prestack depth migration, crosses a 10 × 15 kilometer and 800-meter-high hill presumed previously to be basement, but that instead is composed of stratified rock superimposed on the slope sediment. This image and multibeam bathymetry illustrate a slide block that could have sourced the 1946 near-field tsunami because it is positioned within a distance determined by the time between earthquake shaking and the tsunami arrival at Scotch Cap and is consistent with the local extent of high runup of 42 meters along the adjacent Alaskan coast. The Unimak/Scotch Cap margin is structurally similar to the 2011 Tohoku tsunamigenic margin where a large landslide at the trench, coeval with the Tohoku earthquake, has been documented. Further study can improve our understanding of tsunami sources along Alaska’s erosional margins.

Alaska↗

USGS2013

Alaska earthquake source for the SAFRR tsunami scenario: Chapter B in The SAFRR (Science Application for Risk Reduction) Tsunami Scenario

Tsunami modeling has shown that tsunami sources located along the Alaska Peninsula segment of the Aleutian-Alaska subduction zone have the greatest impacts on southern California shorelines by raising the highest tsunami waves for a given source seismic moment. The most probable sector for a M w ~ 9 source within this subduction segment is between Kodiak Island and the Shumagin Islands in what we call the Semidi subduction sector; these bounds represent the southwestern limit of the 1964 M w 9.2 Alaska earthquake rupture and the northeastern edge of the Shumagin sector that recent Global Positioning System (GPS) observations indicate is currently creeping. Geological and geophysical features in the Semidi sector that are thought to be relevant to the potential for large magnitude, long-rupture-runout interplate thrust earthquakes are remarkably similar to those in northeastern Japan, where the destructive M w 9.1 tsunamigenic earthquake of 11 March 2011 occurred. In this report we propose and justify the selection of a tsunami source seaward of the Alaska Peninsula for use in the Tsunami Scenario that is part of the U.S. Geological Survey (USGS) Science Application for Risk Reduction (SAFRR) Project. This tsunami source should have the potential to raise damaging tsunami waves on the California coast, especially at the ports of Los Angeles and Long Beach. Accordingly, we have summarized and abstracted slip distribution from the source literature on the 2011 event, the best characterized for any subduction earthquake, and applied this synoptic slip distribution to the similar megathrust geometry of the Semidi sector. The resulting slip model has an average slip of 18.6 m and a moment magnitude of M w = 9.1. The 2011 Tohoku earthquake was not anticipated, despite Japan having the best seismic and geodetic networks in the world and the best historical record in the world over the past 1,500 years. What was lacking was adequate paleogeologic data on prehistoric earthquakes and tsunamis, a data gap that also presently applies to the Alaska Peninsula and the Aleutian Islands. Quantitative appraisal of potential tsunami sources in Alaska requires such investigations.

Alaska↗

USGS2012

Geophysical investigation of a Suture Zone: The Border Ranges Fault of southern Alaska

The Border Ranges fault separates structurally complex accreted Cretaceous rocks from less deformed middle or late Paleozoic and younger rocks in the Cook-Shelikof basin. Of the five types of geophysical data used to investigate this fault, gravity data give the clearest indication of its presence and crustal structure. For at least 400 km along the fault, gravity anomalies include a +20 to +30 mGal peak along the fault's upper plate and a −40 mGal trough along the lower plate. The paired anomaly can be modeled satisfactorily by a simple step, in a deep dense layer, that lies within 3 km of the projected offshore location of the fault. Relatively low-density rocks lie along the fault's lower plate to a depth of about 10 km, and the upper part of the fault dips within 20° of vertical. Satellite altimetry data show that two circular geoid lows lie along the Border Ranges fault and coincide with lows in free air gravity data. Seismic refraction and seismic reflection data suggest that the large-scale density anomalies that cause both types of lows must lie at depths greater than about 1 km within the margin. Three regional magnetic anomalies (Knik Arm, Seldovia, and Shelikof) terminate at the Border Ranges fault, suggesting that the fault truncates obliquely rocks that lie along its northwest side. Six seismic reflection lines cross the Border Ranges fault, but none of them shows reflections from it. The absence of such reflections probably results from the fault's steep dip and from the presence of strong water bottom multiples in the data. From the Late Jurassic until the early Late Cretaceous, the magmatic arc near the Cook-Shelikof basin was inactive, and we infer that the predominant motion along the Border Ranges fault was strike slip. Resurgent Late Cretaceous magmatism was contemporaneous with uplift of rocks along the northwest side of the Border Ranges fault and with deformation of turbidite sequences in the fault's lower plate. We propose that during the Late Cretaceous, motion along the Border Ranges changed from strike slip to reverse. Cenozoic rocks near the fault show no evidence for post-Cretaceous fault movement.

Journal of Geophysical Research Solid Earth↗

USGS2012

Reflections from midcrustal rocks within the Mesozoic subduction complex near the eastern Aleutian Trench

Seismic reflection data collected in 1973 by Western Geophysical Company show that highly reflective rocks make up the midcrust of the convergent margin adjacent to the eastern Aleutian Trench. These rocks form an arch that strikes obliquely across the strongly expressed northeast-southwest structural grain of exposed Mesozoic rocks. In an earlier report we proposed that the deep events mark the location of either the presently subducting plate or the top of underplated rocks. However, the short radius of curvature of this arch precludes the possibility that the deep events indicate the igneous oceanic crust. Instead, the deep reflections could be from underplated rocks that have been arched by the imbrication or underplating of strata below the reflective rocks. High-amplitude water layer and interbed multiples prevent precise connection of shallow and deep geology. For example, the Border Ranges fault, a suture between major tectonostratigraphic terranes, is not revealed on any of our seismic sections. We speculate, however, that one band of reflections that rises toward but does not reach the surface is from the Eagle River thrust fault, which separates Late Cretaceous melange from deformed turbidite sequences of the same age.

Journal of Geophysical Research Solid Earth↗

USGS2012

Tsunami hazards to U.S. coasts from giant earthquakes in Alaska

In the aftermath of Japan's devastating 11 March 2011M w 9.0 Tohoku earthquake and tsunami, scientists are considering whether and how a similar tsunami could be generated along the Alaskan-Aleutian subduction zone (AASZ). A tsunami triggered by an earthquake along the AASZ would cross the Pacific Ocean and cause extensive damage along highly populated U.S. coasts, with ports being particularly vulnerable. For example, a tsunami in 1946 generated by a M w 8.6 earthquake near Unimak Pass, Alaska (Figure 1a), caused significant damage along the U.S. West Coast, took 150 lives in Hawaii, and inundated shorelines of South Pacific islands and Antarctica [ Fryer et al. , 2004; Lopez and Okal , 2006]. The 1946 tsunami occurred before modern broadband seismometers were in place, and the mechanisms that created it remain poorly understood.

Eos, Transactions, American Geophysical Union↗

USGS2012

Subducting plate geology in three great earthquake ruptures of the western Alaska margin, Kodiak to Unimak

Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.

Alaska↗

USGS2012

History of earthquakes and tsunamis along the eastern Aleutian-Alaska megathrust, with implications for tsunami hazards in the California Continental Borderland

During the past several years, devastating tsunamis were generated along subduction zones in Indonesia, Chile, and most recently Japan. Both the Chile and Japan tsunamis traveled across the Pacific Ocean and caused localized damage at several coastal areas in California. The question remains as to whether coastal California, in particular the California Continental Borderland, is vulnerable to more extensive damage from a far-field tsunami sourced along a Pacific subduction zone. Assuming that the coast of California is at risk from a far-field tsunami, its coastline is most exposed to a trans-Pacific tsunami generated along the eastern Aleutian-Alaska subduction zone. We present the background geologic constraints that could control a possible giant (M w ~9) earthquake sourced along the eastern Aleutian-Alaska megathrust. Previous great earthquakes (M w ~8) in 1788, 1938, and 1946 ruptured single segments of the eastern Aleutian-Alaska megathrust. However, in order to generate a giant earthquake, it is necessary to rupture through multiple segments of the megathrust. Potential barriers to a throughgoing rupture, such as high-relief fracture zones or ridges, are absent on the subducting Pacific Plate between the Fox and Semidi Islands. Possible asperities (areas on the megathrust that are locked and therefore subject to infrequent but large slip) are identified by patches of high moment release observed in the historical earthquake record, geodetic studies, and the location of forearc basin gravity lows. Global Positioning System (GPS) data indicate that some areas of the eastern Aleutian-Alaska megathrust, such as that beneath Sanak Island, are weakly coupled. We suggest that although these areas will have reduced slip during a giant earthquake, they are not really large enough to form a barrier to rupture. A key aspect in defining an earthquake source for tsunami generation is determining the possibility of significant slip on the updip end of the megathrust near the trench. Large slip on the updip part of the eastern Aleutian-Alaska megathrust is a viable possibility owing to the small frontal accretionary prism and the presence of arc basement relatively close to the trench along most of the megathrust.

Alaska. California↗

USGS2011

Discussions of ODP Leg 205 and drilling of Middle America Seismogenic Zone

Erosional convergent margins, where material is removed from the base of the upper plate and subducted on the lower plate, are fundamental features of the Circum Pacific. The erosional Middle America Trench convergent Pacific margin, remarkable for its broad diversity of dynamic environments, is a natural laboratory for studying convergent margin processes and seismogenesis. These environments include a shallow and deep trench axis, shallow-to-steep-dipping plate interfaces, abnormally hot-to-cold subducting plate temperatures, and a subducting plate with smooth morphology bordering basement ridges and seamounts. The subducting topography accelerates erosion and localizes seismicity.

Middle America Trench, Pacific Ocean↗

USGS2010

Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (∼19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-medium-sized prisms have formed (∼16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (∼8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (∼43,500 km) is calculated to be 1.5 km³/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km³/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km³/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km³/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km³/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km³/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km³/yr to a total of 1.1 km³/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km³/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted material is estimated to range from 1.3 to 1.8 km³/yr. Subcrustally subducted material is either returned to the terrestrial crust by arc-related igneous processes or crustal underplating or is lost from the crust by mantle absorption. Geochemical and isotopic data support the notion that upper mantle melting returns only a small percent of the subducted material to the terrestrial crust as arc igneous rocks. Limited areal exposures of terrestrial rocks metamorphosed at deep (>20–30 km) subcrustal pressures and temperatures imply that only a small fraction of subducted material is reattached via deep crustal underplating. Possibly, therefore much of the subducted terrestrial material is recycled to the mantle at a rate near 1.6 km³/yr, which is effectively equivalent to the commonly estimated rate at which the mantle adds juvenile igneous material to the Earth's layer of terrestrial rock.

Alaska↗

USGS2009

Implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary (non-collisional) and collisional (suturing) orogens

Arc magmatism at subduction zones (SZs) most voluminously supplies juvenile igneous material to build rafts of continental and intra-oceanic or island arc (CIA) crust. Return or recycling of accumulated CIA material to the mantle is also most vigorous at SZs. Recycling is effected by the processes of sediment subduction, subduction erosion, and detachment and sinking of deeply underthrust sectors of CIA crust. Long-term (>10-20 Ma) rates of additions and losses can be estimated from observational data gathered where oceanic crust underruns modern, long-running (Cenozoic to mid-Mesozoic) ocean-margin subduction zones (OMSZs, e.g. Aleutian and South America SZs). Long-term rates can also be observationally assessed at Mesozoic and older crust-suturing subduction zone (CSSZs) where thick bodies of CIA crust collided in tectonic contact (e.g. Wopmay and Appalachian orogens, India and SE Asia). At modern OMSZs arc magmatic additions at intra-oceanic arcs and at continental margins are globally estimated at c. 1.5 AU and c. 1.0 AU, respectively (1 AU, or Armstrong Unit,= 1 km 3 a -1 of solid material). During collisional suturing at fossil CSSZs, global arc magmatic addition is estimated at 0.2 AU. This assessment presumes that in the past the global length of crustal collision zones averaged c. 6000 km, which is one-half that under way since the early Tertiary. The average long-term rate of arc magmatic additions extracted from modern OMSZs and older CSSZs is thus evaluated at 2.7 AU. Crustal recycling at Mesozoic and younger OMSZs is assessed at c. 60 km 3 Ma -1 km -1 (c. 60% by subduction erosion). The corresponding global recycling rate is c. 2.5 AU. At CSSZs of Mesozoic, Palaeozoic and Proterozoic age, the combined upper and lower plate losses of CIA crust via subduction erosion, sediment subduction, and lower plate crustal detachment and sinking are assessed far less securely at c. 115 km 3 Ma -1 km -1 . At a global length of 6000 km, recycling at CSSZs is accordingly c. 0.7 AU. The collective loss of CIA crust estimated for modern OMSZs and for older CSSZs is thus estimated at c. 3.2 AU. SZ additions (2.7 AU) and subtractions (23.2 AU) are similar. Because many uncertainties and assumptions are involved in assessing and applying them to the deep past, the net growth of CIA crust during at least Phanerozoic time is viewed as effectively nil. With increasing uncertainty, the long-term balance can be applied to the Proterozoic, but not before the initiation of the present style of subduction at c. 3 Ga. Allowing that since this time a rounded-down rate of recycling of 3 AU is applicable, a startlingly high volume of CIA crust equal to that existing now has been recycled to the mantle. Although the recycled volume (c. 9 ?? 10 9 km 3 ) is small (c. 1%) compared with that of the mantle, it is large enough to impart to the mantle the signature of recycled CIA crust. Because subduction zones are not spatially fixed, and their average global lengths have episodically been less or greater than at present, recycling must have contributed significantly to creating recognized heterogeneities in mantle geochemistry. ?? The Geological Society of London 2009.

Geological Society Special Publication↗

USGS2009

Neogene collision and deformation of convergent margins along the backbone of the Americas

Along Pacific convergent margins of the Americas, high-standing relief on the subducting oceanic plate "collides" with continental slopes and subducts. Features common to many collisions are uplift of the continental margin, accelerated seafloor erosion, accelerated basal subduction erosion, a flat slab, and a lack of active volcanism. Each collision along America's margins has exceptions to a single explanation. Subduction of an ???600 km segment of the Yakutat terrane is associated with >5000-m-high coastal mountains. The terrane may currently be adding its unsubducted mass to the continent by a seaward jump of the deformation front and could be a model for docking of terranes in the past. Cocos Ridge subduction is associated with >3000-m-high mountains, but its shallow subduction zone is not followed by a flat slab. The entry point of the Nazca and Juan Fernandez Ridges into the subduction zone has migrated southward along the South American margin and the adjacent coast without unusually high mountains. The Nazca Ridge and Juan Fernandez Ridges are not actively spreading but the Chile Rise collision is a triple junction. These collisions form barriers to trench sediment transport and separate accreting from eroding segments of the frontal prism. They also occur at the separation of a flat slab from a steeply dipping one. At a smaller scale, the subduction of seamounts and lesser ridges causes temporary surface uplift as long as they remain attached to the subducting plate. Off Costa Rica, these features remain attached beneath the continental shelf. They illustrate, at a small scale, the processes of collision. ?? 2009 The Geological Society of America. All rights reserved.

Conference Paper↗