Susan McGeary

Structure and composition of the Aleutian island arc and implications for continental crustal growth

We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a continental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two.

Spatial gaps in arc volcanism: The effect of collision or subduction of oceanic plateaus

Abstract One of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism. The plate-tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone. However, the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains (> 200 km). A significant proportion (~ 30%) of oceanic crust is subducted at these gaps. All but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge. The idea that the collision of such features may have a major tectonic impact on the arc lithosphere, including cessation of volcanism, is not new. However, it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism. Three main factors necessary for arc volcanism are (1) source materials for the volcanics—either volatiles or melt from the subducting slab and/or melt from the overlying asthenospheric wedge, (2) a heat source, either for the dehydration or the melting of the slab, or the melting within the asthenosphere and (3) a favorable state of stress in the overlying lithosphere. The absence of any one of these features may cause a volcanic gap to form. There are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism. The clearest and most common cases considered are those where the feature completely resists subduction, causing local plate boundaries to reorganize. This includes the formation of new plate-bounding transform faults or a flip in subduction polarity. In these cases, subduction has slowed down or stopped and the lack of source material has created a volcanic gap. There are a few cases, most notably in Peru, Chile, and the Nankai trough, where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism. The shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere. The cases which are the most enigmatic are those where subduction is continuous, the oceanic plateau is subducted along with the slab, and the dip of the slab is clearly steep enough to allow arc volcanism; yet a volcanic gap exists. In these areas, the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself. The presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates. Alternatively, the subduction of the plateau may change the chemical processes at depth if either the water-rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different.

Crustal construction of a volcanic arc, wide‐angle seismic results from the western Alaska Peninsula

[1] Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5- to 10-km-thick upper crustal carapace of low velocity (2-5.8 km s -1 ) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (∼7.0 km s -1 ) of variable thickness, for a maximum arc crust thickness of,25-30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s -1 ) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ∼1, and the total arc crust volume implies a magmatic flux of ∼67 km 3 km -1 m.y. -1 , more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ∼6.4 km s -1 , suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.

Cenozoic normal faulting and the shallow structure of the Rio Grande rift near Socorro, New Mexico

Migrated versions of a deep seismic section across the Albuquerque basin in the southern Rio Grande rift (COCORP Abo Pass line 1 and Socorro line 1 A) are used to develop a detailed interpretation of the shallow structure of the rift. Our interpretation, which is consistent with nearby drill-hole and gravity data, suggests that listric faulting has been the dominant extensional style of faulting in this part of the rift. During the early stages of rifting, blocks of Paleozoic and Mesozoic prerift sedimentary strata overlying Precambrian crystalline basement were offset and rotated along listric normal faults rooted in the basement. Syntectonic deposition of thick sections of mid-Tertiary sedimentary and volcanic rocks filled the developing fault-bounded basins. These units were continuously rotated during ongoing listric faulting and subsequently buried by later Tertiary and Quaternary basin fill. Present surface fault traces may reflect continued movement on the deeper listric faults or a more recent minor episode of shallow normal faulting. In contrast to models of crustal extension proposed for nearby areas in the Rio Grande rift and Basin and Range province, no clear evidence is seen for two time-distinct stages of faulting characterized by early closely spaced faulting with large stratal tilts followed by wide-spaced high-angle faulting. The large listric faults flatten into basement or converge upon an apparent detachment surface at a depth of about 5 km, posing intriguing questions about the connection between shallow structure and deeper extension within the crust.

Reflections from mantle fault zones around the British Isles

British Institutions Reflection Profiling Syndicate (BIRPS) deep seismic reflection surveys show that the upper mantle around the British Isles is reflective, exhibiting discrete, isolated reflecting zones within an otherwise seismically transparent mantle. There are 20 known occurrences of reflectors within the upper mantle across the continental shelf of the British Isles, many delimited by intersecting or closely spaced profiles. Most of the mantle reflectors have similar reflection characteristics: they dip 10{degree} to 30 {degree} and are thin ({approximately}0.5 s ({approximately} 2 km)) and discrete. The geologic history of the continental shelf around the British Isles (an area 1000 x 1000 km) is complex, and the tectonic settings of these reflectors very. Each mantel reflector is associated with a major zone of deformation in the crust. This association, the dips of the reflectors, and their high reflectivity all indicate a structural origin. The BIRPS mantle reflectors include probable early Paleozoic thrusts and Mesozoic normal faults. In contrast to BIRPS data, deep reflection profiles in other areas of the world, in a variety of geologic settings, have only rarely imaged reflectors within the upper mantle.

Seismic reflection imaging of ultradeep roots beneath the eastern Aleutian island arc

Seismic reflection data show that the eastern Aleutian Arc is characterized by reflectors that extend continuously from the lower arc crust to >50 km depth, which is considerably deeper than the crustal thickness of 27–35 km previously inferred from coincident wide-angle seismic surveys. Because the upper mantle is commonly homogeneous, and therefore nonreflective, relative to the overlying crust, we interpret these reflectors to be gabbro, garnet gabbro, and pyroxenite intrusions within two 50-km-wide roots that represent a >25-km-thick heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks. We suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle-derived melts into the subarc lithosphere, and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined Moho. Because these deep roots are not evident beneath the central Aleutian Arc, either the roots form sporadically, perhaps as a consequence of an elevated magmatic supply, or such roots ultimately founder into the underlying mantle due to their relatively high mass density.

Allochthonous terranes in Alaska: Implications for the structure and evolution of the Bering Sea shelf

Abundant evidence has been gathered in support of the concept that much of the Pacific margin of North America consists of allochthonous terranes that were accreted during Mesozoic and early Tertiary time. In particular, southern Alaska is almost completely composed of separate terranes whose stratigraphy and paleomagnetism indicate distant origins. The Bering Sea continental shelf is among the largest shelves in the world, almost half as large as Alaska itself. Some of the allochthonous terranes in Alaska probably continue beneath the continental shelf, but their distribution beyond the shoreline has not been determined. We believe that much of the continental shelf is probably composed of allochthonous terranes, in much the same way as southern Alaska. Geophysical data from the Bering continental shelf have been used to test this hypothesis. Although inconclusive without extensive drilling, multichannel seismic profiles and magnetic data indicate the composite character of the shelf basement. The magnetic anomalies can be separated into specific domains of limited extent. These magnetic variations suggest a corresponding variation of rock terranes beneath the shelf. In addition, multichannel seismic profiles show possible thrust faults within the shelf basement that could be sutures between separate terranes.

High Resolution Seismic Reflection Images of New Jersey Coastal Aquifers

A high‐resolution seismic reflection experiment was conducted on the barrier islands of New Jersey to study the stratigraphy and physical properties of four regionally important aquifers. Five multichannel profiles, totaling 5.4 km in length, were collected from Island Beach State Park to Shipbottom. Careful selection of acquisition and processing parameters produced very high resolution profiles with penetration depths to 186 m. The average wavelet frequency of 225 Hz provided average quarter‐wavelength resolution of 1.9 m; in some places, recorded frequencies of up to 400–425 Hz allowed individual sand and clay layers less than a meter thick to be resolved. Synthetic seismograms were generated from geophysical logs from nearby wells for comparison with the seismic data and to confirm interpretations.All aquifers and confining units of interest are resolved in detail on the profiles. Typical aquifer responses include strong, continuous reflection peaks at the tops of sand bodies and a less distinguishabl...