John B. Diebold

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.

Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge

New seismic observations of crustal structure along the Juan de Fuca Ridge indicate that the axial rift topography reflects magma-induced deformation rather than alternating phases of magmatism and tectonic extension, as previously proposed. Contrary to predictions of the episodic models, crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2.1–2.6 km. The shallow rift valley or axial graben associated with each Juan de Fuca segment is ∼50–200 m deep and 1–8 km wide and is well correlated with a magma body in the subsurface. Analysis of graben dimensions (height and width) shows that the axial graben narrows and graben height diminishes where the magma body disappears, rather than deepening and broadening, as expected for rift topography due to tectonic extension. We propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor. In this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

Frozen magma lenses below the oceanic crust

The Earths oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust. Thermal modelling, tomography, compliance and wide-angle seismic studies, supported by geological evidence, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of ∼1,500 km of multichannel seismic data collected across the intermediate-spreading-rate Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

Wide-angle seismic imaging across accreted terranes, southeastern Alaska and western British Columbia

Abstract This study addresses the question of crustal, Moho, and uppermost mantle structure across an accreted terrane, continental arc, and fold and thrust belt in southeastern Alaska and western British Columbia. The 186-km-long Portland Canal line of the ACCRETE wide-angle seismic dataset across the Coast Mountains is analyzed using a combination of travel-time analysis techniques, including delay-time tomography, turning ray tomography, reciprocal time analysis, intercept-time inversion, and forward ray tracing. Additional constraints on the model are obtained from a coincident vertical-incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section. The study reveals moderately high crustal velocities, low-velocity gradient in the middle crust, and decreasing average crustal velocity and a north-deepening Moho. Termination of crustal reflectivity across a vertical zone (the Coast Shear Zone, CSZ) indicates that the CSZ is most likely a strike-slip fault associated with a transpressive tectonic regime. A mid-Cretaceous thrust system mapped on the surface to the west of the CSZ is imaged by several groups of mid- to lower crustal reflectors extending close to the Moho indicating it was a thick-skinned thrust system. NE-dipping fabric imaged within the Coast Mountains batholith (CMB) is associated with a ductile deformation during Early Eocene crustal extension. The crustal section under the CMB, which has an average velocity of 6.55 km/s and shallower than average crustal thickness of 31 km, can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high-velocity tonalite to gabbro sills.

Offshore sedimentary effects of the 12 January 2010 Haiti earthquake

Although the 12 January 2010 Haiti earthquake was one of the deadliest earthquakes in history, it left no clear geological evidence of rupture on land. As a tectonic event, the earthquake was complex; even the faults involved remain unclear. Using geophysical and coring data, we document direct evidence of the sedimentation generated by the catastrophic 12 January 2010 earthquake offshore. These studies document submarine paleoseismology methods that can be used for assessing seismic risk in this and other tectonic settings such as the California San Andreas fault, where deeper buried blind thrusts may exist. Shaking by the 12 January main shock triggered sediment failures and turbidity currents from coastal sources to deep-water sinks. An ~0.05 km 3 turbidite was deposited in the Canal du Sud basin (1750 m water depth) over 50 km 2 . Almost 2 months after the main shock, a 600-m-thick sediment plume was still present in the lowermost water column at this location. The turbidite was time correlated to the 12 January earthquake by the excess 234 Th in the sediments. With a half-life of 24 days, its presence documents an infl ux of terrigenous sediment mixing with marine sources derived from the basin slopes. This turbidite, and older ones observed beneath it, displays complex cross-bedded and fi ning-upward stratigraphy indicative of long waves and seiche oscillations that are consistent with locally reported tsunamis. This 12 January sedimentary record highlights the potential for submarine paleoseismology to unravel the seismic history of continental transform boundaries such as the Enriquillo‐Plantain Garden fault in the Dominican Republic, Haiti, and Jamaica, as well as other tectonic settings where no clear land-based evidence for a rupture exists.

Active deformation and shallow structure of the Wagner, Consag, and Delfín Basins, northern Gulf of California, Mexico

Oblique rifting began synchronously along the length of the Gulf of California at 6 Ma, yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern Gulf. Instead, multichannel seismic data show a broad shallow depression, ∼70 × 200 km, marked by active distributed deformation and six ∼10-km-wide segmented basins lacking well-defined transform faults. We present detailed images of faulting and magmatism based on the high resolution and quality of these data. The northern Gulf crust contains a dense (up to 18 faults in 5 km) complex network of mainly oblique-normal faults, with small offsets, dips of 60–80° and strikes of N-N30°E. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfin Basins. These subparallel basins developed along splays from a transtensional zone at the NW end of the Ballenas Transform Fault. Twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone. A structural connection between the two Upper Delfin Basins is evident in the switching of the center of extension along axis. Sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material, atypical of mid-ocean ridges. On the basis of the near-surface manifestations of active faulting and magmatism, seafloor spreading will likely first occur in the Lower Delfin Basin. We suggest the transition to seafloor spreading is delayed by the lack of strain-partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover.

Chapter 20 Tectonic and stratigraphic development of the eastern caribbean: New constraints from multichannel seismic data

Abstract New high-resolution multichannel seismic reflection data collected in the eastern Caribbean during R/V Ewing cruise 9501 imaged both the crustal structure and overlying stratigraphic successions. On the basis of these new MCS data, we define the geologic development of the Beata Ridge and Venezuelan Basin. The proto-Caribbean crust was formed by seafloor spreading in Late Jurassic-Early Cretaceous time. Prior to the Senonian, widespread and rapid eruption of basaltic flows began in concert with extensional deformation of the proto-Caribbean crust. Large divergent volcanic wedges observed along the rough-smooth B″ boundary are coincident with the abrupt shoaling of Moho and appear to be bounded by a large northwest-dipping fault system. The location of the major extensional deformation migrated through time from the Venezuelan Basin to the western flank of the Beata Ridge. Extensional unloading of the Beata Ridge footwall caused uplift and rotation of the ridge and collapse of its hangingwall (i.e., Hess Escarpment). Sediment thicknesses and stratal geometry observed across the Venezuelan Basin and Beata Ridge suggest that the majority of the deformation in this region occurred soon after the emplacement of the volcanics. Minor fault reactivation in the Neogene along the eastern flank of the Beata Ridge is associated with an accommodation zone (i.e., tear fault) that records a change in the deformation style from bending and subduction of the Caribbean Plate along the Muertos Trough south of Puerto Rico to compressional deformation and obduction of the Caribbean Plate south of Hispaniola. We propose that this difference in deformational style is, in part, a consequence of the thicker crust on the Beata Ridge, which is more resistant to subduction. Coincident with the rough-smooth crustal boundary in the Venezuelan Basin is a marked change in sediment thickness. The stratal geometry and spatial distribution of the basal sequence suggest that the Late Cretaceous to Early Miocene sediments are terrigenous deposits derived from South America. We propose that during the Late Cretaceous to Early Miocene the dominant drainage in South America was a northward-flowing axially parallel fluvial system that drained the Maracaibo-Peruvian foreland basin east of the Andes and supplied sediment to the Venezuelan Basin. The middle Tertiary uplift and deformation along the northern South America Plate boundary blocked this axial-parallel fluvial network. As a result of the blockage, the axial-parallel rivers were dammed or diverted eastward. This blockage, together with the renewed uplift of the Central and Eastern Cordillera, supplied abundant sediment to the developing foreland basin and other paleo-structural lows to the east. The consequent regrading of the fluvial systems as the foreland basin was filled allowed drainage systems to flow east across the South American continent (e.g., Amazon and Orinoco) and deliver sediment to the Atlantic Ocean. The seismic reflection data also imaged an Eocene-Early Miocene current-controlled drift deposit which reflects the movement of bottom currents from the eastern Pacific to the Caribbean during this period. The gradual shoaling of the Central American Isthmus in Late Oligocene-Early Miocene time closing the gateway is consistent with the diminished occurrence of current-controlled features upsection and paleo- deep and intermediate water geochemistry in DSDP cores.

Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data

“Wave” fi elds have long been recognized in marine sediments on the fl anks of basins and oceans in both tectonically active and inactive environments. The origin of “waves” (hereafter called undulations) is controversial; competing models ascribe them to depositional processes, gravity-driven downslope creep or collapse, and/or tectonic shortening. Here we analyze pervasive undulation fi elds identifi ed in swath bathymetry and new high-resolution multichannel seismic (MCS) refl ection data from the Sea of Marmara, Turkey. Although they exhibit some of the classical features of sediment waves, the following distinctive characteristics exclude a purely depositional origin: (1) parallelism between the crests of the undulations and bathymetric contours over a wide range of orientations, (2) steep fl anks of the undulations (up to ~40°), and (3) increases in undulations amplitude with depth. We argue that the undulations are folds formed by gravity-driven downslope creep that have been augmented by depositional processes. These creep folds develop over long time periods (≥0.5 m.y.) and stand in contrast to geologically instantaneous collapse. Stratigraphic growth on the upslope limbs indicates that deposition contributes to the formation and upslope migration of the folds. The temporal and spatial evolution of the creep folds is clearly related to rapid tilting in this tectonically active transform basin.

Deformation of the Caribbean region: One plate or two?

New deep-penetrating high-resolution multichannel seismic reflection data collected in the eastern Caribbean during R/V Ewing cruise EW9501 imaged both the crustal structure and overlying stratigraphic successions. On the basis of this new multichannel seismic data, we define the geologic development of the Beata Ridge and Venezuelan basin. The Caribbean crust was formed by seafloor spreading in Late Jurassic–Early Cretaceous time. Prior to the Senonian, widespread and rapid eruption of basaltic flows began in concert with extensional deformation of the Caribbean crust. Thick volcanic wedges characterized by divergent reflectors are observed along the boundary that separates rough from smooth oceanic crust, are coincident with an abrupt shallowing of the Moho, and appear to be bounded by a large, northwest-dipping fault system. The locus of major extensional deformation migrated through time from the Venezuelan basin to the western flank of the Beata Ridge. Extensional unloading of the Beata Ridge footwall caused uplift and rotation of the ridge. Sediment thicknesses and stratal geometry observed across the Venezuelan basin and Beata Ridge suggest that the majority of the deformation in this region occurred during and soon after the emplacement of the volcanics. Minor fault reactivation in the Neogene along the eastern flank of the Beata Ridge is associated with an accommodation zone (i.e., tear fault) that records a change in the deformation style from bending and subduction of the Caribbean plate along the Muertos Trough south of Puerto Rico to compressional deformation and obduction of the Caribbean plate south of Hispaniola. We propose that this difference in deformational style is, in part, a consequence of the thicker crust on the Beata Ridge, which is more resistant to subduction.

Oligocene development of the West Antarctic Ice Sheet recorded in eastern Ross Sea strata

Seismic-reflection data from the easternmost Ross Sea image buried scour-and-fill troughs and flat-topped ridges interpreted as having formed by glacial erosion and deposition during the Oligo-cene. The NNW-SSE orientation of the troughs and lack of similar Oligocene glacial features within the central Ross Sea suggests that the ice issued from the highlands of Marie Byrd Land located 100 km away and that portions of the West Antarctic Ice Sheet formed earlier than previously accepted. Existing global climate models (GCMs) do not produce West Antarctic ice caps for the Oligocene, in part due to low elevations modeled for that time. Evidence for Oligocene ice beyond the paleocoast suggests a higher elevation for the early Cenozoic Marie Byrd Land and Ross Embayment than at present.