Kristin M. M. Rohr

Asymmetric deep crustal structure across the Juan de Fuca Ridge

A multichannel line was shot across the Endeavour segment of the Juan de Fuca Ridge. Careful processing of 25 km of the line centered about the rise axis shows that young crustal structure is asymmetric. An intermittent reflector from the top 1 km of crust follows topography on both the Pacific and the Juan de Fuca plates. It could be either the pillow basalt-diabase dike contact or a metamorphic front within the pillow basalts. A reflection exists 2-3 km below the rift-valley floor; diffractions from its edges indicate that the rift valley9s velocity structure is markedly different from the structure of the flanks. The deep crustal structure is highly asymmetric. Moho exists within 4 km of the rift valley on the Pacific plate, in contrast to the Juan de Fuca plate, which shows a reflector at intermediate depths dipping away from the rise to reach Moho depth 12 km from the rift valley. This reflector9s origin is enigmatic; we speculate that the thermal history and structure of the two plates differ because of local differences in mantle structure caused by either ridge migration or a melting anomaly.

Increase of seismic velocities in upper oceanic crust and hydrothermal circulation in the Juan de Fuca plate

Two multi-channel seismic reflection profiles have been analyzed for interval velocities and thicknesses of seismic layer 2A in crust 0–4.5 Ma created at the Endeavour segment of the Juan de Fuca plate. Stacking velocities were interpreted using the Dix assumption for interval velocity and thickness. Stacking velocities were picked from constant velocity stacks as well as normally moved-out gathers at an interval of .2 to 2 km. Interval velocities are 3000–3500 m/s close to the ridge axis and begin to increase at 0.6 Ma when the crust is covered with a few hundred meters of Pleistocene sediments. Velocities are over 5000 m/s by 1.2 Ma, 20 km from the nearest basement outcrop. This increase is associated with an increase in basement temperature from 2–25°C to 30–50°C as heat flow values approach the predicted conductive cooling curve and the basement hydrothermal regime changes from fully open to mostly closed. Mineralisation in the abundant porosity of the upper crust is probably triggered by these thermal changes and results in increased seismic velocity. This is the first study to correlate an increase in seismic velocity with the hydrothermal regime in upper oceanic crust and demonstrates that such changes can occur in a remarkably short time.

Ephemeral plate tectonics at the Queen Charlotte triple junction

Three plate boundaries, the Queen Charlotte transform, the Cascadia subduction zone, and the Juan de Fuca Ridge, meet in a complex triple junction offshore Vancouver Island. Some interpretations of the plate tectonics of the region have included the Explorer ridge, the Dellwood Knolls, and the Tuzo Wilson volcanic field as part of the spreading system, requiring up to three triple junctions in the region. New SeaBeam bathymetry interpreted with existing regional data sets indicate that the Dellwood Knolls and Tuzo Wilson volcanic field are not independent plate boundaries, but the result of leaky transform tectonics between two overlapping transform faults. Seismicity shows that the Explorer plate is being deformed by Pacific–North American relative motion as a new transform plate boundary forms and cuts off the Explorer ridge. The system is evolving to a single triple junction at the northern terminus of the Juan de Fuca Ridge where it meets the Nootka deformation zone. Thus the Explorer microplate, which was spawned ≈5 Ma, is an ephemeral adjustment to mechanical difficulties at the triple junction. This new model implies that the Explorer subduction zone is no longer active. Ephemeral oceanic microplates have also existed at Pacific–North American triple junctions off southern California and Baja California.

Queen Charlotte basin and Coast Mountains: Paired belts of subsidence and uplift caused by a low-angle normal fault

The Queen Charlotte basin and adjacent Coast Mountains are paired belts of synchronous subsidence and uplift that formed inboard of the Queen Charlotte fault in Neogene time, accompanied by regional basaltic volcanism. We propose that a combination of pure and simple shear on a lithosphere-scale, low-angle normal fault could have been responsible for the observed vertical motions. Extensive crustal thinning in the basin decreases toward the Coast Mountains and has resulted in net subsidence of as much as 6 km since 20 Ma. East of the basin, in the Coast Mountains, more than 3.5 km of surface uplift has taken place since 14 Ma, probably because the upper mantle lithosphere has been locally thinned and replaced with less-dense asthenosphere. Magmatic activity in the basin and mountain belt could have been caused by decompression melting in the deformed lithosphere and upwelling mantle.

REGIONAL PATTERNS OF HYDROTHERMAL ALTERATION OF SEDIMENTS AS INTERPRETED FROM SEAFLOOR REFLECTION COEFFICIENTS, MIDDLE VALLEY, JUAN DE FUCA RIDGE

Reflection coefficients of the seafloor have been calculated from three multi-channel seismic reflection profiles across Middle Valley of the Juan de Fuca ridge. Seafloor reflection coefficients in this sedimented rift valley are high over an active hydrothermal vent and adjacent to major offset faults. Comparison of our measurements to drilling results from Leg 139 shows that high reflection coefficients over an active vent mound are produced by cemented sediments. Large reflection coefficients adjacent to major faults may have a similar origin and indicate that ongoing faulting creates pathways for hydrothermal fluids which alter the sediments and result in higher densities and velocities. Since 30 Hz seismic energy responds to the top 50 m of sediments, we are looking at the integrated response of hydrothermal alteration over tens of thousands of years. This is the first time seafloor reflection coefficients have been used to identify highly altered sediments in a region of deep-water hydrothermal activity.

First results on velocity analyses of multichannel seismic data acquired with the icebreaker Araon across the southern Beaufort Sea

One thousand two hundred twenty kilometres of multichannel seismic data were acquired in the Beaufort Sea in 2013 and 2014 to interpret shallow sedimentary structures associated with the upper Cenozoic Iperk and Shallow Bay depositional sequences. Seismic velocity analyses suggest a remarkably consistent regional velocity-depth trend on the slope within the upper 4 s two-way traveltime. A separate velocity trend was not defined beneath the shelf in this region, where data are influenced by the occurrence of permafrost. Deviations from this trend were noted at unconformities including an upper erosional unconformity. The seismic data in the Mackenzie Trough region suggest a different velocity-depth trend within the upper section and the region is marked by a large erosional unconformity, likely the base of the Shallow Bay sequence. Velocity analyses suggest the removal of up to 425 m of overburden; however, this is an overestimate of erosion as differential compaction from the glacial history has not yet been taken into account. In deeper water (>750 m) a bottom-simulating reflector is present, characterized by the occurrence of free gas and a low-velocity zone. Analyses of three fluid-expulsion features on the slope indicate that the Pokak fluid-expulsion feature and the Triple-Mound fluid-expulsion feature are linked to underlying anticline structures. A flat-topped fluid-expulsion feature at the flank of an equivalent anticline was also examined, but the occurrence of shallow gas creates a blank zone beneath this structure. Pronounced changes in the velocity-depth function at these fluid-expulsion features are linked to occurrence of free gas, and/or fluidized mud extrusions.

Origin and late quaternary tectonism of a western Canadian continental shelf trough

Abstract Analyses of high resolution and multi-channel seismic profiles from the central continental shelf of western Canada ascribe a late Quaternary glacial origin to large-scale troughs. Along the margins of Moresby Trough, one of three large-scale cross-shelf bathymetric depressions in Queen Charlotte Sound, seismic profiles within Quaternary sediments show a divergence of reflectors, thickening and folding of seismic units, and concavity of reflectors suggestive of drag. Compactional subsidence, growth faulting, and compaction faulting are also observed. Fault traces commonly terminate below the seabed. Deformation of Quaternary sediments due to faulting is plastic in nature and maximum offset of reflectors is 2.5 m. The observed Quaternary deformation appears to be a product of rapid deposition, loading and subsidence of late Quaternary sediment, which is unrelated to seismic activity. In addition, Quaternary faulting was probably activated by post-glacial loading and isostatic rebound of consolidated Tertiary strata along the margins of continental shelf troughs. The presence of mass movement (slump or debris flow) deposits overlying lithified Tertiary strata along the flanks of Moresby Trough provides the only evidence of seismic activity in the study area. The lack of a mud drape over these deposits implies a late Holocene age for the timing of their emplacement. The Quaternary troughs are incised into Tertiary-aged sedimentary fill of the Queen Charlotte basin. Previous workers had interpreted seafloor escarpments paralleling the trough margins to indicate that the location of Moresby Trough was controlled by renewed or continued activity on Tertiary-aged faults. A multi-channel seismic line across Moresby Trough shows that such an escarpment on the seafloor does not correlate to faults either in the Tertiary basin fill or the underlying basement. Tertiary reflectors are continuous underneath Moresby Trough; the seafloor escarpment is an erosional feature and was not created by reactivation of Tertiary structures. Trough erosion and subsequent fill (up to 175 m thick) are entirely of Quaternary age.