Jeffrey Matthew Babcock

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.

An examination of along-axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13°30′N and 12°20′N

We investigate the along-axis variations of magma chamber width and crustal structure along the East Pacific Rise (EPR) from 13°30′N to 12°20′N through reprocessed common depth point (CDP) reflection profiles. The magma lens is, predominantly, a continuous feature in the study area with an average width of ∼500 m as determined from migrated cross-axis CDP profiles. This value is similar to widths estimated elsewhere along the EPR, suggesting that the axial magma chamber (AMC) width is not spreading rate dependent once the threshold for a steady state magma chamber is reached. The axial morphology of the 13°N area is generally not a good predictor of magma lens width or continuity. A fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply. In fact, between 13°05′N and 13°01′N a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity. This shoaling is similar to that observed near the 17°26′S region of the EPR where the rise axis summit is domed and highly inflated. Generally, the thickness of seismic layer 2A beneath the ridge crest is uniform and comparable to that estimated for 9°N, 14°S, and 17°S on the EPR, suggesting that the axial extrusive layer is invariant along fast spreading ridges. Uniformity of layer 2A thickness along-axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex (seismic layer 2B). Contrary to expectations of decreasing melt sill depth with increasing spreading rate, the average thickness of seismic layer 2B is slightly less (∼165 m) at 13°N than at the faster spreading, more robust 9°N area. Finally, geochemical/petrologic boundaries, which may delineate separate melt supply regions, occurring at the 13°20′N and 12°46′N devals (deviation in axial linearity) are observed to coincide with subtle changes in AMC and layer 2A reflection characteristics.

Long-term tectonic control on Holocene shelf sedimentation offshore La Jolla, California

A high-resolution Compressed High-Intensity Radar Pulse (CHIRP) survey reveals shore-parallel variations in the Holocene sediment thickness offshore La Jolla, California. Sediment thicknesses decrease from >20 m in the south near Scripps Canyon to zero in the north approaching Torrey Pines. In addition to the south-to-north variation in sediment thickness, the transgressive surface observed in seismic lines shoals from Scripps Canyon to the north. Despite these dramatic shore-parallel subsurface changes, the nearshore bathymetry exhibits little to no change along strike. A left jog (i.e., a constraining bend) along the Rose Canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface, the decrease in relief on the transgressive surface away from the left jog, and the Holocene sediment thickness variation. This tectonic deformation is shore parallel, and thus the accommodation can be separated into its tectonic and eustatic components.

A high-resolution seismic CHIRP investigation of active normal faulting across Lake Tahoe Basin, California-Nevada

We measured extension rates across Lake Tahoe Basin for the last 60 k.y. based on measured displacement of offset marker surfaces across three active faults beneath Lake Tahoe. Seismic chirp imaging with submeter accuracy, together with detailed multibeam and light detection and ranging (LIDAR)-derived bathymetry, was used to measure fault offset, thickness of earthquake-derived colluvial wedges, depth of wave-cut paleoterraces, and other geomorphic features. An analysis of these features provides refined estimates of extension rates and new information on Holocene faulting, and places Tahoe Basin deformation into the larger context of Walker Lane and Basin and Range tectonics. Measured offset marker surfaces include submerged wave-cut paleoterraces of Tioga age (19.2 ± 1.8 ka), McKinney Bay slide deposits (ca. 60 ka), and a winnowed boulder surface of Tahoe age (ca. 62 ka). Estimated vertical offset rates across submerged geomorphic surfaces are 0.43-0.81 mm/yr for the West Tahoe fault, 0.35-0.60 mm/yr for the Stateline-North Tahoe fault, and 0.12-0.30 mm/yr for the Incline Village fault. These offset rates indicate a combined east-west extension rate across Lake Tahoe Basin, assuming 60° fault dips, of 0.52-0.99 mm/yr. This estimate, when combined with the Genoa fault-slip rate, yields an extension rate consistent with the magnitude of the extension deficit across Carson Valley and Lake Tahoe Basin derived from global positioning system (GPS) velocities. The Stateline-North Tahoe, Incline Village, and West Tahoe faults all show evidence for individual Holocene earthquake events as recorded by either colluvial wedge deposits or offset fan-delta stratigraphy.

New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California

High-resolution seismic compressed high intensity Radar pulse (CHIRP) data and piston cores acquired in Fallen Leaf Lake (FLL) and Lake Tahoe provide new paleoseismic constraints on the West Tahoe-Dollar Point fault (WTDPF), the western- most normal fault in the Lake Tahoe Basin, California. Paleoearthquake records along three sections of the WTDPF are investigated to determine the magnitude and recency of coseismic slip. CHIRP profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event (MRE) on the WTDPF. Three faults are imaged beneath FLL, and the maximum vertical offset observed across the primary trace of the WTDPF is ∼3:7 m. Coregis- tered piston cores in FLL recovered sediment and organic material above and below the MRE horizon. Radiocarbon dating of organic material constrained the age of the MRE to be between 3.6 and 4.9 k.y. B.P., with a preferred age of 4.1-4.5 k.y. B.P. In Lake Tahoe near Rubicon Point, approximately 2.0 m of vertical offset is observed across the WTDPF. Based on nearby core data, the timing of this offset occurred be- tween ∼3-10 k:y: B.P., which is consistent with the MRE age in FLL. Offset of Tioga- aged glacial deposits provides a long-term record of vertical deformation on the WTDPF since ∼13-14 k:y: B.P., yielding a slip rate of 0:4-0:8 mm=yr. In summary, the slip rate and earthquake potential along the WTDPF is comparable to the nearby Genoa fault, making it the most active and potentially hazardous fault in the Lake Tahoe Basin.

Paleoseismic history of the Fallen Leaf segment of the West Tahoe–Dollar Point fault reconstructed from slide deposits in the Lake Tahoe Basin, California-Nevada

The West Tahoe–Dollar Point fault (WTDPF) extends along the western margin of the Lake Tahoe Basin (northern Sierra Nevada, western United States) and is characterized as its most hazardous fault. Fallen Leaf Lake, Cascade Lake, and Emerald Bay are three subbasins of the Lake Tahoe Basin, located south of Lake Tahoe, and provide an opportunity to image primary earthquake deformation along the WTDPF and associated landslide deposits. Here we present results from high-resolution seismic Chirp (compressed high intensity radar pulse) surveys in Fallen Leaf Lake and Cascade Lake, multibeam bathymetry coverage of Fallen Leaf Lake, onshore Lidar (light detection and ranging) data for the southern Lake Tahoe Basin, and radiocarbon dates from piston cores in Fallen Leaf Lake and Emerald Bay. Slide deposits imaged beneath Fallen Leaf Lake appear to be synchronous with slides in Lake Tahoe, Emerald Bay, and Cascade Lake. The temporal correlation of slides between multiple basins suggests triggering by earthquakes on the WTDPF system. If this correlation is correct, we postulate a recurrence interval of ∼3–4 k.y. for large earthquakes on the Fallen Leaf Lake segment of the WTDPF, and the time since the most recent event (∼4.5 k.y. ago) exceeds this recurrence time. In addition, Chirp data beneath Cascade Lake image strands of the WTDPF offsetting the lake floor as much as ∼7.5 m. The Cascade Lake data combined with onshore Lidar allow us to map the WTDPF continuously between Fallen Leaf Lake and Cascade Lake. This improved mapping of the WTDPF reveals the fault geometry and architecture south of Lake Tahoe and improves the geohazard assessment of the region.

RIO ROSO a Robotically Installed and Online Remote Ocean Seafloor Observatory

We describe a system for ocean observatories which streams data from the seafloor to shore in near real-time without a cable or moored surface buoy. The system utilizes a Wave Glider® (Liquid Robotics, Inc.); an autonomous surfboard-sized maritime vehicle that harvests wave energy for propulsion and solar energy for electrical power. The Wave Glider acts as a communications gateway as it hovers on the surface above the observatory connecting the acoustic telemetry through the ocean column and satellite telemetry to the shore. Several deployments of a prototype system demonstrated the feasibility of this concept. We also demonstrated that a wave glider could tow a suitably designed ocean bottom package with acceptable loss of speed. In this paper, we describe such a system that can be deployed autonomously and provide real-time telemetry of data from seafloor sensors.