Clarion and Murray Fracture Zones in the Central Pacific
Geomorphic Analysis and Characterization of Features
Multibeam sonar data for the Clarion Fracture Zone were collected by E/V Nautilus during expedition NA102 in October, 2018. The Expedition Lead was Lindsay Gee, a Science & Mapping Manager at the Ocean Exploration Trust.
Data for the Murray Fracture Zone were sourced from the EX1707 expedition by NOAA Ship Okeanos Explorer. The Expedition Coordinator was Elizabeth Lobecker, of the NOAA Office of Ocean Exploration and Research. The expedition was primarily focused on the Musician Seamounts to the northwest of the fracture zone, but multibeam sonar data used in this study were collected as part of a strategic transit leg.
The purpose of this study was to analyze and characterize interesting geomorphic features indicated by the bathymetric data, supplemented by intensity backscatter mosaics generated with the obtained data.
The Clarion Fracture Zone, south of Hawaii, will be examined first, and then the Murray Fracture Zone, which is to the north of Hawaii.
The study area includes the indicated segments of the Murray and Clarion Fracture Zones, their locations shown here relative to Hawaii. The Murray Fracture Zone site is roughly 900km north of Hawaii, while the Clarion site is about 500km southeast of Hawaii.
CLARION FRACTURE ZONE STUDY AREA
This study area is comprised of multibeam data gathered from the western tip of the Clarion Fracture Zone by E/V Nautilus expedition NA102.
Two study sites were examined in this study area. The Central Clarion Fault Scarp was chosen due to its interesting geological structure. The second site is the smaller fault scarp to the south and includes one of the shallowest areas of the dataset, in the form of a raised, flat seabed cut by a canyon structure.
CLARION SITE 1: CENTRAL FAULT SCARP
The Central Fault Scarp site is characterized by a 70 km long east-west high-slope (25-40°) escarpment with a vertical relief of 300 m (500 m in the deeper western end of the base). The northern, higher side of the fault scarp is characterized by vertical, north-south oriented ridges, which are of inconsistent width and slope, but of similar lengths (approx. 7,000 m).
Left: Bathymetry (Depth), Right: Classified Intensity
Abyssal ridges occur on the top of the fault scarp, most easily visible in Slope and Aspect views (see yellow box).
Left: Slope (degrees), Right: Aspect (degrees)
3D Bathymetry, Classified Backscatter Intensity, Slope, and Aspect surfaces are shown here, with views looking westward along the fault scarp’s face (VE=2.0x).
High-intensity substrate is located at the base of the large fault scarp, where the slope is less than 3° (see yellow polygons in the 3D Slope and Classified Intensity views), and medium to low intensity occurs along the face of the fault scarp. The seabed along the base is notably featureless.
Profiles across the fault scarp (A-A’, C-C’) show the steep 30-35° slope. B-B’ and D-D’ run perpendicular to and along the top of the fault scarp, across the north-south oriented abyssal ridges. Profiles are shown at the same scale, with 1.5x vertical exaggeration.
CLARION SITE 2: SOUTHERN FAULT SCARP
The Southern Clarion Fault Scarp site is characterized by a shorter (23 km) Fault Scarp of similar 30° slope and 300 m vertical relief as the Central Fault Scarp.
Instead of abyssal ridge-like structures on the top of the scarp, this fault scarp has a single large, flat-topped feature and a canyon to the north. Abyssal ridges are less obvious at this site and occur on the dropped block.
The fault scarp’s base is ~ 6100 m for the length of the formation with a slope < 4°. The large feature on the up-side is very flat with a steep western side and terraced to the east.
The Aspect surface reveals very low relief abyssal ridges on the dropped block, south of the fault.
Large feature has almost no higher intensity returns on the backscatter view. Highest intensity substrate readings are present at the base of the fault scarp, in a relatively flat region, rather than on the high-slope fault scarp itself. This site exhibits nadir effect in the center of the medium-intensity zone, but the medium intensity is still most prevalent in the dropped block. The large scarp-edge feature is dominated by medium intensity returns.
Ground-truth imagery collected using National Geographic deep sea camera systems deployed from E/V Nautilus during mapping. Depths of the seabed photographed are between 5500 and 6000 m in the dropped block of the fault (yellow box on map below).
MURRAY FRACTURE ZONE STUDY AREA
This study area is comprised of multibeam data gathered from a western area of the Murray Fracture Zone by NOAAS Okeanos Explorer during the EX1707 mission.
Study sites in this study area are centered around the two large fault scarps running east-west across the ocean floor. The Northern Murray Fault Scarp Site contains highly visible abyssal ridges, to the south of the fault scarp itself. The Southern Murray Fault Scarp Site contains several volcanoes to the north of its primary fault scarp.
MURRAY SITE 1: NORTHERN FAULT SCARP
The Northern Murray Fault Scarp study site is along a normal fault, with a deeper, dropped-block region shown in the central area of the bathymetric surface. Numerous abyssal ridge features occur on the south, up-side of the fault. Two large circular volcanoes are also included, in the south-east corner.
The focus of this site is an examination of the abyssal ridges present on the up side of the south fault scarp. Curved ends on the ridges indicate western motion along strike-slip fault. Two highly circular volcanoes are also present in the southeast.
Slope (Left) and Aspect (Right) views
The abyssal ridges are extremely visible in the slope and aspect views. Note in the slope view on several of the ridges that the eastern side is steeper than the western side.
The volcano that shares the up side of this fault scarp with the abyssal ridges is very circular, and flat on the top, due to effusive eruptions at the high pressures present at these depths. It has a vertical relief of approximately 800 m, and is approximately 5,000 m wide.
The classified backscatter intensity view below shows significant high-intensity striping that follows the eastern slopes of the abyssal ridges.
Profile A-A’ is a view of the slope from the top of the fault scarp on the southern side of the dropped block down to the base, crossing the curved tip of an abyssal ridge. Profiles B-B’ and C-C’ give views of the abyssal ridges at the top of the fault scarp to the north, as well as the slope down to the base. Profiles D-D’ and E-E’ showcase the circular nature of the volcanoes in this region, as well as their flat tops and relatively steep sides (30°). All profiles are shown with VE = 2.5x.
Examination of the abyssal ridges’ classified backscatter shows that the eastern slope of each ridge exhibits a relatively high intensity, while most western slopes have medium to low intensity.
Profile R-R’ (below) taken along the abyssal ridge (map, above) is shown with a classified intensity bar along the same profile. A relationship between the eastern slopes of several abyssal ridges and high intensity returns (white areas of the intensity bar) is evident.
While the high-intensity areas of the intensity bar primarily coincide with eastern slopes of these abyssal ridges, low-intensity returns (dark purple areas) primarily occur within troughs between each ridge, suggesting a buildup of unconsolidated sediment between the ridges. Areas of high intensity are highlighted with yellow boxes, and areas with low intensity are highlighted with purple boxes.
The eastern slope of each ridge exhibits a relatively high intensity, while many western slopes have medium to low intensity.
MURRAY SITE 2: SOUTH FAULT SCARP
This area is characterized by a 1.4 km fault scarp face that stretches the width of the site to the north of the dropped block (~6100 m deep). A secondary fault scarp with less dramatic vertical relief is located on the south side of the dropped block and has several curved abyssal ridges. Four circular, flat volcanoes are on the up-side block in the north.
The north scarp (white box in 3D view) has vertical relief of 1400m, and abyssal ridges are clearly visible on both fault scarps, above the deeper, flat dropped block.
More abyssal ridges are observable on both fault scarps in this site, all on the up-side of each fault.
Aspect view shows intensely curved features within the dropped block, suggesting more tectonic movement.
Classified backscatter intensity shows a striping pattern of high and medium intensity that does not appear to correlate with slope.
Profile locations for Murray South Fault Scarp.
Profiles of the fault scarp (A-A’, B-B’) illustrate the 1400m vertical relief with 30° slope (shown with VE=1.6x). Also depicted is the depression at the scarp base and flat areas at the top with small volcanic features.
Profiles collected along the fault scarp’s edge (C-C’, D-D’, shown with a different color scale) show several north-south abyssal ridges that characterize much of the Murray Fracture Zone.
DATA ANALYSIS
Slope and Intensity data were gathered from one location in each study area, and a linear regression was performed for each, showing no correlation between slope and intensity returns for either the central fault scarp in the Clarion Fracture Zone, or the abyssal ridges in the north fault scarp site in the Murray Fracture Zone. (R^2 < 0.0787)
SUMMARY
Murray and Clarion are prominent Fracture Zones, which have similar geomorphologic features despite their 1400 km distance from each other, including abyssal ridges associated with fault scarps. Abyssal ridges are most easily visible in the Murray Fracture Zone’s Northern Murray Fault Scarp site. These ridges clearly curve eastward at their northern end, likely due to westward strike-slip tectonic motion along the fractures, similar to those present in the Mendocino Fracture Zone examined in previous BEAMS program studies by Will Hefner and Dr. Sautter. Elsewhere, abyssal ridges are more subtle and are best seen in Slope and Aspect surface views.
Several circular, flat-topped the volcanoes present in the Murray Fracture Zone resemble features present near the Mendocino Fracture Zone in previous BEAMS studies. Further investigation and comparison warranted.
Clarion Fracture Zone abyssal ridges do not curve as dramatically, suggesting less tectonic motion along the fault scarps observed. Notably, at Clarion’s Southern Fault Scarp site the only abyssal ridges occur on the dropped block (Aspect view). These ridges are very low-relief, and did not curve in any consistent direction, suggesting a lack of similar strike-slip tectonic motion along this particular fault scarp.
Neither Clarion nor Murray’s surfaces indicate a correlation between slope and backscatter intensity. However, the Murray Fracture Zone’s “stripes” of high-intensity along the eastern slopes of the abyssal ridges warrant further investigation, to collect more sonar data with tracklines perpendicular to those collected in the original data. Also, an ROV dive at one of the abyssal ridges would help to ascertain why the eastern slopes have higher intensity readings and would provide important ground-truth imagery for the Murray Fracture Zone.
References
Thomas A. Morrow, Eric Mittelstaedt, Seung-Sep Kim, 2019, Are segmented fracture zones weak? Analytical and numerical models constrain anomalous bathymetry at the Clarion and Murray fracture zones, Earth and Planetary Science Letters, Volume 512, Pages 214-226.
NOAA Ocean Exploration and Research, 2017, Musicians’ Seamounts Telepresence Mapping: Musicians Seamounts Telepresence Mapping: NOAA Office of Ocean Exploration and Research (Accessed April 2021).
Nautilus Live Ocean Exploration Trust, 2018, Clarion-Clipperton Fracture Zone: Cruise | Nautilus Live (Accessed April 2021).
William Hefner and Dr. Leslie Sautter, 2016a, Geomorphic Analysis of Seamounts Adjacent to the Mendocino Fracture Zone, Department of Geology and Environmental Geosciences, College of Charleston
William Hefner and Dr. Leslie Sautter, 2016b, Analysis of Gorda Escarpment Geomorphology, Mendocino Fracture Zone, Department of Geology and Environmental Geosciences, College of Charleston
Acknowledgements
We would like to thank the College of Charleston BEAMS Program and the College of Charleston Department of Geology and Environmental Geosciences. Also, we are grateful to, CARIS, Google Earth, Lindsay Gee and the participants of the E/V Nautilus expedition NA102, the participants of the NOAA Okeanos Explorer expedition EX1707, and the National Geographic for providing deep-sea camera system imagery.
