Geomorphology of the Northern Mariana Arc & Forearc Region

The Mariana Trench is most often known for being the site of the deepest spot in the world’s ocean, the Challenger Deep. This site is located within the southern portion of the trench and as a result, the southern Mariana region is extensively mapped, explored, and researched. The goal of the project presented here was to draw on a vast body of recent multibeam sonar data to characterize and better understand some of the unique geologic and ecologic characteristics of the understudied northern part of the Mariana Trench region.

Multibeam sonar data were collected during NOAA Ship Okeanos Explorer’s EX1605 mission in 2016 from April through July 2016. This ship is operated by the NOAA Office of Ocean Exploration and Research (OER) for the primary purpose of exploring the deep sea. These data were used to generate bathymetric (depth) and slope 'surfaces', or maps. Additionally, backscatter intensity information was collected to map variations in seafloor character, as the intensity of the acoustic return signal typically increases with harder seafloor substrates.

A Kongsberg EM302 multibeam system was used for data acquisition.

The remotely operated vehicle (ROV) Deep Discoverer was deployed from the ship (unmanned) for up-close investigation of locations of interest, collecting thousands of hours of high-definition video footage.

EX1605L3 Dives 5, 12, and 14 were used in this study.

This domed structure is a seamount 2 km high and 25 km wide, composed of cool, soft serpentinite muds pushed through faults from the mantle (NOAA OER 2016a). Mantle xenoliths, occasionally home to epifaunal hexactinellid sponges and crinoids, dot its low gradient surface. This particular mud volcano is no longer active (NOAA OER 2016a).

CROSS-SECTIONAL PROFILES:

Profiles A –A’ and B-B’, VE=2.0x, illustrate the mud volcano's geomorphology in cross-section. Interestingly, the volcano is much broader E-W than N-S (27 and 18 km, respectively). The mud volcano’s crest is a nearly flat plateau.

BACKSCATTER INTENSITY-SLOPE DATA ANALYSIS:

Quantitative data analysis was performed by first collecting slope and backscatter intensity values at thirty points along a chosen profile transect at each study site. These data were then graphed and an R2 value calculated to ascertain degree of correlation between slope and intensity.

Backscatter intensity and slope showed a weak, positive correlation correlated along profile A-A', suggesting a possibility that steeper areas may have harder substrate.

DIVE FOOTAGE from EX1605L3 Dive 17:

This dive was located on the upper portion of the serpentinite mud volcano, moving slowly up the gentle slope. The bottom was mostly covered in light mud, with chunks of serpentinized mantle rock scattered randomly. These hard surfaces are home to a variety of epifaunal suspension feeders, including hexactinellid sponges, stalked crinoids, and benthic ctenophores. Cusk-eels (family Ophiidae) are some of the few vertebrates that frequent these deep, dark bottom waters. (NOAA OER 2016a)

The low gradient (<10°) of the Serpentinite Mud Volcano underscores the relatively low viscosity of serpentinite mud, which Fryer (1992) described as “the consistency of cream cheese.” Such a low viscosity material would flow and spread widely, forming a low gradient structure. A 17 km wide serpentinite mud volcano can have as little as 1 to 2 km vertical relief (Fryer 1992).

The arcuate scarping, one of the mud volcano’s most obvious features, indicates immense areas have slumped, shifting nearly half the mountain’s surface outwards and downwards from its center. Concentrically arranged scarps reflect normal faults that would have formed as the lower slopes slumped outwards. The westernmost scarp, somewhat west of the mountain’s center, is the highest point on the volcano, and location of the ROV dive site. Considering that serpentinite mud volcanoes build outwards from a central point (hence the relatively neat roundness), the summit should be closest to the center. These slumping events, however, have caused the original summit to subside, leaving the western extremity of the slumping as the structure’s highest point.

Steeper slopes may have slightly harder substrate, though the reason is unknown and additional study is needed.

ROV dive footage indicates that hard-substrate-loving epifauna are counterintuitively common on this ‘soft’ bottom.  Serpentinized mantle xenoliths dot the muddy seabed, hosting long-stalked crinoids, hexactinellid sponges, and other taxa. In at least six cases, the ROV observed Relicanthus anemones commensally clinging onto hexactinellid stalks (NOAA OER 2016a). The xenoliths appear to be a crucial influence on the biotic community. Hypothetically, they could also affect the backscatter intensity, which is highest near the mound’s summit, although dive observations from near the base of the mud volcano are needed to verify.

This vast rifting zone consists of a network of horst-and-graben normal block faulting, manifested as steep slopes covered in unconsolidated mud and tumbled rocks. With rifting perpendicular to the trench axis, a consensus tectonic explanation for its existence has yet been found (NOAA OER 2016b).

CROSS-SECTIONAL PROFILES:

Profiles C-C’, D-D’, and E-E’, VE=5.0x, illustrate faulted block shapes. Profiles are aligned by the approximate rift axis (dashed line).

Note the small, steep walls forming a stepped pattern southeast of the main rifting zone, dipping steeply southeast (visible toward the right end of each profile). 

A detail of the blue bracketed portion of D-D’ is also shown, at a VE=1.0x.

BACKSCATTER INTENSITY-SLOPE DATA ANALYSIS:

A weak, positive correlation was found along C-C" (illustrated, below), indicating that the steeper scarp faces had harder substrate, likely due to the lower accumulation of sediments.

DIVE FOOTAGE from EX1605L3 Dive 14:

This dive started on flat ocean bottom and climbed the slope along a steep escarpment formed by extensional normal faulting, overlooking the floor of the rifting network at a location named Explorer Deep. Talus piles of broken rock have accumulated near the slope base, dotted by bamboo corals and hexactinellid glass sponges. At the base and proceeding up the 45o slope collections of boulders were found, often encrusted with manganese oxide (NOAA OER 2016b).

This region is cut by extensive normal faulting striking mainly southwest-northeast and superficially has the distinct morphology of a classic rift-valley, with faults dipping towards a dropped segment between the rifting halves. However, the apparent southern/southeastern rim of the rift valley might be better considered as simply a horst in a larger network of divergence. Normal faults, dipping away from the rim but striking parallel to the main rifting axis, occur immediately to the south. Scarping from these additional faults is small but clear in cross-section. The cause of the rifting remains unknown, though Fryer et al. (1985) tentatively suggested it was related to flexure of the Mariana Trench region, as the trench becomes more curved over time. At any rate, similar structures that likewise strike perpendicular to the trench axis have also been observed on the overriding plate associated with the Tonga Trench (Stalvey and Sautter 2021) and may have formed by similar means.

Statistical analysis uncovered a moderate positive correlation (R2=0.3075) between backscatter intensity and slope at this site, suggesting that the relatively steep scarp faces (>30o) tends to promote removal of unconsolidated sediments resulting in consolidated sediments and bare rock substrates. 

The ROV dive at Explorer Deep encountered mainly unconsolidated muds, even at considerable gradients, yet also revealed substantial talus piles of boulders at the scarp base. Occurrence of these rock piles supports a regime shaped by gravity and slope failure, as well as possibly indicating the existence of bare rock outcrops upslope. As with the rocks on the serpentinite mud volcano, these boulders provided holdfasts for deep-water epifauna like delicate hexactinellids and anemones, as well as haunts for deep-water fish (NOAA OER 2016b).

These seamounts are formed by arc volcanism adjacent to the subduction at the Mariana Trench to the east. Bottom substrate types vary from fine volcanic ash to pyroclastic gravel to bare volcanic rock. The seabed is often dotted with hydrocorals and/or split by columnar joints, which offer homes for reclusive fish and crustaceans. At many locations, active hydrothermal vent communities support Alvinoconcha snails which live off hydrothermal fluids via chemosynthetic endosymbionts (NOAA OER 2016c).

CROSS-SECTIONAL PROFILES:

Profiles F-F’, G-G’, H-H’, and I-I’ (VE=3.0x), illustrating cross-sectional seamount shapes. Most become steeper further up the slopes, closer to the summit, producing slightly concave flanks.

BACKSCATTER INTENSITY-SLOPE DATA ANALYSIS:

Backscatter intensity and slope were not correlated along any of the profile locations examined (H-H' is illustrated, below) at the Mariana Seamount Chain site.

DIVE FOOTAGE from EX1605L3 Dive 5:

This dive was located on the upper slopes of the Ahyi Seamount, in the Mariana Seamount Chain study site. Ahyi is an active volcano, covered in many places by volcaniclastic material of all sizes, from fine ash to gravel and occasional boulders. These substrates alternate with exposures of bare dark volcanic rock, often featuring columnar jointing. In this active, ever evolving, dangerous environment, slow-growing epifauna like corals and sponges are rare. However, a variety of mollusks and deep-water fish, as well as crustaceans like squat lobsters, frequent the mountain slopes (NOAA OER 2016b).

All the volcanic seamounts of this chain have steeper slopes (~15-30°) than the serpentinite mud volcano of the forearc region (see above). Unlike the unnamed mud volcano, the arc volcanoes of the Mariana Seamount Chain show no association between slope and backscatter intensity, as large patches of high intensity, occur somewhat randomly on the slopes of the arc volcanoes, and may indicate exposed lava flows. In contrast, the serpentinite mud volcano shows highest backscatter at the summit, decreasing with the falling bottom in every direction.

ROV dive footage from Ahyi Seamount shows that substrate types on one of these volcanic mountains is highly variable: from the finest ash and mud, to volcaniclastic gravel, to cliffs of sheer volcanic rock. Together with ecosystem types, substrates vary considerably with each particular volcano, its emissions, and degree of activity. Ahyi Seamount shows a striking absence of corals and other slow-growing, sessile epifauna, likely due to its high level of volcanic activity (NOAA OER 2016b).

Comparisons of the serpentinite mud volcano and classic volcanic arc seamounts offer potential insight into their respective materials and evolution over time

Note how the mud volcano shows much lower steepness/slope overall than the arc volcanoes, as well as its incredible size.

This study showcases the startling geological and biotic diversity of the under-studied northern Mariana region. Unique features such as serpentinite mud volcanoes may evolve with time in striking ways, raising new questions. For instance, what is the cause of higher backscatter closer to the mud volcano’s summit? Future ground-truth investigations on the lower slopes of the mud volcano could suggest and support answers to this question.

Future expeditions should also map more extensively to the south of the Anomalous Rifting Zone, attempting to determine the limits of the rifting. Other areas at similar distances from the trench axis, should also be mapped more extensively to determine if such anomalous rifting occurs through the Marianas forearc more broadly. This would be predicted if it is caused by increasing flexure of the subduction zone. The resulting data might also shed light on the tectonic processes shaping this rifting.

In the case of forearc seamounts, future expeditions should seek to explore in more detail the complex and highly dynamic relationships between volcanic activity and organic life, which vary dramatically from one volcano to the next. On one volcano, hydrothermal vents may nurture unique chemosynthesis communities, while on another regular eruptions exclude many organisms. How these interrelations change with eruption, rise and fall in activity, and physical substrate variation should be further characterized.

Amon, D., Fryer, P., Glickson, D., Pomponi, S. A., Lobecker, E., Cantwell, K., Elliot, K., & Sowers, D. (2017). Deepwater Exploration of the Marianas [in special issue: New Frontiers in Ocean Exploration: The E/V Nautilus, NOAA Ship Okeanos Explorer and R/V Falkor Field Season.

Fryer, P. (2012). Serpentinite mud volcanism: observations, processes, and implications. Annual review of marine science, 4, 345-373.

NOAA OER (2016a) OKEANOS EXPLORER ROV DIVE SUMMARY. Retrieved March 24, 2021, from https://oer.hpc.msstate.edu/okeanos/ex1605l3/EX1605L3_DIVE05_20160622_ROVDiveSummary_Final.pdf

NOAA OER (2016b) OKEANOS EXPLORER ROV DIVE SUMMARY. Retrieved March 24, 2021, from https://oer.hpc.msstate.edu/okeanos/ex1605l3/EX1605L3_DIVE14_20160701_ROVDiveSummary_Final.pdf

NOAA OER (2016c) OKEANOS EXPLORER ROV DIVE SUMMARY. Retrieved March 24, 2021, from https://oer.hpc.msstate.edu/okeanos/ex1605l3/EX1605L3_DIVE12_20160629_ROVDiveSummary_Final.pdf

Stalvey, A.D., and L. R. Sautter (2021). “Tonga Ridge” (research poster).

We would like to thank the College of Charleston BEAMS program and Department of Geology and Environmental Geosciences for enabling this project, as well as A. D. Stalvey for personal communication and collaboration in the pursuit of science.