SO301 SCIROCCO: Seismic Imaging at the Central Indian Ridge: Structure and Formation of Oceanic Core Complexes

Begin of project: July 1, 2023

End of project: December 31, 2025

Status of project: March 1, 2024

Cruise logo SO301 SCIROCCO

Objectives of SCIROCCO project

The main target of cruise SO301 is a seismic image of the Central Indian Ridge in order to better understand structures and processes of magmatism and crustal extension.

At slow spreading ridges, phases of magmatic accretion alternate with phases of tectonic stretching and consequently faulting on different scales. The long-living large detachement faults lead to the formation of Oceanic Core Complexes (OCC), which represent exhumed lower crustal or upper mantle material being raised to the seafloor. Results will allow to validate competing conceptual OCC models by seismic imaging – the missing tool in order to resolve the crustal structure and fault system at depth. Faults are potential path­ways for fluids which might develop to hydro­thermal systems and mineral deposits in combination with magmatic heat sources. Here, we study the link between OCCs, fault systems and hydrothermal vents by seismic imaging.

The survey area at 25 °S, 70 °E is located at the Central Indian Ridge (CIR) directly north of the Rodriguez Triple Junction (RTJ), where three mid-ocean ridges meet (Central, Southwest, and Southeast Indian Ridge: CIR, SWIR, SEIR). The working area hosts a prominent OCC, which extends approximately 10 km by 20 km and rises 1500 m high, as well as active hydrothermal fields, including the well-known Kairei field.

Research cruise SO301

These mid-ocean ridge structures were geophysically investigated with the research vessel RV SONNE on expedition SO301, lasting from November 22, 2023, to January 4, 2024. Figure 1 of the working area provides an overview of the acquired profiles and the station network.

Figure 1: SCIROCCO working area at 25°S, 70°E near the Rodriguez Triple Junction (RTJ), where the Central, Southwest and Southeast Indian Ridge meet. The oceanic crust with the oceanic core complex (OCC) and the Kairei hydrothermal field was surveyed with 29 seismic reflection lines (red), 86 ocean bottom seismometer (OBS) stations (yellow, blue and purple dots), and refraction profiles (yellow lines). Bathymetry derived from the multibeam echosounder data of RV SONNE Source: BGR

The refraction seismic survey carried out a total of 86 stations with 50 ocean bottom seismometers (OBS) from GEOMAR and BGR, which were deployed i) along two long refraction profiles across the plate boundaries, ii) regionally as a 3D array around the OCC, and iii) locally at the Kairei hydrothermal field. The seismic source of the diving waves was an airgun array operating along refraction profiles and in curves between reflection profiles with a shot interval of one minute. Refracted phases can be recognized up to a distance of about 40 km, including shear waves, and also Moho reflections. A first model derived from the data depicts the oceanic core complex (OCC) as a body of higher velocities.

The reflection seismic survey operated for the first time in German research history with an 8 km long hydrophone cable. A dense rectangular network of 29 profiles was acquired in two weeks with 50 m shot point spacing. The demanding topography and sediment-free lithology require special processing. Near-surface structures and faults can already be recognized in the area of the OCC. The expected refracted phases also occur in the last two kilometers of the long streamer.

The potential field methods magnetics and gravimetry recorded a dense data grid on all profiles, which allows a new detailed view on anomalies in the working area and around the OCC in particular - in addition to a comparison with data sets from previous surveys that show inaccuracies in the IGRF reference model.

The high data quality of the bathymetry data set acquired with the EM122 multibeam echosounder demonstrates the advantages of multiple coverage from different azimuths. It becomes the starting point for the seismic 3D migration to eliminate the diffractions caused by side reflections.

Secondary user project KABA

The research cruise SO301 also included the project KABA entitled "Ramifications of Venting at the Kairei Hydrothermal Field for Bathypelagic Microbial Processes in Consort With Carbon and Metal Cycling". The objective of this program is to determine how the ramification of the Kairei hydrothermal plume fluids affects microbial life and biogeochemical cycling in the bathypelagic ocean.

In 28 CTD deployments, the KABA project recorded the expansion of the hydrothermal plume above the Kairei hydrothermal field using temperature, redox-potential and turbidity measurements as well as water samplers and in-situ pumps. More than 200 samples were taken to examine microorganisms and for gene and genome sequencing; over 1000 samples were taken to quantify the trace metal input and its dispersion in the water column.

Literature:

• SO 301 Cruise Report
• Research Vessel SONNE: Cruises No. SO299/2 - SO301 15.08.2023 - 04.01.2024 Expedition brochure (2023) (PDF)
• Weekly Report Nb. 1 (22/11 – 26/11)
• Weekly Report Nb. 2 (27/11 – 03/12)
• Weekly Report Nb. 3 (4/12 – 10/12)
• Weekly Report Nb. 4 (11/12 – 17/12)
• Weekly Report Nb. 5 (18/12 – 24/12)
• Weekly Report Nb. 6 (25/12 – 31/12)

Partner:

GEOMAR Helmholtz Centre for Ocean Research Kiel (Dr. Anke Dannowski)

Promotion / document number:

03G0301A (BMBF)