(Department of Geology and Geophysics, West Main Road, University of Edinburgh, Edinburgh, UK, EH9 3JW)

On the voyage of 160, two mud volcanoes, Milan and Naples, were drilled in the southern part of Crete in the eastern Mediterranean. These mud volcanoes are located on an accretionary complex dominated by ooze on the back of the Mediterranean Sea ridge, which is caused by the northward subduction of deep-sea sediments from Neogene to Holocene in the African plate under the Eurasian plate. Only by drilling can the age and underground structure of mud volcano be determined. The main conclusions are as follows: the active period of the two mud volcanoes is > > 1Ma, which is mainly composed of multi-stage debris flow (deposition) composed of sandstone and limestone debris with muddy matrix. These basements may come from the overpressure fluid-rich ooze in the Lower Late Miocene (Messina) of the Mediterranean ridge accretion complex in the subduction detachment zone. On the contrary, the petrochemical debris mainly comes from the middle Miocene strata, and by that time, the petrochemical debris may have become a part of the overlying accretionary complex. The understanding of the cause of debris flow in "muddy breccia" has changed the viewpoint of early viscous ooze intrusion.

Mediterranean mud volcano; Accretionary wedge; ocean drilling program

1 Introduction

During the Beijing International Geological Congress, People's Republic of China (PRC) is seriously considering becoming a member of the ocean drilling program. ODP was discussed at the seminar of the International Geological Congress, which was warmly supported by the representative of China. The first author submitted a paper summarizing the drilling results of two mud volcanoes on the Mediterranean ridge in southern Crete. This is one of the two voyages of 1995 (Figure 1) in the Mediterranean Sea in spring and summer. Voyage 160 has two objectives: structure and paleo-ocean. Paleooceanography involves the origin of organic-rich ooze in the deep sea from Pliocene to Holocene, that is, sapropelic mud. The goal of the structure is twofold. The first one involves the collision process of Eratosthenes seamount, which is a carbonate platform from late Mesozoic to Neogene, bordering South Cyprus along the active plate boundary of eastern Mediterranean Africa and Eurasian plate. The second purpose is to summarize the origin of the peculiar mud volcano located on the Mediterranean ridge, which was caused by the subduction of the African plate under the Eurasian plate during Neogene-Holocene, and was a accretionary wedge dominated by soft mud. This study took 10 days. Following the voyage 160, the voyage 16 1 was carried out in the western Mediterranean, and the sapropelic study and the investigation of crustal extension related to the opening of the Albern Sea were carried out.

Figure 1 ODP 160 The tectonic environment and location map of two mud volcanoes in Milan and Naples at 970 and 97 1.

This voyage has two objectives: tectonics and paleoceanography. The objective of this structure is to study the environment of Eratosthenes seamount in southern Cyprus and the genesis of mud volcano, as described in this paper. Paleooceanography involves deep-sea sediments rich in organic matter, that is, the origin of sapropelic mud accumulated in the whole Mediterranean deep-sea basin during the last 5Ma.

This article follows a new viewpoint. The original text was actually written at sea, describing some exciting discoveries about the ocean during deep-sea drilling at that time. The ocean drilling program provides a rare opportunity for the cooperation and new discoveries of the international marine science community. See the original report of Voyage 160 and related preliminary results for details.

2 regional geological background

In the late 1980s, Italian scientists discovered the dome structure on the Mediterranean ridge150km south of Crete. The Mediterranean Ridge (500km long and 100km wide) is interpreted as an accretive prism, which was formed in the last 25Ma and was the result of the northward subduction of the African plate under the Eurasian plate. The study of seismic reflection shows that there are many hill-like structural features on the seabed. After sampling, the piston core contains strange empty soft mud, which is called mud gravel and has a structure like custard. They are composed of soft sand and soft and hard rock fragments in clay-rich matrix. These mounds were originally interpreted as diapiric structures, which are relatively viscous substances that invade the seabed upwards. Using underwater photography, they found at least one structure, the dome of Naples, which is still spewing liquid and surrounded by a bacterial pad. * * * Cold spring community includes crustaceans. The deep towed seismograph also found that lahar radiated from the central nozzle. Therefore, a question is raised: is the ooze structure a viscous intrusion similar to the dome or volcanic cone of clastic argillaceous deposits?

The result of Milan mud volcano

The first drilling structure is the Milan dome in the east (Figure 2A). The seismic reflection data show that it has the shape of a wide-brimmed hat, and the central volcanic cone is surrounded by inclined flanks ending at the seabed. The seismic reflection layer under the wing of the dome inclines inward, and a bowl-shaped depression is formed under the volcano. How did this unique structure come into being? Drill a shallow hole (less than 200 meters) from the outside of the two structures to the ridge. After drilling several meters deep in calcareous ooze, a large number of sediment cores were collected, and scattered hard and soft rock fragments were found in hard sandy clay (Figure 3). Under this, we found several thin layers of deep-sea sediments, which were covered by clay-rich sand, silt and gravel layers in turn, and some of them represented evidence of turbidity current deposition. To our surprise, the lowest age of deep-sea sediments below the gravel layer determined by paleontologists on board is1.75ma. Milan's structure is very active, at least periodically, with a long period. Further information comes from the logging curves of geophysical methods. Stratigraphic microscope scanner found a layered structure formed by mud debris flow and consolidated debris with a thickness of at least half a meter.

Fig. 2/Comprehensive lithostratigraphic map of Milan mud volcano (a) and Naples mud volcano (b) on voyage Kloc-0/60.

They show the seismic reflection layers visible under two mud volcanoes. There are inward inclined reflection layers under the two wings of Milan and Naples argillaceous structures, which are used to indicate the gradual subsidence process during mud volcanism. The depth of the ordinate refers to the depth below the seabed.

At first, it was thought that mud-rich breccia represented mud-rock flow in mud pits. How far can they be distributed from the eruption center? To solve this problem, we drilled another hole from the side of the structure. We have drilled through the typical sediments on the surrounding seabed (i.e. semi-deep sea sediments, phosphate ooze and sapropelic mud), and the results show that the eruption flow of mud volcano is less than 1km from the eruption center of Milani volcano. Therefore, we drill closer to the argillaceous structure, and the core in the inner wing shows evidence of another mud flow, so the ridge area is composed of sandy materials, which can form a central plugging structure.

When the ship's geochemists processed their data, they were surprised to find that some cores collected from ridge points were obviously full of gas. A mixture of icy methane and water, called clathrate compound (gas hydrate), exists at a depth of 30 ~ 40m below the seabed. The salinity of the solution collected from the ridge is relatively low, which means that the inclusion is decomposed and brought to the surface during sampling. In contrast, at deeper depths, the salinity of pore water is usually much higher than that of Mediterranean bottom water: this is because of the dissolution of evaporated salt in Messina period. This salt was widely precipitated in the late Miocene, about 6 ~ 5 Ma ago, when the sea level in the Mediterranean was much lower than it is now [9].

Fig. 3 Typical example of thick and relatively homogeneous muddy debris flow collected by Milan mud volcano.

The upper part is a typical "muddy breccia" structure, which is interpreted as debris flow; Dark clastic rocks (middle) are timely sandstone, which is interpreted as turbidite; The homogeneous debris near the bottom of the photo is large pieces of debris and sandstone; 970A point, core 10X, profile156 ~ 82.5cm.

The result of the mud volcano in Naples

We once again used the method of drilling a borehole profile (Figure 2B). Firstly, the drilling casing is positioned out of the side of the core to be mined, and then the age of the pinnate edge of the mud volcano cone is determined. We are on the target: under the thin deep-sea sediment, we drilled through the muddy breccia similar to the mud volcano in Milan, and then directly entered the standard deep-sea sediment, and determined its age to be 1.5 ~ 1.2 Ma or older. In order to find out the thickness of these breccias, they were drilled into a channel-like fault depression near the bottom of the central volcanic cone. After drilling to 180m, the core penetration rate is very low, and it is still in mud-rich debris flow. Here, signs of oil and natural gas are found: the sample is exposed to ultraviolet light and emits strong fluorescence. So we are faced with a question: should we close this drilling hole for safety reasons? However, there are no active signs of oil and gas migration here, and we can continue to carry out our planned maximum depth. Due to poor drilling conditions, geophysical logging was tried, and only part of the drilling was successful. However, we found clear strata in muddy breccia interpreted as mud flow.

Drill into the top of the ridge area. The sediments there are full of gas and contain irritating hydrogen sulfide gas. Due to the high gas pressure, safety precautions have been taken, and its composition shows that it is a relatively shallow source related to bacterial degradation. This can't constitute a safety accident. We can sample dozens of meters as planned. Soon, the soft mud we collected began to dry and frost into crystalline salt, and some even contained small pieces of crystalline stone salt, presumably in Messina period. Next, we relocated the ship to the ridge area of the active crater discovered by the 1993 expedition team for sampling. The sampling went well. When the third core reached the deck, it exploded without proper treatment. Mud splashes everywhere! The reason is that the volume of gas expands rapidly due to the increase of temperature. Fortunately, no one was injured except the person who made the sample and some packaging bags were dirty. This is a safety accident. Although we don't want to do this, we have no choice. When we put away the casing, this exciting mud volcano exploration in the eastern Mediterranean ended. Fortunately, these are the last holes we planned to drill, and almost no data was lost.

5 discussion

Now we know that Milan Dome is a submarine mud volcano, which began to form at least before 1.5Ma, and the Naples structure was at least 1.5 ~ 1.2 Ma (Figures 4A and b). Relative to the center of the volcano, the seismic reflection layer inclines inward, indicating that the volcanic cone has gradually collapsed. The volcanic cone of unstable clastic sediments, including argillaceous clastic flow and turbidite, was formed by early eruption. Then mud flows began to flow out on a large scale, and deep-sea aggregates were scattered everywhere, eventually forming the current volcanic cone.

Gas hydrate can only be formed in a limited temperature/pressure range. If a fluid with a relatively high temperature overflows from the depth, it will be unstable. Consistent with this, natural gas hydrate was found to be related to the inactive Milan mud volcano. In contrast, Naples volcano, which actively ejects fluid and gas, seems to lack gas hydrate.

What drives mud volcanism near the northern edge of the Mediterranean ridge? As a remnant of the Mesozoic Tethys Sea, the deep Mediterranean basin in this area is in the final stage of collision and closure between the African plate and the Eurasian plate, and the northern edge of accretionary wedge is inserted northward under the continental crust of southern Crete. This compression zone may be the location of overpressure fluid-rich deposits. Usually, the thick deep-sea sediment layer obviously accumulated on it will prevent this material from moving upward. However, when the thrust fault action of the Mediterranean ridge is triggered, it is used as a way for overpressure materials to move upward to the seabed (Figure 5). At least part of the filler may come from the fossil-free ooze of Messina in the late Miocene in the compression zone. The possible reason of limestone and sandstone debris is that they are squeezed out of the overlying Miocene accretionary sediments by the mixed action of hydraulic crushing and physical erosion. Materials rich in mud and debris are brought to the top and finally sprayed out of the seabed, forming multi-stage debris flow.

Fig. 4 Interpretation profiles of Milan mud volcano (a) and Naples mud volcano (b)

Muddy debris flows out of the central crater and gathers in the depressed valley depression or adjacent areas; 1-ejection zone; 2- Mud volcano structure; 3- seismic reflection layer; 4— Late Miocene "M" reflection layer

Fig. 5 development stages of mud volcanoes on Mediterranean ridge

In the first stage, the early eruption formed a clastic volcanic cone; In the second stage, multi-clastic manifold became the main structure of mud volcano, which was related to the gradual subsidence of marginal valleys and depressions.

6 conclusion

Through the investigation of mud mounds 10 days in the eastern Mediterranean, we have made important discoveries about deep-sea mud volcanism. It is known that the mud domes in Milan and Naples are submarine mud volcanoes, which may be similar to the environment under the convergence edge, like the Barbados subduction complex. For magma volcanoes, these mud volcanoes seem to have a cycle of millions of years. More mud volcanoes can be found in other submarine tectonic active areas. There is no doubt that other scientists will continue to investigate this strange geological phenomenon soon.

Without the support of the captain, crew and marine technicians, nothing will be accomplished in all deep-sea drilling voyages.

(Translated by Zhou Lijun, Xu Dong Language School)

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