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<title>deformation – JOIDES Resolution</title>
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<title>deformation – JOIDES Resolution</title>
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<title>Biscuits and Gravy</title>
<link>https://joidesresolution.org/biscuits-and-gravy/?utm_source=rss&utm_medium=rss&utm_campaign=biscuits-and-gravy</link>
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<dc:creator><![CDATA[Naomi Barshi]]></dc:creator>
<pubDate>Wed, 17 Aug 2016 18:36:24 +0000</pubDate>
<category><![CDATA[core]]></category>
<category><![CDATA[deformation]]></category>
<category><![CDATA[drilling]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[Sumatra Seismogenic Zone]]></category>
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<description><![CDATA[Drilling can alter the core in a variety of ways. This morning, the core on the description tables is made... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/biscuits-and-gravy/" title="Continue reading Biscuits and Gravy">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>Drilling can alter the core in a variety of ways. This morning, the core on the description tables is made of “biscuits and gravy”: coherent chunks of compacted sediments separated by a muddy matrix of ground up core that gets squeezed into the <a class="glossary-term" href="http://archive.joidesresolution.org/glossary/9#term412"><dfn title="Look up the definition of Core Barrel.">core barrel</dfn></a> in between the biscuits. Why does this happen and what can it tell us?</p>
<p class="p1">*edited again 20 August, with additional corrections.</p>
<p class="p3"><span class="s1">The variety of ways drilling can alter the core deserves a handbook. The geologists making the core descriptions have the beginnings of one, and their observations will help us turn the changes to the core because of the drilling process into useful information about the way these sediments might behave when they get to the subduction zone about 200 km to our East. After all, that is one of our goals on Expedition 362: Sumatra Seismogenic Zone. </span></p>
<p class="p3"><span class="s1">If you’re just joining us, here’s a reminder about what we’re after. Along the margin between the down-going plate and the overriding plate in a subduction zone, sediments can collect in a big wedge. This is called an “accretionary wedge” or “accretionary prism” because the sediments get scraped off the down-going plate and stuck onto (accreted to) the overriding plate. New sediments are always being brought into the system because they’re traveling on the incoming, eventually subducted tectonic plate*. Usually these accreted sediments are quite soft, filled with water between the grains, and not likely to break into faults until they reach high pressure and elevated temperature much farther into the subduction zone. </span></p>
<p class="p3"><span class="s1">The accretionary wedge at the North Sumatra margin is weird for a few reasons. It’s very thick and not wedge-shaped (it doesn’t have an approximately triangular cross section). The 4-5 km of sediment traveling in on the incoming Indian Plate that get accreted create a plateau up to 10 or even 15km thick from the seafloor surface down to the subducting plate. Recent earthquakes have made us rethink its role in how the subduction margin functions. Instead of the typical triangular prism shape, it’s a flat plateau that’s almost rectangular if you cut it in cross section perpendicular to its long-axis (hamburger, not hot-dog). That is, if you were traveling along the seafloor from the eastern Indian Ocean, you’d come across a steep slope of accreted sediment. Once you climb the slope, it would be a fairly flat walk to the Aceh Basin on the eastern side. When you’re standing in the Aceh Basin, you’re above the area where we usually find subduction zone earthquakes. After crossing this wide valley, you’d reach the island of Sumatra, where the volcanic arc is forming above the subduction zone. </span></p>
<p class="p3"><span class="s1">On the North Sumatra margin, the M 9.1 earthquake on 26 Dec. 2004 happened further west beneath the plateau area, which made the width of the earthquake rupture across the margin much larger. This is part of the reason the 2004 earthquake was so big. The ruptured area where the two plates slid past each other was also extremely long, possibly as long as 1600 km (1000 miles)! The bigger the area that slips during an earthquake, the more powerful the earthquake. Where the rupture starts influences how far it can go in different directions, and therefore how big the earthquake could get. </span></p>
<p class="p3"><span class="s1">Typically, the large subduction zone earthquakes start at depth of tens of kilometers and then break through across and along the plate boundary interface over a very large area. In 2004, the earthquake started under the seafloor offshore of Sumatra, at ~40 km depth and may have continued to a depth nearly as shallow as 5 km. It is the very large area of the fault that moved that gives the earthquake so much power. (More on what earthquake magnitude means in a later post.) We’re curious what controls the location of the plate boundary fault between the down-going plate and the accretionary plateau as well as the changes the sediments undergo between deposition hundreds of kilometers away from the trench and when they reach the subduction margin, carried by the moving plate.</span></p>
<p class="p3"><span class="s1">We’d like to see the sediments as fresh and pristine as possible so we can know everything about them before they start to change in the extreme conditions of the subduction zone. But drilling into the ocean floor is tricky business. We started coring using the <a class="glossary-term" href="http://archive.joidesresolution.org/glossary/9#term846"><dfn title="Look up the definition of Advanced Piston Corer (APC).">Advanced Piston Corer (APC)</dfn></a> method. This sends a core barrel down and sucks up a tube-shape of sediment. We can tell the orientation of these cores in space—that is, not just which way is up but also which way is north. Sucking the core into the core barrel causes deformation to the sediments if they’re not very well compacted. When the sediments become too stiff to use the APC method, we switch to a different coring technique. We no longer have orientation information aside from up, but we can drill through more compacted sediment. </span></p>
<p class="p3"><span class="s1">Because <a class="glossary-term" href="http://archive.joidesresolution.org/glossary/9#term861"><dfn title="Look up the definition of XCB.">XCB</dfn></a> uses rotating parts, the core gets sheared around its outside and can get broken up into pieces inside. This happens when the material is fairly coherent and can survive as chunks, like well-compacted mud. The sheared outer stuff is very soft and gets pushed in between the chunks. It’s a bit like biscuits and gravy, apparently, so this type of drilling deformation gets this silly (but informative) name! </span></p>
<p class="p3"><span class="s1">How easily the sediment gets broken into blocks can be a clue about the strength of the sediments. If they’re stronger—more resistant to deformation—they’ll break apart less easily and remain as bigger blocks. But if the sediments are not well compacted and weak—they get sheared easily—then they might end up being entirely gravy. This is useful for the structural <a class="glossary-term" href="http://archive.joidesresolution.org/glossary/9#term390"><dfn title="Look up the definition of Geology.">geology</dfn></a> team as they play time forward in their minds, moving the Indian Plate toward the Sumatra subduction zone: What will these sediments behave like when they reach the subduction zone and accretionary plateau and experience stress from the tectonic plates moving toward each other? As Peter, the leader of the structural geology team, says, “That’s the $64,000 question.” </span></p>
<p class="p3"><span class="s1">*If you have visited the San Francisco Bay Area or Big Sur in California, you may have seen ancient accretionary wedge rocks. I was just there on a field trip, so I’ll make a separate post about that soon!</span></p>
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<title>CORE DESCRIBERS : 3 teams, 3 specialities, 1 goal ! episode 3 Structural Geol. con't</title>
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<dc:creator><![CDATA[Marion Burgio]]></dc:creator>
<pubDate>Thu, 14 Jan 2016 05:30:37 +0000</pubDate>
<category><![CDATA[deformation]]></category>
<category><![CDATA[exp360]]></category>
<category><![CDATA[magmatic veins]]></category>
<category><![CDATA[Structural Geology]]></category>
<guid isPermaLink="false">https://joidesresolution.org//core-describers-3-teams-3-specialities-1-goal-episode-3-structural-geol-cont</guid>
<description><![CDATA[]]></description>
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<title>CORE DESCRIBERS : 3 teams, 3 specialties, 1 goal ! EPISODE 2 : STRUCTURAL GEOL.</title>
<link>https://joidesresolution.org/core-describers-3-teams-3-specialties-1-goal-episode-2-structural-geol/?utm_source=rss&utm_medium=rss&utm_campaign=core-describers-3-teams-3-specialties-1-goal-episode-2-structural-geol</link>
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<dc:creator><![CDATA[Marion Burgio]]></dc:creator>
<pubDate>Sat, 02 Jan 2016 08:29:52 +0000</pubDate>
<category><![CDATA[cores_119]]></category>
<category><![CDATA[deformation]]></category>
<category><![CDATA[exp360]]></category>
<category><![CDATA[magmatic veins]]></category>
<category><![CDATA[Structural Geology]]></category>
<guid isPermaLink="false">https://joidesresolution.org//core-describers-3-teams-3-specialties-1-goal-episode-2-structural-geol</guid>
<description><![CDATA[STRUCTURAL GEOLOGISTS Mike, Jeremy, Oliver, Carlotta, Gustavo. Another team to describe the rocks. They are interested in the organization of... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/core-describers-3-teams-3-specialties-1-goal-episode-2-structural-geol/" title="Continue reading CORE DESCRIBERS : 3 teams, 3 specialties, 1 goal ! EPISODE 2 : STRUCTURAL GEOL.">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>STRUCTURAL GEOLOGISTS</p>
<p>Mike, Jeremy, Oliver, Carlotta, Gustavo. Another team to describe the rocks. They are interested in the organization of the minerals.</p>
<p><img fetchpriority="high" decoding="async" class="alignnone size-full wp-image-23903" src="https://joidesresolution.org//wp-content/uploads/2016/01/C8U8983.jpg" alt="" width="640" height="427" srcset="https://joidesresolution.org/wp-content/uploads/2016/01/C8U8983.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/01/C8U8983-300x200.jpg 300w" sizes="(max-width: 640px) 100vw, 640px" /></p>
<p>Background : Mike ; left : Gustavo ; right : Carlotta</p>
<p>Carlotta, observes the magmatic fabric, she has to determine if minerals are preferentially oriented because of the magmatic flow. If the minerals are randomly distributed, the rock is isotropic and the fabric intensity is “0”, otherwise “1” means the minerals have regular shape and are weakly oriented, “2” to “3” mean phases are moderately to strongly oriented.</p>
<p>Gustavo is focused on crystal plastic fabrics. He tracks the deformation<br />
shown by minerals that have changed their original shape due to<br />
modifications in pressure and temperature due to a process known as<br />
solid-state flow. The intensity of the crystal plastic fabric is related<br />
to the degree of deformation and/or recrystallisation of the rock: “0”<br />
means that the rock is undeformed, while “5” represents a fully<br />
recrystallised rock. This information helps to understand the way<br />
deformation is distributed and further localized.</p>
<div></div>
<p>The other way to deform a rock is the brittle deformation when the mineral are broken. It happens in a different context, for example when temperature is lower. Jeremy’s work is to identify and describe such deformation.</p>
<p><img decoding="async" class="alignnone size-full wp-image-23905" src="https://joidesresolution.org//wp-content/uploads/2016/01/DSC02209e.jpg" alt="" width="640" height="427" srcset="https://joidesresolution.org/wp-content/uploads/2016/01/DSC02209e.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/01/DSC02209e-300x200.jpg 300w" sizes="(max-width: 640px) 100vw, 640px" /></p>
<p>Left: Oliver; center : Jeremy; right : Mike</p>
<p>Mike and Oliver observe the veins…</p>
<p>Mike is examining the core for intrusive magmatic veins, magmatic<br />
contacts and cross-cutting relationships to better understand the<br />
relative timing of magma intrusion and crustal faulting. He recognizes<br />
the type of boundary as intrusive or faulted or as an internal magmatic<br />
contact, describes what they look like and measures the dip and dip<br />
direction of the feature. it’s the rich interplay of these two processes<br />
that ultimately define how ocean crust grows.</p>
<p>There is another type of veins, the alterations ones. They are due to<br />
fluids percolation and are characterized by new crystallization. Oliver<br />
is specialized in these veins.</p>
<p><img decoding="async" class="alignnone size-full wp-image-23904" src="https://joidesresolution.org//wp-content/uploads/2016/01/C8U8984.jpg" alt="" width="640" height="460" srcset="https://joidesresolution.org/wp-content/uploads/2016/01/C8U8984.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/01/C8U8984-300x216.jpg 300w" sizes="(max-width: 640px) 100vw, 640px" /></p>
<p>Mike Cheadle !</p>
<p>The complementary expertises of the members of this team, allow them to work individually and efficiently !</p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23907" src="https://joidesresolution.org//wp-content/uploads/2016/01/Ship-tour.jpg" alt="" width="640" height="480" srcset="https://joidesresolution.org/wp-content/uploads/2016/01/Ship-tour.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/01/Ship-tour-300x225.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p>Structural team + Jakub, Natsue and Chris, co-chief, during the ship tour.</p>
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