Ocean, Basalt, Volcano 19.2. Guy Lang – What determines the width of a volcanic margin? The Eastern North American Margin
It was recently published in jgr and uh. In this talk, i will mainly focus on the long strike variation in the in the width of the eastern north, american volcanic continental margin, uh and i’ll start with a short introduction. Um um often passive margins are divided into two end members. The the magmapur margin which is um, has a highly complex Music high, offset low angle, normal falls and um usually described as tend to be or might be, uh quite wide. When, when i mean wide, i mean the distance between the stretch domain uh, which is faulted but not thinned and the oceanic domain, so the that might reach up to 350 kilometers in wide in width. The other end member is the volcanic margin, which is characterized by large additions of volcanic and magmatic rocks in the form of sewer, dipping reflectors or break up results, and for quite a lot of time it was thought that volcanic margins tend to be narrow and uh. I mean the necking zone from the making domain between the stretch domain and the igneous domain. Uh is usually uh less than 100 kilometers, and that was um um that the explanation was either because of nexus. That uh, that caused drifting uh due to higher denominator, would tend to localize strain and creating this sharply neck margin or that because of the sharp necking for rheological reasons or strain rate, we get this sharp index margin structured, neck lithosphere that causes convection, Music and but Either way that would we would uh predict to have a relatively narrow margin compared to imagine but uh.
Indeed, volcanic margins are narrow. For example, the southeast inland margin has necking zone of 100 kilometers, as you can see in this section, but other margins. Volcanic margins, like the one gwen was referring to the more imaging, could be quite wide, and actually this section um shows uh more than 300 kilometers or about 300 kilometers, a tone of crustal thinning. The way it is usually explained is that, as when she said that that drifting was protracted, it was. It took a long to allow the lithosphere to regain its thickness and uh and strength, thus forcing the rift to the failed rift to migrate. Laterally a long time and our volcanic drifting breakup occurred, it was juxtaposed next to five failed rift so um this is. It says that whenever we have a very protect long, protracted rifting, we would expect to have a wide volcanic margin and when we would have short, relatively continuous drifting phase that predated volcanic breakup, we will have relatively narrow, uh continental volcanic continental margin. In order to test this hypothesis, we we went to the eastern north american margin, which is uh volcanic uh and, most importantly, it experienced relatively short that’s about 30 million years of thrifting, and it was relatively continuous. But, most importantly, it was well uh well constrained. Well defined due to the numerous work that have been done on the offshore and offshore scenery strata. So we know it ended uh with uh. It started during the late triassic ended in the early jurassic and that uh it um followed the allegheny neurogeny that ended about 270 million years ago um with the accretion of several uh accreted terrain that constitute the prairie basement for the rifting of the eastern north american Margin um – and we have two main segments that i will talk on today: the george’s bank basin, which is undelayed by the meguma accreted meguma terrain, which is a sediment prone, relatively weak felsic terrain and the avalon terrain that underlays the baltimore canyon trough here, which is More of has a more intermediate composition, which is thought to be speculated to be uh more resilient to deformation lifting ended with the emplacement of the east coast, magmatic province that you can see its geophysical manifestation, which is the east coast magnetic anomaly.
That goes all along. The margin and correspond both to the continent ocean transition and to the magmatic additions that that were in place during the final stages of rifting. In order to investigate the structure of the margin, we used a dense grid of seismic reflection, uh data that was constrained by 40 wells and and several uh refraction lines that crosses the margin and in order to get a better perception of the crystal thickness in some Areas we also use published data of receiver function. Professional function results now, let’s talk a little bit about the long strike variation of the margin. This is an isopacma that displays the total positive sediments of the margin, and we do when looking at those two segments, the baltimore canyon trough and the george’s bank basin. We immediately see that the baltimore canyon trough accumulated much more sediments up to 18 kilometers of sediments. In northern baltimore canyon trough, while the george’s bank basin is more moderate and and soft and and went through less probably less subsidence and can accumulate less sediments to investigate the crustal structure of the margin of the segmentation. Let’S, look at the strike line going from the george’s banks down south to the baltimore canyon trough and we’ll start looking at the george’s bank basin first. So this is the george’s bank basin and uh. We immediately see these beautiful rift basins, uh that are delineated by normal falls, um and above them we see this sag basin of jurassic, cretaceous and tertiary age.
If we go further to the west, we see the long island platform that separates the george’s bank basin from the baltimore canyon trough, which is deeper, has a bigger sag basin and is underlined by both thin rift and sewer. Dipping reflectors – and we know the um – the composition of of the crust here, uh based on refraction, the published refraction data uh. What we don’t see here, um as opposed to the georgia’s bank, basing we don’t see as many faults. They probably are faults, but they’re, probably covered and and concealed by the magmatic cover, underneath the baltimore canyon trough, and this is how it looks in a structural map. This is the structural map of the top basement and you can see that in the george bank basin we have numerous faults, very complex patterns, but the moderate diffuse depression of the top basement, while in the baltimore canyon track, we don’t see as much fall folding, but We do have this deep trough: uh asymmetric, that reach up to eight seconds deep and it’s deeper part deepest part. Uh next step was to construct crustal thicknesses along the margin, and for that we used all of our available data. That includes um, deep seismic reflection and published refraction data and receiver function, data uh, using that we were able to constrain moho and then crustal thickness. So this is an isopark of the crust of thickness along the margins, the margin and we do see um thickness, minimum underneath the baltimore canyon trough and the george’s bank basin.
If we take the gradient of the crustal structure, we clearly can delineate between two domains: the unthin domain. Here in blue, we don’t see any crustal thickness changes and the necking domain, which is the domain in which the cross thins uh towards the continent ocean boundary. Here in um, yellow, green and red, and the red line in between them is the hinge line that separates the stretch domain or the unthin domain from the necking domain. Next step was to uh map the sewer, dipping reflectors, those um exclusive results, uh breakup, results, um and and the most accurate place to pinpoint them was actually looking at the landwarmost uh extent, and we did this using all available. Uh seismic reflection, data and let’s allow the correlation between the seaway dipping reflectors and the east coast magnetic anomaly: there’s a nice correlation in between them, and if we overlay this with the crustal hinge line, we see two different patterns between the baltimore canyon trough. Where we see the close uh relation between the steel dipping reflectors and the hinge line, that means that um, the thin domain is almost entirely covered with basalt, while in the george’s bank basin, the there is a a 200 kilometer wide zone that separates the crustal hinge Line from the continent, ocean transition and the seaward dipping reflectors um, so the necking domain in the joist bank basin is about 200 kilometers, while in the baltimore canyon traffic it moves from 100 kilometers to about 60 kilometers in the southern baltimore canyon crop.
So two different styles of thrifting and two different um relations between magmatism and and crustal uh, thinning um. If we overlay the this is gon na interrupt. This ten minutes are over just as a heads up right, um. If we overlay this map with the crustal um, the crustal terrain boundary, we see that the george’s bank basin is underlaid by by the meguma, meaning that all the crustal thinning and and unfolding actually occurred on the maguma um terrain, while the avalon teran remained intact. We know that there are also no onshore rift basins, uh onshore of the george’s bank. Basically, this is like, like the meguma, took all the strain to itself, as it was the weakest link in the chain, as opposed to the baltimore canyon trough. Where we do, we know of famous rift basin, for example the new york basin um onshore uh, but we don’t see a lot of faulting going on right east of the hinge line. So, to sum it up: let’s let’s, um, look at it and at this cartoon of the two segments, one is the baltimore canyon trough and one is uh george’s bank basin. So in the baltimore canyon trough uh, the prairie setting at the end of the orogeny was that the avalon terrain was juxtaposed on top of old appalachian terrains. While in the george bank basin, we had three um terrain stats, one upon each other, the meguma, the avalon and gander terrain once we start stretching um in the in the baltimore canyon trough, where we have more resilient crust, the um the formation mostly occur in in The reactivation of all the appalachian thrusts forming um, basing like the newark basin or the connecticut valley basins, while in the george’s bank basin, we don’t see any faulting onshore, but we do see a lot of faulting offshore where the george bank basin is today, and it Is accommodated with crustal thinning, which did not happen during this phase in the baltimore canyon trough.
So this is the prayer magmatic drifting, but once rifting become more magmatic strain tend to localize towards the place of of um final breakup and what happens in the in the baltimore canyon trough. It occurs on the avalon terrain, strain localizes, and then we start seeing the thin crustal thinning um correlated with uh, with the magmatic edition at the same location in the george’s bank basin. This occurs right next to the to the area of thin continental crust and we see and we get a wide wide continental margin, uh next to a zone of uh sewer, dipping reflectors and in oceanic crust, while in the baltimore canyon trough thinning is limited to the Place of uh magma, assisted drifting and uh magmatic additions. So, to conclude, the baltimore canal trough is: is a narrow volcanic margin with 50 to 80 kilometers of necking zone, the george’s bank exhibits a broad 200 kilometer zone of thinning uh, and this is the main point of disturb prematumatic thinning determine the widths of the of The future volcanic margin we propose that the cause for the variation in in premature thinning is actually the heterogeneous peripherality of the electrospill, specifically the differences in strength between the avalon and the ghuma terrain. Thank you very much. Excellent thanks a lot that’s time for one or two questions, and we have one question already: liane hello. Thank you. That was a great talk, um i’m interested in just going back to how you identified the continental material um associated with the baltimore canyon trough underlying the central atlantic area there as avalonia.
Thank you yes, uh, so it’s base, basically based on previous publication by hatcher and and i think what what they do know is actually what’s going on onshore getting to know exactly. No one could actually uh sample this data, but they do have some will in the salisbury embayment which they identified as avalon terrain and they basically based on geophysical data, extrapolated it to uh to the offshore okay and another question uh muhammad hi, thanks sasha thanks guy For the excellent talk, i was wondering in your uh definition of the narrowness of the margin, especially the baltimore kenyan transect. You mentioned that it’s narrow transect. But if you look at the conjugate side, you have much wider, actually continental crust and you have uh on on the african side. You have a very wide uh basins there. Actually that tells you that you have actually like a little bit wider continental crust than on the american side. So are you referring to the the gila the killer, transect yeah? So if you look at the dafla transects, actually you have a whole uh onshore basin. Actually, with a lot of cinematic structures there, and if you look at the seismic refraction that has been done on that side, they seem to hint actually that you have much more Music crystal thinning actually than on the conjugate side. So basically, if you take it, both sides, like the margin, is not as narrow as it appears on the american side.
So if i remember right, the the southernmost, the gila transect, that actually is the conjugate for the baltimore clinic isn’t that uh wide what’s, the with the wider transaction or the one further to the north, that are the conjugate of nova, scotia there’s, no conjugate for the Georgia’S bank basin, but actually the southernmost one which um they turned the conjugate of baltimore kendra, if i remember correctly, is actually quite narrow as well. You go. You don’t have a lot of magmatic addition. You just have some, but basically the necking is, if i’m ever right, it’s quite narrow, but we can. We can check it out afterwards, if, if you like, yeah, okay, okay, thanks yeah, actually we’re kind of running out of time, but that question by uh e chen is so interesting that we have to discuss it uh again, could you unlute and ask yourself yeah um? I i’d like to ask about the geological properties of each domain. I wonder if you have some quantitative estimates on that, for example, viscosity yearly streams, something like that that’s a great point. We do not have that actually uh. We have um actually descriptions of those domains. Coming from uh from the onshore – and we know we have one sample of the meguma in the georges bank basin, that is supposed to be a meta sediment um, while while the um, the avalon terrain, is supposed to be more intermediate island arc kind of crust.