Unless otherwise noted, all images are courtesy of Professor Burchfiel.
In this lecture we return to the Late Devonian/ Early Mississippian to discuss the Antler Orogeny. In this image we see the sheared and folded deep water sediments of the distal Antler marine basin thrust over and emplaced on top of the shallow water deposits of the continental margin. It is uncertain how and why these deep water sediments were thrust up and over the continental edge extending from southern California to Alaska. We have a good idea that they were not transported much more than a few hundred kilometers as the sediments contain fossils of North American affinity but lack and volcanic or Arc materials. To understand the story of what happened when, we rely on the sediments deposited at the time that the events were occurring. The Antler foredeep (introduced in a previous lecture) records the sediments derived from the mountain building event that occurred as the deep-water sediment overrode the shallow sediments. In the foredeep, we see both sediments derived locally off the topographically high Antler thrust sheet and those from the long traveling continental rivers. The Antler sediments, transported from the west are composed of conglomeratic deposits composed of cherts, shales and other low-grade metamorphic rocks. This is the first time we see sediments derived from the west- a feature that indicates the first tectonic activity and mountain building in the west. Transported from the east, we find finer-grained, quartz-rich sediments that have traveled great distances (during transport, other minerals are eroded away, weathered or deposited, leaving quartz grains preferentially carried over large distances).
In this image we see the Robert’s Mountain thrust contact up close. Above the contact we see the Early Missisppian conglomerates (but shouldn’t this be the distal sediments) and below we see a thick band of sheared rocks. To constrain the age of a thrust fault like this, you need to know the age of the youngest rocks in the footwall and the oldest age of the sediments produced by the deformation (the oldest age of the foredeep sediments). For the Roberts Mountain thrust, we know that the fault was active over roughly 10 million years.
A possible young analog for the Antler Orogeny.
In this image you can see a big thick stratigraphic section on the eastern edge of the foredeep where continental sediments and carbonates continued to do be deposited. Here there is no record of the Antler foredeep or orogeny.
If we move west from the previous slide we see the continental margin sediments sourced from the east inter-finger with the conglomerates sourced from the Antler Orogeny in the foredeep. In the central part of the foredeep, the sediments are 5-6 km thick, but if you go east, out of the foredeep, toward the continentally derived sediments, you find for the same time period sediments that are 300-400m thick.
In this image we see where early Pennsylvanian sediments were deposited over the eroded interior of the mountains of the Antler Orogeny. This suggests that the topography created by the Antler was eroded to low relief that the Antler Orogeny was complete by the early Pennsylvanian.
In this image we see folded carbonates that were deposited west of the Antler Orogeny at the same time that that deformation was occurring. This suggests that there was no major continental or island arc collision that created the Antler Orogeny.
In this image we see the ashes and sediments associated with the late Paleozoic island arcs that have experienced epidote alteration. That is what gives them their blue-green color. Though these volcanically derived deposits were created at the same time as the Antler, there are no evidence of ashes or volcanic fragments in the Antler-related sediments, suggesting that the Arcs were still far away from the edge of North America during Antler time. -------------- An interesting thing to know about the sediments created in Arc Complexes is that most of the volume of material that we see is not the primary eruptive deposits but rather the materials eroded off the volcanic edifices. Thus they are mostly sedimentary deposits derived from volcanic materials.
In this image we see a student contemplating the rocks further west of the previous image. There you find the ocean floor on which the Arc was deposited. The structurally lowest rocks we find below oceanic crust are tectonically layered dunite (the olivine-rich mantle) and peridotite (olivine-pyroxene).
Though these rocks are the source of the rocks that we traditionally think of as igneous, we would call these rocks metamorphic because they are under temperatures and pressures where they are ductilly deformed. In this image, the bands that we see in this rock are due to ductile flow in the mantle adjacent to the mid-oceanic ridge.
In this image we see the layering of crystals that accumulate and precipitate out of the melt in a magma chamber. There are light colored trondhjemites and mafic peridotite but most of the cumulate in the chamber is a coarse gabbro. So in this image the layering is not by tectonics or metamorphism but rather my crystals setting out of the melt through time. The mafic minerals precipitate first, then sequentially other minerals rain out of the melt and accumulate on the floor of the magma chamber. Banding or repeated crystal precipitation events suggests that the chemistry of the melt is changing in time as new magma enters the chamber or as the melt slowly cools. Though there can be variations due to the influx of new magma, the general progressive trend toward cumulates that are more feldspar rich provides geologists with good right-way-up indicators, an important tool for geologists mapping in field areas with extensive deformation.
As you move up through the oceanic crust, you leave the magma chamber and enter the sheeted dike complex. This region is composed of a collection of vertically-oriented basalts in dikes, but there are so many that this region is almost entirely composed of dikes. In this image you can see one of these darker dikes cutting up and left though a basaltic rock that is just composed of dikes itself. The darker rocks are along the dike margins due to rapid cooling against lighter-colored rocks near the middle of a previously intruded dike. The extension responsible for creating the fractures that the dikes travel up is due to the tectonic instability of the region adjacent to the mid-ocean ridge.
In this image, we see the part of the oceanic crust just above the sheet dikes complex: The Pillow Basalts. The production of rocks is fed from below by the dikes emerge into the deep sea and erupt on the surface of the seafloor. The bulbous form that we traditionally see is due to the eruption of magma fed from the dikes below, but rapidly cooled on the sea floor. The forms are rounded on top and pinched at the base as they fill the convex space between those basalts already erupted. In the cordillera we can see portions of the Cambrian ocean floor.
In this image we see folded and deformed cherts that were deposited above the pillow basalts. This picture is from the area west of the Roberts Mountain Thrust in the Klamath Mountains of northern California. The typical stratigraphy of the ocean floor has to so with the changing depth of the seafloor as oceanic crust is created at the spreading ridge and moves away. Near the spreading ridge, the crust is warm and buoyant and at shallow enough depths that carbonate deposits are preserved. As the crust cools and moves away from the spreading center it is less buoyant and is found at deeper depths. As the depth increases, the temperature of the bottom waters decrease. The sediments found in these deep-water regions are mainly silicious deposits, such as chert, shown above. This is because carbonate’s solubility increases with decreasing temperatures. Though throughout the ocean, carbonate detritus created by marine microorganisms rains out of the water column, the deep parts of the ocean (which are cooler) lack carbonate sediment because the falling detritus dissolves before it can be deposited. The depth at which this happens is called the CCD or the Calcite Compensation Depth. Thus the stratigraphy of ocean floor sediments typically starts out as carbonates, but later become silicious. There sediments complete the makeup of oceanic crust. If we find all these components present on land, then we can call it an Ophiolite Sequence. Lets review. Peridotites and Dunites with metamorphic ductile fabrics are at the contact between the mantle and the crust. Above there are magna chambers with cumulate textures due to the progressive settling of crystals from the melt. Fed from the magma chamber, the sheeted dike complex is typically composed of many, many generations of near-vertical dikes. These dikes reach the surface and provide the magma erupted at the ocean’s floor as pillow basalts. Pelagic sediments form the top of the oceanic crustal sequence.
In this image, we see some of those arc volcanics deposited above seafloor sediments.
This next image portrays the timing of different depositional events around the time of the Antler Orogeny. You can see that the Arc Volcanism continued throughout the time that the Antler was created. You can also see that the sediment found in the Antler Allochthon were experiencing active deposition until ~360ma. The filling of the foredeep then starts ~355ma, suggesting that the material had been thrust over the sinking continental margin. By ~350ma, we find sediments overlapping the Antler Orogeny, suggesting that it had been significantly eroded. The dates in this figure are old, and though we do not need to know the exact numbers right now, it is important to think about the different forms of geologic evidence available for constraining when certain events happened. Note that deposition continues in the Havallah basin east of the arc and west of the developing Antler Mountain belt. This suggests there was no arc collision during Antler time.
Three additional events are important during Mississippian-Permian time in western North America: the formation of the ancestral Rockies east of the Cordilleran belt, the formation of the Havallah Basin and the strike-slip formation of the southwest continental margin. This latter event set the stage for Mesozoic subduction beneath the continental crust in the southwest Cordillera. The Havallah sediments, deposited in a distal marine basin, were a lot like the Antler sediments before they were thrust over the western edge of North America. In the Permian, these marine sediments were thrust up and over the western edge of North America, overlapping the rocks of the Antler Orogeny. No oceanic crust in found in either the rock of the Antler or the Havallah Orogenies. The sediments could have been scraped off a down-going plate like in Accretionary wedges. The difference between the two events is that the accretion of the Havallah basin sediments was driven by an Island Arc collision. The island arc emplaced west of the Havallah sediments had an accretionary wedge on its western edge that contains marine sediments that range in age between Cambrian and Permian. These accretionary wedge sediments contain a fossil assemblage that is not of North American but rather of Asian affinity. This fossil evidence suggests that during arc formation vast areas of oceanic crust were subducted beneath the west side of the arc, some coming from the Asiatic side of the PaleoPacific. We know that the Havallah basin was close to North America as its fossils are of North American affinity. Some workers debate whether the Havallah basin did not directly collide with North America, but rather was transported laterally into position along large transverse faults like those found on the South Island of New Zealand today. An onlap sequence of sediments of Triassic age cover the Havallah and Antler rocks, demonstrating that the mountains created by these accretionary events were eroded away and subdued enough to have shallow marine rocks deposited over them.
In this slide we see the igneous rocks that intruded the edge of North America exposed by the late Paleozoic truncation fault. These granites are ~252ma and are overlain along an unconformable contact by conglomerates composed of the same granitic materials. This granite suggests that the edge of south-western North America were subject to subduction following truncation and formed the beginnings of a Continental Andean arc sometime before 252ma.
