Field Geology I

Lecture 7 Image Gallery

We begin this lecture at the end Paleozoic. Up to now, the tectonic assemblage of western North America has been dominated by the accretion of terranes over the passive margin. After the period of passive margin sedimentation, the Antler material overthrust the margin, then the Havallah (with a small island arc) overthrust the Antler, then finally the Sonoma Terrane overthrust the Havallah. These events were broadly experienced across the edge of western North America as we find remnants of the tectonic events from southern California to Alaska. At the southern end of this continental margin we recognize that this sequence may have been truncated by what we believe to have been a large left lateral transform fault that offset the southern extension of the continental margin sequence into the region near Caborca, Mexico. Though the sequence is poorly exposed due to extensive cover beds of Cenozoic volcanics, there are exposures of rocks like those found in the western US. This truncation fault is important because it exposed the region the southeastern corner of the US as a continental margin. This continental margin was very different compared to the margin further north where the Passive Margin, Antler, Havallah and Sonoma materials bordered, covered and insulated the continental basement of North America. In the area where the truncation happened, the continental basement was directly exposed at the new raw margin and also was not covered by thick sequences of sediment. At this time, the continental margin also began to switch from a ‘West-Pacific’ style margin (where the tectonics are dominated by irregular collisions of island arcs and continental fragments) to an ‘Andean’ style margin (where the tectonics are dominated by steady subduction of oceanic crust beneath continental curst resulting in arc volcanism and back arc or marginal basins). In these cross sections we compare the differences between the northwest and the southwest part of the continental margin. Along the northwest part of the margin (W NV), we see the red Sonoma terrane accreted to NA and the fold and thrust belt active further inland. At the same time (moving horizontally across the figure), the SE part of the margin has experienced rifting or a transform fault that has juxtaposed the thick Precambrian crust (white) covered with thin passive margin sediments against thin oceanic crust. Later in time we see in the NW that the polarity of subduction switched from west dipping to east dipping, effectively changing the plate margin from a ‘South-Pacific’ style margin to a ‘Andean’ style margin. As well, in the backarc environment, there is extension and deposition in the West Nevada Triassic basin. Along the SW portion of the plate margin, east dipping subduction is also active, but because the continental crust is dominantly thick, buoyant Precambrian crust, there is no back-arc extension (as in the case in W NV) and instead, there are structures indicative of compression. In this image we can see in the North the Sonoma orogen bounded on the East by the Sonoma thrust front. In the SW you can see the beginning of the arc intrusions above the east dipping subduction. The Permo-Triassic Front shown by the dashed line is the eastern extent of the compressional deformation occurring contemporaneously with the subduction. In this image we can see the mélange rocks that were scraped off the subducting oceanic crust and accumulated in accretionary wedges at the subduction zone present all along western North America following the Sonoma orogeny. This is another image of mélange. As you can see the rocks are highly deformed, sheared and folded. Only a mother could love these rocks. Within the mélange, we find mixed exotic fossils including those of both North American and Asian affinity. This image shows some of the coherent oceanic sediments found in the accretionary wedge. These are deep-water silicious cherts. Again, in these rocks we find mixed exotic fossils including those of both North American and those of Asian affinity. In this image we can see some of the elements we have discussed previously: the distribution of Havallah and related basinal (oceanic) sediments thrust onto western North America, the arc rocks accreted at the end of the Paleozoic and the dark blue arc accretionary prism rocks west of the arc. Note the termination of all these tectonic elements in the southwest U.S. This is another image showing the accretionary wedge material in blue. The black spots are areas of oceanic crust or partial ophiolite sequences. This material confirms that there was persistent Andean-style Arc subduction along the western edge of NA during early Mesozoic time. This slide shows the different thicknesses of the sediments in the fore-deep basin. You can see that the thickest part of the basin is in the west, adjacent to the front of the fold and thrust belt. In the south you can see the paleo-Gulf of Mexico. This extension extends barely northward into southern Arizona. Sedimentary deposits were thickest in the west and thinning to the east. In this image, the purple, Star Peak basin is where the West Nevada Triassic basin sediments were deposited in the back arc environment following development of the early Andean-style continental magmatic arc. The green-gray rocks are the arc volcanics along the edge of NA. South of the Canadian border, the rocks are all forming in situ, but north of the border, island arcs continue to collide with western British Colombia through the Jurassic and Cretaceous. Localities that have a ‘t’ next to them have oceanic sediments of Tethyan origin, indicating Asiatic origin. The ‘bs’ signifies the location of high pressure and low temperature metamorphic rocks characteristic of a subduction environment. We now consider the time between the middle Jurassic (Jr) and the late Cretaceous (K). In this image you can see the red magmatic arc mountain belt extending up the entire western edge of NA. Within the U.S., the sequence of geologic environments is relatively consistent along this margin. From west to east, the regions are: Accretionary Prism / Fore Arc / Arc Intrusions / Metamorphic zones related to the intrusions / back arc fold and thrust belt / Fore-Deep Basin. The eastern extent of this fold and thrust belt is shown by the outline labeled as Sevier Belt. The grey-brown coloring indicates the location of the fore-deep east of the fold and thrust belt. Note that this deformational and depositional style does not extend south to the Mojave region. The situation in Canada is different—arc collisions continue into the Cretaceous and the subduction zone and its associated magmatic arc migrate to the west. In this image we see that the western margin of NA in CA today has outcropping blocks of Cretaceous rock in mélange. In this image you can see the arc intrusive rocks well exposed in Yosemite Valley in California. In this image we can see the Cordilleran fold and thrust belt in Canada. Some have described the structural form of arc environments using the following unique terminology. The accretionary wedge is called the ‘Synthetic Thrust Belt,’ where the direction of shear on the down-going plate is the same as the direction of shear within the wedge. The fold and thrust belt in back arc is referred to as the ‘Antithetic Thrust Belt’ because the direction of the shear is opposite to the direction of shear in the subduction zone. Here is a cross section of the antithetic thrust belt in Canada that involves the Paleozoic sediments of the passive margin and young Mesozoic sediments. In this image you are looking west at the basal detachment of the fold and thrust belt where the fault is relatively parallel to the stratigraphy. In this image of the Keystone Thrust near Las Vegas, Cambrian limestones are thrust over Jurassic sandstones. These thinly bedded, fine-grained tilted sedimentary deposits are the turbidite deposits found in the deepest portion of the foredeep. These coarse conglomerate deposits are located in close proximity to the thrust front. In this slide, we see the location of the eastern limit of the arc-related plutons. They trace across the entire western edge of the plate margin, suggesting that Andean-style subduction was active throughout most of the Mesozoic. An over-riding theme in this story of the Arc environment is HEAT. This is the driving factor for most of the structures that we observe. In the subduction zone environment, the isotherms are depressed where cold rocks of the subducting plate go down but to the east they are juxtaposed against the hot intrusive Arc environment. Moving further in-board from the arc, the rocks progressively cool to within the fold and thrust belt, finally returning to normal temperatures in the undeformed basement. This heat in the arc provides a possible mechanism for consuming the foot-wall of the antithetic fold and thrust belt, but it is hard to locate where this might be happening. In a lot of geologic thinking, we too often expect compressional deformation to be accordion-style where all movement is in the direction of compression. But we need to consider lateral and oblique movement as well. In the Cordillera, we find large number of Mesozoic strike-slip faults capable of facilitating large magnitude displacements. These structures are dominantly right lateral and trend toward the northwest, commensurate with the interaction between the subducting oceanic crust and North America. The magnitude of the displacements on these structures is hotly debated to this day. In this image we can see a change in the behavior of the Arc environment. To the north in Canada we find the intrusive arc continues to emplace large igneous bodies. This is also true in the Mojave region in the southwest corner of north America, but as discussed earlier, the fold and thrust belt active along much of western NA is becoming less active to inactive in its central segment. In the southwest U.S., because most of the passive margin sequence was sheared off by the late Paleozoic truncation, the deformation is complicated by the presence of thick continental basement. The deformation is accomplished by much more ductile shear and metamorphism and a discontinuous and irregular fold and thrust belt. During the end Cretaceous, in between the Canadian arc and the Mojave region, the arc intrusions stop but the mélanges deposited in the accretionary wedges continue. Subduction remains active but the arc extinguishes…Why? Looking further east, you can see that large scale inter-continental deformation is occurring via block uplifts of Precambrian basement and deposition of sediments in adjacent basins within the cratonic rocks east of the former Paleozoic passive margin. It is suggested that these observations are a consequence of a change in the dip of the subducting slab. As the slab-dip shallows, subduction continues to build the accretionary wedge, but the down-going slab is not deep enough or hot enough to melt/dewater and produce igneous activity in the overlying mantle wedge. Where the slab does begin to dip, heat is concentrated against the base of the crust, but because the heat is applied to the base of thick continental crust, the deformational activity is expressed as the block uplifts rather than Arc volcanics forming the Laramide Colo-Wyoming Rocky Mountains. It is not known why the slab changed from steeply dipping to shallowly dipping, but research in northern Argentina (where there is also shallow-slab subduction), may shed light on this process. In this image you can see the Block uplifts of Precambrian basement with Paleozoic sediments draped up and over the blocks in the Wyoming Rocky Mountains. This image, taken near Cody, Wyoming also shows the deformation associated with the Laramide orogeny. Here Precambrian rocks are thrust up and over Paleozoic rocks. This image demonstrates the role of the basement rocks in the deformation patterns in the Cordillera, showing locations of the eastern limits of the Laramide deformation and the thrust and fold belt of Mesozoic age. This map view image of North America demonstrates that following the Laramide orogeny, Arc volcanism migrated southwestward and northwestward as the slab steepened and swept back westward. You can see that arc volcanism was reestablished along the western margin of North America by ~20ma. During the slab roll back and the migration of arc volcanism, large magnitude extension occurred in western NA, creating numerous metamorphic core complex structures (orange blobs). The thinning and heating during slab roll back and gravitational potential energy stored in a thickened crust during Mesozoic shortening are considered much of the driving mechanism for the later basin and range extension. During later Cenozoic time right oblique shear between the Farallon and North American plates also contributed to the extension that created the Basin and Range Province. This image shows all the basin and range faults. You can see the releasing bend in Baja California as the western edge of NA is shifted to the NW. This change may have reactivated extension in the Rockies and in the Rio Grande Rift. The oceanic ridge between the Farallon and Pacific plates was subducted beneath N.A. and the boundary between the Pacific and N.A. plates became right-lateral strike-slip ultimately leading to the formation of the San Andreas fault system and parallel faults across the ~500 km of western N.A. Remnants of the Farallon plate are present off northwest U.S. and southern Mexico. In this final slide, you can see the present configuration of the western edge of NA. Arc volcanism continues in the Cascades above the subduction zone there. A hot-spot impinging on the crust has created the volcanism in the Snake River Plain that migrated eastward and presently drives deformation and high heat flow in and around Yellowstone.