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Insights into the tectonic development of the Klamath Mountains Province from thermal data and modeling
- 1. Insights into the tectonic development of the Klamath Mountains Province from thermal data and modeling Rachel Piotraschke Penn State Department of Geosciences Graduate Student Colloquium March 19, 2011
- 2. 2
- 3. Based on Ernst et al., 2008 3
- 4. Modified from Kelsey et al., 1994 4
- 5. What drove (is driving?) topographic development in the Klamath Mountains Province? Is it related to the northward migration of the Mendocino Triple Junction? 5
- 6. When did the modern high topography of the KMP develop? Are there post-accretionary structures that might tell us something about timing and mechanisms of topographic development? 6
- 7. Hypothesis: Unusual high topography of the Klamaths is a recent development Extensional faulting has played an important role in the Cenozoic tectonic development of the Klamath Mountains 7
- 8. La Grange detachment 8 Modified from Cashmanand Elder, 2002
- 9. Approach: Timing of exhumation Patterns and rates of exhumation—erosional vs. tectonic 9
- 10. Apatite Fission Track (AFT) and apatite (U-Th)/He cooling ages from: Plutons in lower plate of detachment Plutons distal from detachment Plutons in upper plate? Thermal history 10
- 11. Vitrinite reflectance (thermal maturity) data from: Cretaceous rocks from Western Klamaths (distal from detachment) Miocene rocks (from closer to detachment) Thermal history 11
- 12. Previously existing apatite (U-Th)/He data from lower plate plutons indicated Late Eocene exhumation (Data from Batt et al., 2010) 12
- 13. New data shows north-to-south younging trend 13
- 14. …indicating ages were locked in by removal of the upper plate along the detachment 14
- 15. Back to the hypotheses: Extensional faulting was a significant driver of Cenozoic exhumation in the eastern Klamath Mountains Does this correspond with playing a significant role in the overall tectonics? 15
- 16. Back to the hypotheses: After unroofing by detachment, plutons were shallower than ~2.5 km depth Requires average erosion rates over last 20 Ma of less than ~0.13 mm/yr Is high topography of the Klamaths a recent development? Depends on modern erosion rates… 16
- 17. Thanks to: Kevin Furlong Sue Cashman at Humboldt State University Peter Kamp and Martin Danišík at University of Waikato 17
- 18. Apatite Fission Track (AFT) constraints Currently available data (Batt et al., 2010) gives ~80-100 Ma ages with large (~20 Ma) error Constrains depth at onset of detachment faulting to <4 km Limits thickness of detachment Requires some exhumation after pluton emplacement More data is forthcoming 18
- 19. Vitrinite reflectance VR data from coal at Big Bar (western KMP) indicates <4km burial & exhumation since Great Valley deposition Generally consistent with other data 19
Editor's Notes
- My talk today focuses on the Klamath Mountains of northern California, and what their thermal and exhumational history can tell us about their tectonic development. In the background here is a picture I took while doing field work in the Klamaths last summer, so you can get a sense of what the landscape is like today…. Pretty high elevation and a lot of relief, which I’ll talk about in a minute.
- The Klamath Mountains are located in an interesting part of the world. Here’s where we’re talking about. The Klamath Mountains Province is shaded in gray here and you can see that its southern edge just about lines up with the Mendocino Triple Junction, which is currently migrating north as the Pacific plate moves north relative to North America. The fact that they’re so close to the triple junction makes the Klamaths one of the prime places to study the how the overriding North American plate is affected by this spatial and temporal transition from subduction to translation.[MTJ/SAF formed around ~30 Ma;MTJ is migrating at the same rate as the rate of Pacific motion relative to N. America (attached to Mendocino FZ) (something like 5 cm per year)]
- I also want to quickly give an overview of the regional geology…The Klamath Mountains Province itself is made up of a series of east-dipping accreted terranes bounded by thrust faults. The Klamath rocks are also the oldest in the region; they’re surrounded by young Cascade volcanics to the north, and other relatively young rocks to the south. The Cretaceous Great Valley Group down here in pink is a forearc basin deposit that onlaps older Klamath bedrock in the south, and there are also a few isolated remnants of this Great Valley Group preserved in the southern Klamaths, which I’ll talk a little about later.
- In addition to being located just north of the triple junction, the Klamaths are also unusual because they have distinctly high topography compared to the rest of Cascadia. You can see that here, with the middle line, the “combined coast range” line, which is the average elevation over a band along the Pacific coast about 75 kilometers wide, and you can see that the area of the Klamaths represents the highest topography. Now this is average topography, the highest peaks in the Klamaths are around 2.5 kilometers above sea level.[western and eastern coast range zones do not overlap; KMP actually extends slightly further south]
- So together these observations beg two big picture questions—why are the Klamaths high? What drove that topographic development? And is it related to processes associated with the migration of the triple junction? These are pretty big questions, too big to answer with a masters thesis…
- But these are pretty big questions, so to address them we’ve broken them down into smaller questions that may be easier to answer. To know what drove the development of the Klamath’s high topography, it’s pretty essential to know when it happened, and currently that’s not well-constrained. Another thing we want to look at is whether there are structures in the Klamaths that are not present in the other, lower-elevation parts of Cascadia, because it then follows that those may tell us something about how the high topography developed. I specify “post-accretionary” because a lot of the structure in the Klamaths is these roughly north-south trending thrust faults that bound the accreted terranes, and we’re interested in a more recent part of the history so we want to differentiate from that.
- So our hypotheses are basically answers to those last two questions: we think it’s likely that the high topography is a relatively recent development—and what I mean by “recent” here is still being refined, but let’s go with the most generous definition of “since the Late Miocene”; and the hypothesized answer to the second question is that yes, there are these extensional structures in the Klamaths that have played a major role in their tectonic history, which may or may not turn out to be a clue to what’s driven the high topography. I’m going to focus first on the 2nd hypothesis because that’s where our data are currently most conclusive, but the same data set does eventually end up shedding light on the 1st hypothesis.*** Why are these two things related? Bringing up extensional faulting doesn’t come out of nowhere—even though our big question is “was topographic development recent,” it’s important to understand the extensional faulting piece since our approach involves exhumation
- So here’s the primary extensional structure I’m referring to. There is evidence of normal faulting throughout the Klamaths, particularly the eastern part, both in the form of a few small grabens, but also in this set of lower-angle faults that form this regional detachment. Here the upper plate of the detachment is shown in green, the lower plate in white, and based on fault surface striations the direction of offset is roughly south to south-southeast, which is shown by this orange arrow. Now there are multiple localities around this region where very low angle faulting with younger on older offset is observed, but the really spectacular exposure is at the La Grange Mine on Highway 299, marked by this orange star and here’s a picture of it. You can see the black fault rock, the fault is dipping off to the left and somewhat towards the camera. If you were at Sue Cashman’s colloquium talk last month, she talked a little about this same fault and she passed around a chunk of this cataclasite that caps the fault surface… that’s this black rock here. Take note of these plutons that are exposed in the lower plate because those are where the data I’m presenting today come from.[EK terrane = 3 subterranes, from N to S: Yreka, Trinity (lower plate/ultramafic/arch) + Central Metamorphic + Stuart Fork (Fort Jones), Redding]
- Now just to take a step back and review how I approached this problem: what we ultimately want to determine is the tectonic history of the Klamaths. It’s closely linked to their exhumation history, which is somewhat easier to determine because we can use the thermal history to get at it. Basically, we are using the thermal history to assess the timing, patterns, and rates of exhumation, and those will indicate likely mechanisms for exhumation, which then tells us something about the tectonic history. … Here’s another view of the detachment, both an up close view of the fault surface at the La Grange Mine locality, and a cross-sectional view of the detachment fault model in this region. We have north on the left, south on the right and the plutons are emplaced in the Trinity terrane, in this “Trinity arch” here, where today they’re exposed at the surface. With this model we expect the La Grange detachment to be a significant driver of exhumation in this region.
- So I’m using a couple different tools to look at the thermal history. The primary tool is low temperature thermochronology, specifically apatite fission track and (U-Th)/He. Both give us “cooling ages,” which just means they tell us when the sample cooled through a certain temperature; fission track records a hotter part of the temperature history than helium, so it gives older ages. The overall sampling strategy was to look at the thermal history of intrusive rocks in the lower plate, those are shown in pink… those in the upper plate, which may or may not include the Shasta Bally pluton (shown here in blue) but also includes some older intrusives we sampled over here [just east of Shasta Bally]… and also to get a couple samples from plutons that are relatively distal from the detachment, and therefore may have not been affected or unroofed by it, or perhaps affected less… those are up here in purple.[3 sample locations in WC (2 in SW end, 1 in NE end), 2 in Slinkard, 2 in NW end of SB, 1 in MM]
- We also have a couplevitrinite reflectance values that help constrain the thermal history, one from Cretaceous Great Valley Group rocks at Big Bar [point it out], one of those little Great Valley outliers I mentioned earlier, and one from the Miocene Weaverville Formation, over here in the Reading Creek graben.[GV sample is late Valanginian, ~138-136 Ma, based on marine fossils; Weaverville is Early to Middle Miocene, ~24-14 Ma, based on pollen and spore flora]
- So let’s look at the data. First, the helium ages. These correspond to cooling through a depth of about ~2.5 kilometers… we’ll be able to constrain that depth better with future modeling. The ages I’m showing here are multi-grain apatite helium ages published last year. You can see the sampling locations on this map, the small black squares. These ages indicate that there was some Late Eocene exhumation in this area, but don’t tell us a ton else… there’s a southward younging trend that may be visible, but it’s not totally clear.
- So last summer I sampled four other plutons in the area, and Martin Danisik got (U-Th)/He ages on 9 of these samples for us. These are single grain ages, with four to five grain ages per sample, so that’s why there are so many more points on this plot… but it represents 9 additional samples. The sample locations are shown up here on this map as the white circles. And as you can see, these new ages really solidify that southward younging trend seen in these central plutons, and they also extend it further north and south. This pattern really seems to indicate that these cooling ages indicate cooling that happened as detachment faulting was happening, as the upper plate was being removed north-to-south.
- The new data also allow us to start quantifying this part of the exhumation history. We can now say a few things: the La Grange detachment was active starting around ~40 Ma and continuing to at least ~20 Ma; horizontal displacement rates are on the order of 2-3 km/Ma (consistent w/published rates for detachment faults); and detachment faulting brought the samples through the apatite helium closure depth, like I said before probably around 2.5 kilometers.
- Does this correspond with playing a significant role in the overall tectonics? I’m still putting all the pieces together, but right now I would say yes.
- So can we say the high topography of the Klamaths is a recent development? well, we now have a constraint on the maximum amount of erosional exhumation that’s occurred since the Early Miocene in this area… it can’t have been more than 2.5 kilometers because the helium ages were locked in at that time. how that 2.5 kilometers of exhumation is distributed over time is less clear. There are some data on modern erosion rates available, but none published that I’ve been able to find.0.13 mm/yr = 0.13 km/Myr = 130 m/Myr. Max erosion rate from Tim Anderson Texas Tech thesis based on peneplain elevation & assuming peneplain was uplifted @ 0.7 Ma gives >2 mm/yr. Unpublished UW paper gives cosmogenic 10Be (millenial timescale) erosion rates of ~200-300 m/Myr (0.2-0.3 mm/yr)
- Now to look at a couple other constraints we currently have on the thermal history. Some fission track ages were published as part of the same paper last year for samples from the same lower-plate samples those previous helium ages were from. And partly because of the large error bars, there’s no clear spatial pattern in the ages, but they’re all mid-Cretaceous, in the range of 80-100 million years. Considering that the plutons were emplaced around 140 Ma, and detachment faulting started around 40 Ma, at the onset of detachment faulting the samples had fission track ages of ~40-60 Ma, which means they were retaining enough tracks to have an age that was at least 40 percent of their real age, or their hold time at depth. Thermal modeling shows this means they were held at depths of 4 km or less. Considering that the plutons were emplaced at depths of 7-11 kilometers, this requires some kind of exhumation prior to the mid-Cretaceous, but we can’t say too much more about it yet because of data quality issues. However, I’m hoping to have fission track ages from Peter Kamp’s lab for many of my samples within a few months, so those should be interesting, and will help constrain this earlier part of the history.
- Field photos of coal