scholarly journals Identifying elusive piercing points along the North American transform margin using mixture modeling of detrital zircon data from sedimentary units and their crystalline sources

2021 ◽  
Vol 19 (2) ◽  
pp. 12-21
Author(s):  
J. Clark Gilbert ◽  
Zane R. Jobe ◽  
Samuel A. Johnstone ◽  
Glenn R. Sharman ◽  
Howard Harper
Keyword(s):  
Detrital Zircon ◽  
Mixture Modeling ◽  
Alluvial Fan ◽  
Fault System ◽  
The North ◽  
San Andreas ◽  
Slip History ◽  
San Andreas Fault System ◽  
Ventura Basin ◽  
Transform Margin

The San Gabriel and Canton faults represent early stages in the development of the San Andreas fault system. However, questions of timing of initiation and magnitude of slip on these structures remain unresolved, with published estimates ranging from 42–75 km and likely starting in the Miocene. This uncertainty in slip history reflects an absence of appropriate piercing points. We attempt to better constrain the slip history on these faults by quantifying the changing proportions of source terranes contributing sediment to the Ventura Basin, California, through the Cenozoic, including refining data for a key piercing point. Ventura Basin sediments show an increase in detrital zircon U-Pb dates and mineral abundances associated with crystalline sources in the northern San Gabriel Mountains through time, which we interpret to record the northwest translation of the basin. by dextral strike-slip faulting. In particular, an Oligocene unit mapped as part of the extra-regional Sespe Formation instead has greater affinity to the Vasquez Formation. Specifically, the presence of a unimodal population of approximately 1180 Ma zircon, high (57%) plagioclase content, and proximal alluvial fan facies indicate that the basin was adjacent to the San Gabriel anorthosite during deposition of the Vasquez Formation, requiring 35 to 60 km of slip on the San Gabriel-Canton fault system. Mixture modeling of detrital zircon data supported by automated mineralogy highlights the importance of this piercing point along the San Gabriel-Canton fault system and suggests that fault slip began during the late Oligocene to early Miocene, which is earlier than published models. These two lines of evidence disagree with recent models that estimate greater than 60 km of offset, requiring a reappraisal of the slip history of an early strand of the San Andreas transform zone.

scholarly journals Chlorine-36/beryllium-10 burial dating of alluvial fan sediments associated with the Mission Creek strand of the San Andreas Fault system, California, USA

2019 ◽  
Author(s):  
Greg Balco ◽  
Kimberly Blisniuk ◽  
Alan Hidy
Keyword(s):  
San Andreas Fault ◽  
Alluvial Fan ◽  
Fault System ◽  
Middle Pleistocene ◽  
Slip Rates ◽  
San Bernardino Mountains ◽  
Production Ratio ◽  
San Andreas ◽  
San Andreas Fault System ◽  
Middle And Late Pleistocene

Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar/10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al/10Be pair, 36Cl/10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma that are too young to be accurately dated with the 26Al/10Be pair, and should theoretically be more precise for middle and late Pleistocene sediments in general. However, using the 36Cl/10Be pair is more complex because the 36Cl/10Be production ratio varies with the chemical composition of each sample. We use 36Cl/10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl/10Be production ratio in this lithology, and then apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl/10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derives from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and highlights the potential usefulness of the 36Cl/10Be pair for dating upper and middle Pleistocene clastic sediments.

scholarly journals Chlorine-36∕beryllium-10 burial dating of alluvial fan sediments associated with the Mission Creek strand of the San Andreas Fault system, California, USA

Geochronology ◽  
2019 ◽  
Vol 1 (1) ◽  
pp. 1-16
Author(s):  
Greg Balco ◽  
Kimberly Blisniuk ◽  
Alan Hidy
Keyword(s):  
San Andreas Fault ◽  
Alluvial Fan ◽  
Fault System ◽  
Middle Pleistocene ◽  
Slip Rates ◽  
San Bernardino Mountains ◽  
Production Ratio ◽  
San Andreas ◽  
San Andreas Fault System ◽  
Middle And Late Pleistocene

Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.

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