Principle Investigator Contact Information

Name: Samantha Malavarca

Institution: Colorado State University, Department of Geosciences

Email: Sammy.Malavarca@colostate.edu

Alternate Contact Information

Name: Dr. John Singleton

Institution: Colorado State University, Department of Geosciences

Email: John.Singleton@colostate.edu

Dataset DOI: 10.5061/dryad.8931zcs4w

Dataset Overview

This dataset contains structural geology field data, kinematic analysis data, and K-feldspar ⁴⁰Ar/³⁹Ar geochronology data from the granite of Chokecherry Canyon in the Sangre de Cristo Range, southern Colorado, USA. These data support interpretations of ductile and brittle deformation associated with early Rio Grande Rift extension and later extensional exhumation of the range. Data were collected as part of a 1:10,000-scale geologic mapping and structural analysis of the ~30 Ma granite of Chokecherry granite and surrounding Proterozoic metatonalite.

All structural data were analyzed using Faultkin 8.1 and Stereonet 11.1.1 (Allmendinger et al., 2011; Cardozo & Allmendinger, 2013). K-feldspar ⁴⁰Ar/³⁹Ar step-heating analysis was performed at the Nevada Isotope Geochronology Laboratory (University of Nevada, Las Vegas), and age determinations were calculated using ArArCalc v. 2.7.0 (Koppers, 2002). 

In all data files, cells containing "n/a" indicate that data were not collected or could not be determined for that sample or measurement. This notation does not imply that a measurement is inapplicable, only that it was not obtained. 

None of the files in this repository are included in the journal supplementary materials. All data are available exclusively through this Dryad repository.

Dates of Data Collection

Structural field data: 2022–2025

K-feldspar sample collection: 2022

K-feldspar Ar/Ar analysis: 2024

Funding

This research was funded by the National Science Foundation Tectonics program, award number 2115719.

Related Publications

Malavarca, S., Singleton, J., Hudson, M.R., Ursin, G., Rahl, J.M., Wong, M.S., Primus, M., & Broeder, H. (2026, in revision). Ductile and Brittle Rio Grande Rift Deformation in Oligocene Granite Records a Two-Stage Rift History in Southern Colorado. Tectonics. 

Description of Files

1. StationLocations.csv

   GPS station locations collected during 1:10,000-scale geologic mapping of the granite of Chokecherry Canyon and surrounding Proterozoic metatonalite. Station locations were recorded with a handheld GPS. Coordinates are in UTM (Zone 13N, NAD83).

   Columns:

   Station/sample: A unique identifier for each field station or collected sample. The naming convention encodes the year of collection, month, field area, and sequential station number.

   UTM_X: Easting coordinate in meters (UTM Zone 13N, NAD83)

   UTM_Y: Northing coordinate in meters (UTM Zone 13N, NAD83)

2. FaultData.csv

   Orientations and kinematic data for brittle faults measured in the granite of Chokecherry Canyon, Proterozoic metatonalite, and minor mafic intrusions. The dataset comprises 27 faults: 19 within the granite of Chokecherry Canyon, 11 within the metatonalite, and 2 within a mafic intrusion. Slip sense criteria include R- and T-fractures, slickenfiber steps, and visible offset (Petit, 1987). Incremental shortening (P) and extension (T) axes were determined from paired fault plane and slickenline measurements using FaultKin 8.1. 

   Columns: 

   Station: Unique field station identifier (see StationLocations.xlsx for coordinates).

   Unit: Lithologic unit in which the fault was measured. ChC granitoid = granite of Chokecherry Canyon; Xto = Proterozoic metatonalite, and Gabbro = minor mafic intrusion.

   Fault Strike = strike of the fault plane in degrees, measured using the right-hand rule (RHR) convention, where the fault dips to the right of the strike direction.

   Dip: Dip angle of the fault plane in degrees from horizontal (0–90°).

   Dip Quad: Cardinal or intercardinal quadrant direction of fault dip.

   Slicks Rake (from RHR): Rake of the slickenlines measured from the right-hand rule of strike in degrees (0–180°). Cells containing "n/a" indicate that a rake measurement was not collected for that fault.

   Slicks trend: Trend of slickenlines in degrees (0–360°), measured clockwise from the north.

   Slicks plunge: Plunge of slickenline in degrees below horizontal (0–90°). 

   Slicks-plane misfit (°): Angular misfit in degrees between the measured slickenline orientation and the fault plane. A value of 0 indicates the slickenline lies perfectly within the fault plane. Cells containing "n/a" indicate that a misfit value was not calculated for that fault plane.

   Slip Sense: Interpreted sense of slip on the fault. N = normal; NL = normal with a left-lateral component; NR = normal with a right-lateral component; LL = left-lateral; unknown = slip sense could not be determined from available kinematic indicators.

   Confidence: Confidence level assigned to the slip sense interpretation, based on the quality and number of kinematic indicators observed. 0 = low confidence or slip sense unknown; 1 = moderate confidence; 2 = high confidence.

3. GneissicFoliationData.csv

   Orientations of gneissic foliation measured in the Proterozoic metatonalite surrounding the granite of Chokecherry Canyon. Foliation orientations are reported in strike and dip format. 

   Columns:

   Station: Unique field station identifier (see StationLocations.xlsx for coordinates).

   Fol strike: strike of foliation plane in degrees, measured using the right-hand rule convention.

   Dip: Dip angle of the foliation plane in degrees from horizontal (0–90°). 

4. SampleData.csv

   Summary data for oriented samples collected from shear zones across the study area, including shear zone orientation, lineation orientation, and shear sense. Coordinates are in UTM (Zone 13N, NAD83). The SOS column records the sense of shear. Oriented thin sections were cut in the X:Z plane for microstructural and kinematic vorticity analysis.

   Columns:

   Station: Unique field station identifier (see StationLocations.xlsx for coordinates). Some stations include a letter suffix (e.g., S21a, S21b) where multiple samples were collected at the same station.

   UTM_X: Easting coordinates in meters (UTM Zone 13N, NAD83).

   UTM_Y: Northing coordinates in meters (UTM Zone 13N, NAD83).

   Unit: Lithologic unit or structural domain from which the sample was collected. Entries describe the host rock and fabric type, including variants of quartz vein, granite (ChC granitoid), protomylonite, mylonite, aplitic phase, and metatonalite (Xto), as well as contact zone samples. Cells containing "n/a" indicate unit was not recorded.

   Surface: Description of the structural surface on which the lineation was measured, typically a C-plane or shear zone/vein margin. Cells containing "n/a" indicate that a specific surface description was not recorded.

   RHR Strike: Strike of shear zone margin or C-plane in degrees, measured using the right-hand rule convention. Cells containing "n/a" indicate the measurement was not collected.

   Dip: Dip angle of the shear zone margin or C-plane in degrees from horizontal (0–90°). Cells containing "n/a" indicate the measurement was not collected.

   Dip Quad: Cardinal or intercardinal quadrant direction of dip. Cells containing "n/a" indicate the measurement was not collected. 

   Lineation_trend: Trend of the stretching lineation in degrees (0–360°, measured clockwise from north). Cells containing "n/a" indicate no lineation was measured for that sample.

   Lineation_plunge: Plunge of the stretching lineation in degrees below horizontal (0–90°). Cells containing "n/a" indicate no lineation was measured for that sample.

   SOS: Sense of shear interpreted from field and thin section observations. N = normal (dip-slip); NL = normal with left-lateral component; T = thrust or reverse sense; top-NE, top-SW, top-W = dip-slip shear direction toward the indicated geographic location. Combined entries (e.g., N, top-NE) indicate that both a slip sense and a transport direction were recorded. Cells containing "n/a" indicate that a shear sense could not be determined. 

5. ProtomyloniteData.csv

   Structural measurements from protomylonitic domains in the granite of Chokecherry Canyon, including foliation and lineation orientations and kinematic observations. Approximately 30% of the granite records macroscopic protomylonitic foliation. All orientations are reported using the right-hand rule convention.

   Columns:

   Station: Unique field station identifier (see StationLocations.xlsx for coordinates).

   Fol strike: strike of the protomylonitic foliation plane in degrees, measured using the right-hand rule convention. Cells containing "n/a" indicate the measurement was not collected. 

   Dip: Dip angle of the foliation planes in degrees from horizontal (0–90°). Cells containing "n/a" indicate the measurement was not collected. 

   Dip Quad: Cardinal or intercardinal quadrant direction of foliation dip. Cells containing "n/a" indicate the measurement was not collected. 

   Lin_trend: Trend of the stretching lineation on the foliation plane in degrees (0–360°, measured clockwise from north). Cells containing "n/a" indicate no lineation was observed or measured at that station.

   Line_plunge: Plunge of the stretching lineation in degrees below horizontal (0–90°). Cells containing "n/a" indicate no lineation was observed or measured at that station.

   Acute rake: Rake of the stretching lineation on the foliation plane, reported as either the rake from right-hand rule of strike if <90°, or as 180 minus rake from right-hand rule of strike if rake >90°. Cells containing "n/a" indicate no lineation was measured or rake could not be determined.

   SOS: Sense of shear interpreted from field and microscopic observations. Values indicate the inferred transport direction: top-NE = top-to-northeast shear; top-SW = top-to-southwest shear; top-W = top-to-west shear. Cells containing "n/a" indicate shear sense could not be determined at that station.

6. Vorticity_analysis_for_shear_zones.xlsx

   Kinematic vorticity analysis results for 30 oriented thin sections from shear zones in the granite of Chokecherry Canyon. This dataset also includes orientation data from shear zones including the strike and dip of shear zone margins (c-planes), the strike and dip of foliation, the trend and plunge of lineations, and the sense of shear. The angle θ' between the mean long-axis orientation of dynamically recrystallized quartz grains and the shear zone margin or C-plane was measured from >300 grains per sample. The kinematic vorticity number (Wk) is calculated as sin(2θ'), and percent simple shear as (90−2θ')/90, following Xypolias (2010). Incremental stretching axes (ISA₁) and shortening axes (ISA₃) are reported as trend and plunge. The SOS column records sense of shear.

   Columns:

   Sample: Unique sample identifier (see StationLocations.xlsx for coordinates).

   Average θ': The mean angle in degrees between the long axes of dynamically recrystallized quartz grains and the shear zone margin or C-plane, measured from >300 grains per thin section. This angle is used to calculate kinematic vorticity.

   St. Dev. of θ': One standard deviation of the θ' measurements in degrees, reflecting variability in grain orientations within the sample.

   Vorticity No. (Wk): Kinematic vorticity number, calculated at sin(2θ'). Values range from 0 (pure coaxial strain) to 1 (simple shear). 

   % Simple shear: Percentage of bulk deformation attributable to simple shear for each shear zone, calculated as ((90-2θ')/90)x100, following Xypolias (2010). 

   Shear Zone Margin_Strike: Strike of the shear zone margin or C-plane in degrees, right-hand rule convention.

   Shear Zone Margin_Dip: Dip angle of the shear zone margin or C-plane in degrees from horizontal (0–90°).

   Dip Quad: Cardinal or intercardinal quadrant direction of shear zone margin dip.

   Lineation_trend: Trend of the stretching lineation in degrees clockwise from north (0–360°). 

   SOS: Sense of shear. N = normal (dip-slip); T = thrust or reverse sense;  L = left-lateral (sinistral strike-slip).

   Category Type: Kinematic category assigned to the shear zone based on sense of shear and shear zone geometry. Values: Low-angle top-W/SW = low-angle shear zone with top-to-southwest transport; normal top-W/SW = normal-sense shear zone dipping >30° with top-to-southwest transport; low-angle top-E/NE = low-angle shear zone with top-to-east or top-to-northeast transport; sinistral strike-slip = left-lateral strike-slip shear zone.

   T-axis_trend: Trend of the incremental extension axis (T-axis) in degrees (0–360°).

   T-axis_plunge: Plunge of the incremental extension axis in degrees below horizontal (0–90°). 

   P-axis_trend: Trend of the incremental shortening axis (P-axis) in degrees (0–360°).

   P-axis_plunge: Plunge of the incremental shortening axis in degrees below horizontal (0–90°). 

   Pole to c-plane_trend: Trend of the pole to the shear zone margin or C-plane in degrees (0–360°).

   Pole to c-plane_plunge: Plunge of the pole to the shear zone margin in degrees below horizontal (0–90°).

   Movement Plane_Strike: Strike of the movement plane in degrees, right-hand rule convention. The movement plane contains both the stretching lineation and the pole to the shear zone margin, and defines the plane within which ISA orientations are calculated.

   Movement Plane_Dip: Dip angle of the movement plane in degrees from horizontal (0–90°).

   Rake of lin on MP_RHR: Rake of the stretching lineation on the movement plane, measured from the right-hand rule strike direction, in degrees (0–180°).

   Acute rake: Acute angle of the stretching lineation rake on the movement plane, in degrees (0–180°).

   Direction from: Direction from which the ISA1 rake is measured on the movement plane. E = measured from the east end of strike; W = measured from the west end of strike.

   Rake + θ' (rake of ISA1): Rake of the incremental stretching axis (ISA1) on the movement plane, calculated by adding θ' to the lineation rake, in degrees.

   ISA1_trend: Trend of the incremental stretching axis (ISA1) in degrees (0–360°).

   ISA1_plunge: Plunge of the incremental stretching axis (ISA1) in degrees below horizontal (0–90°).

   Rake + (90 + θ') (rake of ISA3): Rake of the incremental shortening axis (ISA3) on the movement plane, calculated by adding (90 + θ') to the lineation rake, in degrees.

   ISA3_trend: Trend of the incremental shortening axis (ISA3) in degrees (0–360°)

   ISA3_plunge: Plunge of the incremental shortening axis (ISA3) in degrees below horizontal (0–90°).

7. 22-9-ChC-46_K-spar_Inner_temps_and_times.csv

   Inner furnace temperatures and heating times for each step of the ⁴⁰Ar/³⁹Ar incremental step-heating experiment performed on K-feldspar (microcline) from sample 22-9-CHC-46. This file records the laboratory-controlled heating schedule used to incrementally release argon gas from the sample across 40 heating steps. Analysis was performed at the Nevada Isotope Geochronology Laboratory, University of Nevada, Las Vegas. This file is provided as received from the laboratory. 

   Columns:

   #: Sequential heating step number (1–40), identifying the order in which gas was extracted from the sample.

   Inner T (°C): Furnace temperature in degrees Celsius (°C) at which gas was extracted during the heating step. Multiple steps at the same temperature reflect repeated extractions at that temperature to assess completeness of gas release.

   t (min): Duration of heating step in minutes. Longer durations at a given temperature allow more complete degassing of argon from that diffusion domain. 

8. 22-9-CHC-46_ArAr_Data.xlsx

   ⁴⁰Ar/³⁹Ar isotopic data from incremental step-heating analysis of K-feldspar (microcline) from sample 22-9-CHC-46, an unstrained sample from the granite of Chokecherry Canyon. Analysis was performed at the Nevada Isotope Geochronology Laboratory, University of Nevada, Las Vegas. The sample was irradiated at the Oregon State TRIGA Reactor with Fish Canyon Tuff sanidine (FC-2) standards. Age determinations and errors were calculated using ArArCalc v. 2.7.0 (Koppers, 2002). This file is provided largely as output from ArArCalc software; users seeking full documentation of all software-generated variables are referred to the ArAr Calc v. 2.7.0 user documentation (Koppers, 2002). 

   The spreadsheet contains six data sections arranged sequentially within a single sheet, each with its own header row. Each row represents one heating step, identified by a unique laboratory analysis number and the furnace temperature (°C) at which gas was extracted. Two special row types appear across all sections. First, rows where the first column contains "n/a" indicate values that were not calculated or are not applicable for that data section. Second, rows where the temperature column contains "S" represent the sum row for that section, calculated automatically by ArArCalc as the cumulative total across all heating steps.

   The six data sections and their key variables are as follows:

   Relative Abundances: The primary step-heating results. Key variables include the raw signal intensities for each argon isotope (³⁶Ar, ³⁷Ar, ³⁸Ar, ³⁹Ar, ⁴⁰Ar; units: millivolts, mV), each paired with a 1% sigma uncertainty column. Derived quantities include: ⁴⁰Ar(r)/³⁹Ar(k), the ratio of radiogenic ⁴⁰Ar to potassium-derived ³⁹Ar(±2σ); Age, the apparent age calculated from this ratio (Ma, ±2σ);  ⁴⁰Ar(r)%, the percentage total of ⁴⁰Ar that is radiogenic; ³⁹Ar(k)%, the cumulative percentage of potassium-derived ³⁹Ar released at each step used to construct the age spectrum; and K/Ca, the potassium-to-calcium ratio (±2σ), a proxy for the mineralogical domain being degassed. 

   Normal Isochron: Data for reconstructing a normal isochron plot (³⁹Ar(k)/⁴⁰Ar(a) vs. ⁴⁰Ar(a+r)/³⁶Ar(a)), used to evaluate the initial ⁴⁰Ar/³⁶Ar ratio and check for excess argon. The column r.i. is the relative inverse variance weighting used in isochron regression. 

   Inverse Isochron: Data for constructing an inverse isochron plot (³⁹Ar(k)/⁴⁰Ar(a+r) vs. ³⁶Ar(a)/⁴⁰Ar(a+r), an alternative representation that more directly resolves the trapped argon component.

   Degassing Patterns: Breakdown of each argon isotope signal into its source components per heating step. Subscript notation indicates source: (a) = atmospheric, (c) = cosmogenic, (ca) = calcium-derived, (cl) = chlorine-derived. All values in mV with associated 1%σ uncertainties.

   Additional Parameters: Supplementary calculated quantities including isotope ratios, timing parameters, decay corrections, and molar quantities of argon, as generated by ArArCalc.

   Procedure Blanks, Intercept Values, Sample Parameters, and Irradiation Constants: Laboratory metadata sections generated by ArArCalc documenting blank corrections applied during analysis, regression intercept values, sample preparation details (analyst, standard used, J-value, sensitivity, irradiation information), and the interference correction factors used to correct for reactor-produced argon isotopes. 

Access information

This dataset is made available under a CCO 1.0 Universal (CCO 1.0) Public Domain Dedication license.