Fan and Rupture Mapping
Our fan chronology consists of twelve generations of late Pleistocene to Holocene alluvium, and undivided spring and lake deposits based on: 1) alluvial surface descriptions, 2) stratigraphic inset, onlap and burial relationships, and 3) correlations to existing dated lake and alluvial deposits (e.g., McGee et al., 2006; Jayko et al., 2008; Kirby et al., 2016; Sethanant et al., 2019; Regalla et al., 2022). These surface characteristics change rapidly in the first 1-10 ka after deposition, and more slowly in surfaces older than 10 ka. Seven units are Pleistocene to early-Holocene in age (> 80 ka - ~9 ka). These units include Qf1-Qf5 (old alluvium), Qol (old lake deposits), and Qyl (young lake deposits). There are five units that are mid to late Holocene in age (< 9 ka). These include Qf6b, Qf6a, Qf7, Qf8 (young alluvium), and Qa (active or recently active wash deposits). My mapped units broadly correlate to previously mapped units along the Ash Hill fault (Regalla et al., 2022) and the Panamint Valley fault (Hoffman, 2009; Sethanant, 2019).
Infra-Red Stimulated Luminescence
The following IRSL ages constrain deposit ages of Qf5, Qf6b, Qf6a, Qf7, and Qf8 deposits. The spring deposits sampled by PAN2109 yield an age of 20.6 ± 2.9 ka, with an interpreted unit age of 17.7 – 23.5 ka. Previous geochronologic dating of lake deposits in the Ballarat Basin of Panamint Valley indicate a 24 - 14 ka (OIS 2) highstand of Panamint Lake, ranging between elevations of 384 – 518 m, and a persistent wetland until ca. 12,575 – 10,000 14C yr B.P. (Jayko et al., 2008). The age of PAN2109 is consistent with these ages and was sampled from a layer below a fan deposit identified from surface morphology as Qf5. I interpret the maximum age of Qf5 to be ~ 17.7 ka from the underlying PAN2109 age constraint. However, this Qf5 surface is likely younger due to the persistence of the Panamint Lake highstand around ~ 17 – 18 ka (Jayko et al., 2008), as this surface, at an elevation of 397 m, would have been underwater. Qf6b deposits are dated at three locations (PAN2104, PAN2105, PAN2108), yielding ages of 4.20 ± 0.35 ka, 5.38 ± 0.51 ka, and 4.37 ± 0.23 ka (Table 2). I interpret the age of Qf6b to be between 3.85 – 5.89 ka. One Qf6a deposit (PAN2103) yielded an age of 3.58 ± 0.29, providing an age of 3.29 - 3.87 ka for Qf6a. The age of Qf6a at this location is consistent with sediments ponded against, and therefore post-dating it dated to 1.44 - 2.46 ka (PAN2102). One Qf7 deposit (PAN2107) yields an age of 2.36 ± 0.20 ka, or an age 2.16 – 2.56 ka for this unit. This Qf7 deposit likely predates Qf8 surfaces inset and/or onlapping against this unit. Thus, I interpret an age of Qf8 to be younger than 2.16 ka.
Supplemental Table 1. a - Percent water content of field sample. The number in parentheses is the saturated water content. The number in brackets is the water content used to calculate the Environmental Dose Rate DR using the fraction of saturation method (Nelson and Rittenour, 2015; van Genuchten; 1980).
b - Elemental concentrations determined using high-resolution gamma spectrometry (high purity Ge detector).
c - Number of aliquots meeting acceptance criteria (Appendix A, Table A1). The number in parentheses is the total number of aliquots measured.
d - The scatter represents the “over-dispersion” of the De values, defined by the statistical dispersion beyond what is expected for a perfectly bleached sample. Values > 30% are considered to be poorly bleached or post-depositional mixed sediments.
e - Environmental Dose Rate; calculated using the Dose Rate Age Calculator (DRAC; Durcan et al., 2015).
F - Age for fine-grained (180-250 µ), K-feldspar, post-IR IRSL, 100°C, using both the CAM and MAM models. Preferred model and ages are bolded, errors reported at 2σ (95% confidence).
Late Holocene Surface Ruptures
Surface ruptures in the Panamint Valley transtensional relay (PVTR) between the southern Ash Hill fault and the central Panamint fault provide evidence for strain accommodation in the late Holocene (Figure 3). We organize fault traces into three major rupture sequences (Arrays 1 - 3) based on individual fault location and orientation. In the PVTR, deformation is preserved as discontinuous, oblique right-lateral surface rupture (scarps), mole tracks, grabens, and tension gashes in Holocene and late Pleistocene alluvium. Each rupture sequence (array) in the PVTR is 5 – 7 km long, and accommodates both local transtension and transpression. Transtension is locally accommodated by grabens with 1s to 10s of meters of vertical displacement, developed in Pleistocene and Holocene fans. Local transpression is expressed in meter-scale left stepovers in late Pleistocene fans as upward warping of the fan surfaces. In Pleistocene-aged alluvium, ruptures are arranged into less than a dozen, subparallel to en echelon-stepping strands with spacing of 10s to 100s of meters. Along strike, in younger mid-late Holocene deposits, ruptures bifurcate into 17 – 20 subparallel to en échelon strands with smaller spacings of 1 – 10s of meters.
Surface ruptures in the transtensional relay occur as fresh single-event scarps, degraded single-event scarps, fresh rupture of compound multi-event scarps and degraded compound multi-event scarps (Figure 16). Fresh, single event scarps offset alluvium of all generations, have steep, unvarnished to very weakly varnished clasts on scarp faces, and range from pure dextral to oblique-dextral with scarp heights up to ~15 cm. Degraded single-event scarps are similar in height to fresh single-event scarps but with a gentler scarp slope and a moderate to strong revarnish of clasts on scarp faces. Compound multi-event scarps with fresh rupture have variable slopes and varnishes, commonly with a sharp cut in a gentler gradient complemented by a sharp line of unvarnished clasts in an otherwise uniformly varnished slope. Compound multi-event scarps with young ruptures can be up to 10’s of meters high and occur most commonly in Pleistocene-aged units but are found in Holocene units as well. Degraded compound multi-event scarps have similar heights to fresh, compound multi-event scarps, and generally have a uniform scarp varnishes and scarp gradients but may display weak discontinuities in scarp slope from past rupture. Additionally, we used varnish rings on scarp faces with late Holocene ruptures to infer the original ground position, prior to deformation, to determine the relative age of rupture. For instance, very defined varnish rings separating strong varnish with unweathered, unvarnished clast faces indicates a more recent, late Holocene rupture, while a less sharp boundary between the varnish and a strong varnish on the underside of a clast indicates an older Holocene rupture. Some grabens in Pleistocene-aged units have both fresh and degraded multi-event scarps, suggesting reoccupation of only one graben bounding fault in younger events. Presence of four types of surface ruptures in Qf7 - Qf6b units, as well as fresh rupture in Qf8 units, suggest that the spatial and temporal reoccupation of a single rupture plane is variable, and provides evidence for more than one rupture in the late Holocene.
Surface ruptures in the transtensional relay occur as fresh single-event scarps, degraded single-event scarps, fresh rupture of compound multi-event scarps and degraded compound multi-event scarps (Figure 16). Fresh, single event scarps offset alluvium of all generations, have steep, unvarnished to very weakly varnished clasts on scarp faces, and range from pure dextral to oblique-dextral with scarp heights up to ~15 cm. Degraded single-event scarps are similar in height to fresh single-event scarps but with a gentler scarp slope and a moderate to strong revarnish of clasts on scarp faces. Compound multi-event scarps with fresh rupture have variable slopes and varnishes, commonly with a sharp cut in a gentler gradient complemented by a sharp line of unvarnished clasts in an otherwise uniformly varnished slope. Compound multi-event scarps with young ruptures can be up to 10’s of meters high and occur most commonly in Pleistocene-aged units but are found in Holocene units as well. Degraded compound multi-event scarps have similar heights to fresh, compound multi-event scarps, and generally have a uniform scarp varnishes and scarp gradients but may display weak discontinuities in scarp slope from past rupture. Additionally, we used varnish rings on scarp faces with late Holocene ruptures to infer the original ground position, prior to deformation, to determine the relative age of rupture. For instance, very defined varnish rings separating strong varnish with unweathered, unvarnished clast faces indicates a more recent, late Holocene rupture, while a less sharp boundary between the varnish and a strong varnish on the underside of a clast indicates an older Holocene rupture. Some grabens in Pleistocene-aged units have both fresh and degraded multi-event scarps, suggesting reoccupation of only one graben bounding fault in younger events. Presence of four types of surface ruptures in Qf7 - Qf6b units, as well as fresh rupture in Qf8 units, suggest that the spatial and temporal reoccupation of a single rupture plane is variable, and provides evidence for more than one rupture in the late Holocene.
Evidence for four events
Event 3 is constrained by the ages of Qf6a, Qf7, and ponded sediment deposits against an Event 3 scarp. An IRSL age of an offset Qf6a (PAN2103) provides a maximum age bound of 3.29 - 3.87 ka for Event 3 (Figure 21). Three IRSL ages, two from ponded sediments (PAN2101, PAN2102) and one from an inset Qf7 (PAN2107) provide a minimum age bound of 1.44 – 2.56 ka for Event 3 (Figure 21). These bounds provide an age range for Event 3 of 1.66 – 3.58 ka (+0.29/-0.22).
Two events are unconstrained by IRSL ages. Event 2 occurred after the deposition of Qf7 and before the deposition of Qf8. The IRSL age of a Qf7 deposit provides a maximum age of 2.16 – 2.56 ka for Event 2 (Figure 21). Event 2 pre-dates Qf8 surfaces in the PVTR that are correlated to a surface along the Panamint fault with a 14C age of 0.53 – 0.80 cal. ka (Figure 21; Hoffman, 2009; Sethanant, 2019). This suggests that an approximate minimum age of Event 2 is ~0.67 cal. ka (using the mean of the 14C age). Using these approximate ages, I estimate an age range for Event 2 of ~0.67 – 2.36 ka (+0.20/-0.14). Event 1 occurred after the deposition of Qf8 and before the deposition of Qa. No IRSL constraints are available from offset Qf8 deposits in the PVTR for the timing of this rupture. However, using the 14C age of the Qf8 correlated surface 0.53 – 0.80 cal. ka. (Hoffman, 2009; Sethanant, 2019), and an approximate age of a ponded sediment deposit against a scarp in Qf6b with an age of < ~.250 ka, Event 1 likely occurred between ~300 – 700 years ago.
Overall, four events occurred in the PVTR in the mid-late Holocene (since ~ 4-5 ka). Using the age ranges estimated from multiple IRSL samples, Event 4 occurred between 3.58 - 4.20 ka (+0.35/-0.29), Event 3 occurred between 1.66 – 3.58 ka (+0.29/-0.22), Event 2 ruptured between ~0.67 – 2.36 ka (+0.20/-0.14) and Event 1 ruptured between ~300 – ~700 years ago. These events suggest a recurrence interval of ~1.0 - 1.3 ka for PVTR ruptures in the mid-late Holocene.
Two events are unconstrained by IRSL ages. Event 2 occurred after the deposition of Qf7 and before the deposition of Qf8. The IRSL age of a Qf7 deposit provides a maximum age of 2.16 – 2.56 ka for Event 2 (Figure 21). Event 2 pre-dates Qf8 surfaces in the PVTR that are correlated to a surface along the Panamint fault with a 14C age of 0.53 – 0.80 cal. ka (Figure 21; Hoffman, 2009; Sethanant, 2019). This suggests that an approximate minimum age of Event 2 is ~0.67 cal. ka (using the mean of the 14C age). Using these approximate ages, I estimate an age range for Event 2 of ~0.67 – 2.36 ka (+0.20/-0.14). Event 1 occurred after the deposition of Qf8 and before the deposition of Qa. No IRSL constraints are available from offset Qf8 deposits in the PVTR for the timing of this rupture. However, using the 14C age of the Qf8 correlated surface 0.53 – 0.80 cal. ka. (Hoffman, 2009; Sethanant, 2019), and an approximate age of a ponded sediment deposit against a scarp in Qf6b with an age of < ~.250 ka, Event 1 likely occurred between ~300 – 700 years ago.
Overall, four events occurred in the PVTR in the mid-late Holocene (since ~ 4-5 ka). Using the age ranges estimated from multiple IRSL samples, Event 4 occurred between 3.58 - 4.20 ka (+0.35/-0.29), Event 3 occurred between 1.66 – 3.58 ka (+0.29/-0.22), Event 2 ruptured between ~0.67 – 2.36 ka (+0.20/-0.14) and Event 1 ruptured between ~300 – ~700 years ago. These events suggest a recurrence interval of ~1.0 - 1.3 ka for PVTR ruptures in the mid-late Holocene.
Displacement Magnitude Per Event
Kinematics, Magnitude and Rupture Style of Late Holocene Events
Offset geomorphic landforms show that each of the four late Holocene events are associated with ~0.5 – 1.0 m of right-lateral slip, and up to ~0.2 m of vertical slip in a single event. Offset measurements isolating rupture in Qf6b show that Event 4 generated ~1.0 m of dextral slip and up to ~ 0.2 m of dip-slip, for a strike slip to dip slip ratio of 5:1. Using the linear scaling relationships for strike slip dominant faults of Wells and Coppersmith (1994) and Wesnousky (2008), an earthquake that produces ~1 m of total slip would produce a surface rupture ~33 km in length and produce an earthquake of Mw ≈ 6.9. Offset measurements isolating rupture in Qf6a and peaks in the Qf6a COPD plots show that Event 3 generated ~1.0 m of dextral slip and up to ~ .20 m of dip-slip for a strike slip to dip slip ratio of 5:1. Equivalent to Event 4, Event 3 likely produced a surface rupture length of ~33 km and an earthquake of Mw ≈ 6.9 (e.g., Wells and Coppersmith, 1994; and Wesnousky, 2008). Offset measurements and COPD plots show a peak at ~0.8 m of dextral slip in Qf7 deposits not seen in Qf8 deposits, which we interpret as the maximum slip associated with Event 2. Vertical offset associated with Event 2 is poorly constrained, with vertical displacements up to ~3.0 cm for lateral offsets of ~ 0.7 - 0.8 m in Qf7 deposits, for a strike slip to dip slip ratio between 23:1 and 27:1. Rupture with a maximum of ~ 0.8 m of total slip would be approximately ~27 km in length and produce an earthquake of Mw ≈ 6.8. One offset measurement in Qf8 provides a value of ~0.6 m of lateral offset, with negligible dip slip, equating to pure dextral slip for Event 1. Rupture with a maximum of ~ 0.6 m of total offset would produce a rupture ~ 20 km in length, and an earthquake with a Mw ≈ 6.7.
Field observations of scarp morphology and compiled offset data show that subsequent earthquakes in the PVTR rupture both preexisting faults and new fault strands. Additionally, within a single generation of alluvium, only some older faults are reruptured by younger events, while new faults form in lieu of other abandoned older faults. We observe variability for rupture reoccupation from field mapping in locations where: 1) Qf6 deposits are only offset by Event 4 and/or Event 3, indicated by degraded scarps without fresh, unvarnished clasts, 2) Qf6 or Qf7 is only offset by Event 2 and/or Event 1, indicated by fresh, unvarnished clasts and a steep scarp face, and 3) Qf7 and Qf8 deposits are both ruptured by, and truncate faults in a single location.
Field observations of scarp morphology and compiled offset data show that subsequent earthquakes in the PVTR rupture both preexisting faults and new fault strands. Additionally, within a single generation of alluvium, only some older faults are reruptured by younger events, while new faults form in lieu of other abandoned older faults. We observe variability for rupture reoccupation from field mapping in locations where: 1) Qf6 deposits are only offset by Event 4 and/or Event 3, indicated by degraded scarps without fresh, unvarnished clasts, 2) Qf6 or Qf7 is only offset by Event 2 and/or Event 1, indicated by fresh, unvarnished clasts and a steep scarp face, and 3) Qf7 and Qf8 deposits are both ruptured by, and truncate faults in a single location.