Koyna–Warna: fixing a drilling depth and an age for the same basalt flow
A final-year geology batch is working on data from the Koyna–Warna region of Satara district, where thousands of small earthquakes are recorded every year and where a 6 km scientific borehole has been proposed. Two data sets are on the table. (i) A refraction profile along a 60 km line gives a P-wave velocity of 5.8 km/s in the upper layer and 6.9 km/s below it; the crossover distance, where the two travel-time branches meet, is 40 km. (ii) A whole-rock K–Ar determination on a basalt flow near the site gives 92 ± 3 Ma. The same flow lies about 40 m above an intertrappean bed that has yielded Maastrichtian (latest Cretaceous) fossils, and palaeomagnetic work places it in chron C29r. The guide asks the group to state the depth at which the borehole will actually enter the faster layer, and to decide which age should go into the abstract of the report.
1. Calculate the depth to the top of the 6.9 km/s layer and advise on the final drilling depth. State the assumptions your calculation makes.
Read the worked answer
Use the two-layer refraction formula z = (Xc/2)·√((V₂−V₁)/(V₂+V₁)).
z = 20 × √(1.1/12.7) = 20 × 0.2943 = 5.9 km.
Assumptions: a flat interface, laterally constant velocities, first-arrival picks, and a velocity contrast produced by a single sharp boundary rather than a gradual gradient. The last one matters at Koyna, where the crust is faulted and fractured.
Advisory: 5.9 km is the depth at which the borehole reaches the interface, not the depth at which it samples it. A hole ending at 6.0 km would sit almost entirely in the 5.8 km/s rock and prove nothing about the layer below. The target should be set at roughly 6.5 km so that several hundred metres of the faster rock are cored, with casing and mud programmes designed for the 5.9 km transition. Also flag the common error in the report: a jump from 5.8 to 6.9 km/s is an upper-crust to mid-crust transition, not the Moho, which at Koyna would show velocities of about 7.8–8.0 km/s at 35–40 km depth.
2. Should the 92 ± 3 Ma K–Ar result be reported as the age of the flow? Justify your decision and specify the test you would demand before publication.
Read the worked answer
No. The flow is bracketed independently: it lies above Maastrichtian fossil-bearing intertrappeans and within chron C29r, which places it close to 66–65 Ma, in the main Deccan eruptive phase. A whole-rock age older by about 26 Myr is therefore a red flag, not a result.
Two mechanisms explain it. First, excess (inherited) argon trapped in the melt or in xenocrysts from older crust pushes K–Ar ages older; whole-rock basalt is especially vulnerable because potassium sits largely in glass and alteration phases. Second, argon loss from zeolites, clay and devitrified glass pulls ages younger and makes them scattered; either way the age is not a closed-system date.
Decision: report the flow as ~66 Ma on stratigraphic and magnetostratigraphic evidence, and mention the K–Ar value only as an unreliable determination with its reason.
The test to demand is ⁴⁰Ar/³⁹Ar step-heating on hand-picked, fresh plagioclase separates, with an age spectrum and isochron; a flat plateau with a trapped-argon intercept near the atmospheric ratio would be acceptable. Petrographic screening for zeolite, calcite and clay is a precondition, not an optional extra.
3. Small earthquakes here nucleate at 6–8 km depth. Using the velocity structure you derived, explain briefly why this depth range is significant.
Read the worked answer
The nucleation depths cluster just below the 5.9 km interface, where the rock changes from 5.8 km/s to 6.9 km/s. That is consistent with a change from a more fractured, altered volcanic pile to a stronger, more competent basement: the upper layer deforms by distributed small slip, while the stronger layer below can store enough elastic strain to rupture in discrete events.
The practical point for the borehole is that the target interval (below 5.9 km) is also the seismogenic interval, so the hole will cross active fault zones. The drilling plan must therefore include borehole breakouts and image logs to identify fracture orientation and in-situ stress, not just continuous coring. It also warns against reading the 5.9 km interface as a simple flat plane: the 40 km crossover distance has an uncertainty that translates into roughly ±0.3 km on depth, and a dipping interface would shift the computed value further.