Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 4

Section B: Remote Sensing & Active Tectonics.

Many optical land-mapping missions acquire daylight images in the midmorning, roughly 10–11 local solar time, using a Sun-synchronous orbit. This supplies substantial solar illumination while often avoiding the stronger afternoon convective cloud development. It also gives useful relief shadows without the very long shadows and weak illumination of sunrise. Repeated local-time viewing reduces illumination changes when comparing scenes.^[ref]

The time is a mission design choice, not a law applying to every optical satellite; afternoon acquisitions also exist. Sun-synchrony approximately fixes local solar time, not the same civil clock time everywhere, and seasonal solar-angle changes remain.

A band ratio image emphasizes differences in spectral reflectance rather than overall brightness. If two corrected band signals are $L_1=k{\rho }_1$ and $L_2=k{\rho }_2$, with the same multiplicative illumination factor $k$, their ratio is ${\rho }_1/{\rho }_2$: common shading or exposure factors cancel. Materials with different spectral slopes or absorption features can therefore separate more clearly than in either band alone.

The cancellation is approximate. Additive atmospheric path radiance, different angular reflectance responses and different band noise do not cancel, and division by a small denominator amplifies errors. Use calibrated, co-registered bands and mask cloud, deep shadow and invalid pixels before interpreting a ratio as a material or vegetation contrast.

Use a near-infrared/red ratio, or its normalized form, the normalized difference vegetation index:Useful bands are approximately $0.63$–$0.69\mu \mathrm{m}$ in red and $0.75$–$0.90\mu \mathrm{m}$ in near infrared, with exact ranges depending on the sensor. Chlorophyll absorbs strongly in red, whereas leaf structure scatters near infrared efficiently; healthy green vegetation therefore has a large near-infrared/red ratio and positive NDVI. Soil and water have different spectral contrasts.^[ref]

A single value is not a direct census of vegetation biomass or species. Soil background, canopy saturation, season, atmospheric effects and mixed pixels need consideration when mapping the distribution of vegetation.

Trace the subtle, approximately straight strike-slip fault that cuts across the dissected fan surfaces, descending to the right in the image. In normalized map coordinates $(u,v)$ measured from the upper left, a tracing corridor runs approximately from $(0.06,0.16)$ through $(0.35,0.30)$, $(0.65,0.46)$ and $(0.82,0.56)$ toward $(0.99,0.65)$. The broad central river interrupts the surface expression; extend the trace across that gap cautiously rather than following the river downstream.

The systematic jogs of small channels, particularly the conspicuous right-hand drainage, support right-lateral strike-slip faulting: draw arrows parallel to the trace, with the upper-side block toward the image right/down and the lower-side block toward the image left/up. Restore paired channel segments by reversing that sense and check that the fan rills align better. An isolated channel bend can have an erosional origin, so the consistent set of bends is stronger evidence than any one bend. No cardinal azimuth is asserted because Attachment B provides no north arrow.

A useful deflected stream is the conspicuous incised drainage in the right-hand part of the image. It approaches the fault corridor from the upper-right, follows a short segment along the trace near $(u,v)\simeq (0.83,0.60)$, then resumes down the fan toward the lower-left. Mark the two bends and the intervening along-fault reach, rather than labeling the whole channel as a fault.

A candidate beheaded stream is the short downstream gully in the fan immediately to the right of the central wide river, near $(u,v)\simeq (0.56,0.44)$: its head terminates at or near the cross-cutting fault corridor without an obvious continuation into a substantial upstream catchment. Mark it as a candidate abandoned/cut-off channel, and distinguish it from the active continuous drainage farther right. A displaced upstream counterpart, sediment infill or a field profile would strengthen that identification; optical appearance alone cannot uniquely establish every former channel connection.

If a restored channel or fan boundary has accumulated displacement $d$ since it formed, a constant fault slip rate $v=1.6\mathrm{mm}\mathrm{y}{\mathrm{r}}^{-1}$ gives the geomorphic offset-age relationThe right-hand channel jog is of order one to two of the printed $100$ m scale bars, so apparent offsets of order $100$–$200$ m give about $62.5$–$125$ ka. These are scale-based order-of-magnitude estimates, not surveyed piercing-point offsets: the amount of deflection along an eroding channel is not automatically equal to the tectonic displacement. For example, a reliably restored $150$ m marker would imply $93.75$ ka.

These times lie in the late Pleistocene, when global glacial–interglacial changes could alter vegetation, sediment supply and runoff and thereby promote alluvial fan deposition or abandonment. The wide interval spans different climatic phases, so it does not identify one event or prove a climatic cause. More importantly, a channel cut into a fan usually dates channel establishment or abandonment; the fan sediment can be older. Independent optically stimulated luminescence dating, stratigraphic correlation and several matched offsets are needed to assign a depositional age and test a climatic correlation.