In order to understand the significance of detrital grain ages in
sedimentary basins, a new approach is presented. Five characteristic
paths, identified by the change in age of detrital grain populations
combined with the change in lagtime over time, can be related to
different geodynamic settings in the source regions. When lagtime and
grain age increase over time, a change in source must be invoked - this
is usually a direct response to a geological event. A constant cooling
age, a vertical path, associated implicitly with increasing lagtime,
implies erosion of materials that had passed through the closure
temperature rapidly - exhuming sufficient rock to supply detritus over
the time of the path. Constant lagtimes, regardless of the lagtime
itself, are indicative of thermochronological stability in the source
region. This can involve fast or slow cooling. Finally, decreasing
lagtimes support the notion of increasing cooling rates in the source
regions over time. A test study is presented from sediments of the
northern Ecuadorian Sub-Andean Zone where geological events had
previously been identified using alternative methods. The addition of
heavy-mineral studies increased the precision in the interpretation. At
90 Ma, rapidly decreasing lagtimes point to a phase of tectonic
activity. From about 85 Ma until about 60 Ma the lagtimes were
approximately zero. This represents a phase of rapid exhumation of the
source regions correlating with the previously identified Pallatanga
event. An associated increase of metamorphic minerals occurs over this
time span, pointing to increased erosion from deeper horizons. At about
70 Ma, the oldest source region, the shield to the east, was switched
off. This timing correlates with a change from marine to continental
conditions in the basin, a change in palaeocurrent directions from the
east to the west, as well as an associated influx of material from the
growing Cordillera Real. At about 55 Ma, a change in source is
identified by a change in slope of the lagtime curve together with a
change in heavy minerals. From 50 to 35 Ma a renewed period of tectonism
in the source region is correlated with the docking of the Macuchi
terrane which clearly had an effect of increased erosion in the
Cordillera Real bringing in higher grade metamorphic minerals. From
about 32 Ma onwards the lagtime has been somewhat constant at about 30
Myr. This does not imply, however, a steady-state environment as it is
well known from other geological evidence that there have been other
events within this time frame. One must be cautious about
over-interpreting the lagtime as a method to determine steady state in
any region. It is a matter of scale.