The Moho is immutable no more.

The Moho is immutable no more

For decades, geoscientists have thought of the Moho as a simple, immutable dividing line between the earth's crust and mantle. Discovered in 1909 by yugoslavian seismologist Andrija Mohorovicic, the continental Moho resides at depths of 35 kilometers, where the speeds of seismic waves traveling through the earth abruptly change.

In recent years, high-quality seismic reflection profiling - in which sound waves sent into the crust bounce back off contrasting rock types (SN: 12/8/84, p.364) - has provided seismologists with a much closer look at the Moho. And they are beginning to see that it has a more complex structure than once thought. This is leading them to wonder exactly what the Moho is, how it is formed and whether it changes with time.

Some of the latest seismic reflection profiles illustrating the puzzling character of the Moho were displayed last week at the meeting of the Geological Society of American in San Antonio, Tex. The data were compiled by the Consortium for Continental Reflection Profiling (COCORP), one of several research groups in the world doing systematic seismic surveys of the crust. COCORP members discussed some of the more than 2,500 kilometers of new seismic lines taken across the northwest Cordillera, the southern Appalachians and the U.S. Basin and Range Province.

One surprising aspect is evident in the Cordilleran mountain belt, which extends from Alaska to Guatemala. Its northwestern section is a patchwork of different crustal fragments that were carried thousands of kilometers by plate motion and jammed into the North American continent. Yet in profiles of the northwestern Cordillera, the Moho shows up as a flat, bright reflection that extends continuously through this region, oblivious to the sutures and different geologic boundaries that are so evident at the surface. Moreover, in some areas of this region, reflection profiles show that fault-like structures dip into the crust but are cut off at the Moho.

"This all implies that the Moho is a young structure," having re-formed after the terrains were plastered on to the continent, says K. Douglas Nelson of COCORP, which is headquartered at Cornell University. Something has made the Moho straight and continuous, erasing the bottoms of older faults and other geologic boundaries; the new Moho was superimposed on the older crustal fabric.

Some sort of change in the Moho's structure is apparent also in comparisons between the Cordillera and younger mountain belts, such as the Himalayas. There, the Moho is discontinuous, jumping to different depths in different regions. How, asks Nelson, does the Moho structure beneath the Himalayas change to that under the Cordillera? What changed the shape and character of the crust?

One possibility is that the newer, continuous Mohos were created - replacing old ones - when the crust was stretched out. In addition to the northwestern Cordillera, sharp, flat and highly reflective Mohos are seen in the Basin and Range Province, the continental shelf around Great Britain, the Paris basin and the U.S. Atlantic shelf. In most of these places, the last know major tectonic event was crustal extension. Perhaps in this process the crustal fabric was smoothed out, thinned and aligned horizontally. Another suggestion, according to Nelson, is that intrusions of magma into the lower crust left horizontal layers that differ in chemical makeup from surrounding rocks.

While the Moho is clearly defined in seismic profiles of these extensional regions, it is much more diffuse in other areas, such as the North American craton - one of the oldest, nonmountainous, "undeformable" parts of the continent. For example, such diffuse Mohos show up in COCORP profiles of flat regions such as Kansas, Minnesota, Wyoming and the Colorado plateau.

"May geologists have the perception that the processes that made mountain belts in recent times are basically the same processes that made the crust back through the Precambrian," says Nelson. "If that's true, then Precambrian crust in some general way ought to look like what we see [in newer crust]. But our observations suggest that's not true."

Either the processes shaping the earth have changed since the planet's early history, or there are some as-yet-undiscovered processes that act on the crust and the upper mantle over very long time periods, changing the crust from the kind of structure scientists see under the Cordillera to that of the craton just to the east of it, says Nelson.

One way to understand the processes that have formed areas and hope a pattern emerges, he says. "Right now we're just getting a hint of a pattern. But we don't know the reasons for it."